BIND 9.4 Administrator Reference Manual, HTML edition
Copyright © 2004-2007 Internet Systems Consortium, Inc.
("ISC")
Copyright © 2000-2003 Internet Software Consortium.
Table of Contents
- 1. Introduction
- 2. BIND Resource
Requirements - 3. Name Server Configuration
- 4. Advanced DNS Features
- 5. The BIND 9
Lightweight Resolver - 6. BIND 9
Configuration Reference -
- Configuration
File Elements - Configuration
File Grammar -
- acl Statement
Grammar - acl Statement
Definition and Usage - controls Statement
Grammar - controls Statement
Definition and Usage - include Statement
Grammar - include Statement
Definition and Usage - key Statement
Grammar - key Statement
Definition and Usage - logging Statement
Grammar - logging Statement
Definition and Usage - lwres Statement
Grammar - lwres Statement
Definition and Usage - masters Statement
Grammar - masters Statement
Definition and Usage - options Statement
Grammar - options Statement
Definition and Usage - server Statement
Grammar - server Statement
Definition and Usage - trusted-keys Statement
Grammar - trusted-keys Statement
Definition and Usage - view Statement
Grammar - view Statement
Definition and Usage - zone Statement
Grammar - zone Statement
Definition and Usage
- acl Statement
- Zone File
- Configuration
- 7. BIND 9
Security Considerations - 8. Troubleshooting
- A. Appendices
- I. Manual pages
-
- dig — DNS
lookup utility - host — DNS
lookup utility - dnssec-keygen — DNSSEC
key generation tool - dnssec-signzone — DNSSEC
zone signing tool - named-checkconf — named
configuration file syntax checking tool - named-checkzone — zone
file validity checking or converting tool - named — Internet
domain name server - rndc — name
server control utility -
rndc.conf— rndc
configuration file - rndc-confgen — rndc
key generation tool
- dig — DNS
Table of Contents
The Internet Domain Name System (DNS)
consists of the syntax to specify the names of entities in the Internet in
a hierarchical manner, the rules used for delegating authority over names,
and the system implementation that actually maps names to Internet addresses. DNS data
is maintained in a group of distributed hierarchical databases.
The Berkeley Internet Name Domain (BIND)
implements a domain name server for a number of operating systems. This document
provides basic information about the installation and care of the Internet
Systems Consortium (ISC) BIND version
9 software package for system administrators.
This version of the manual corresponds to BIND version 9.4.
In this document, Section 1 introduces
the basic DNS and BIND concepts. Section
2 describes resource requirements for running BIND in
various environments. Information in Section 3 is task-oriented in
its presentation and is organized functionally, to aid in the process of
installing the BIND 9 software. The task-oriented
section is followed by Section 4,
which contains more advanced concepts that the system administrator may need
for implementing certain options. Section 5 describes
the BIND 9 lightweight resolver. The contents
of Section 6 are organized as in a
reference manual to aid in the ongoing maintenance of the software. Section
7 addresses security considerations, and Section
8 contains troubleshooting help. The main body of the document
is followed by several Appendices which
contain useful reference information, such as a Bibliography and
historic information related to BIND and
the Domain Name System.
In this document, we use the following general typographic conventions:
|
To describe: |
We use the style: |
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a pathname, filename, URL, hostname, mailing list name, or |
|
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literal user input |
|
|
program output |
|
The following conventions are used in descriptions of the BIND configuration
file:
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To describe: |
We use the style: |
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keywords |
|
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variables |
|
|
Optional input |
[Text is enclosed in square brackets] |
The purpose of this document is to explain the installation and upkeep
of the BIND software package, and we begin
by reviewing the fundamentals of the Domain Name System (DNS)
as they relate to BIND.
The Domain Name System (DNS) is a hierarchical, distributed database.
It stores information for mapping Internet host names to IP addresses and
vice versa, mail routing information, and other data used by Internet applications.
Clients look up information in the DNS by calling a resolver library,
which sends queries to one or more name servers and
interprets the responses. The BIND 9
software distribution contains a name server, named,
and two resolver libraries, liblwres and libbind.
The data stored in the DNS is identified by domain
names that are organized as a tree according to organizational
or administrative boundaries. Each node of the tree, called a domain,
is given a label. The domain name of the node is the concatenation
of all the labels on the path from the node to the root node.
This is represented in written form as a string of labels listed from
right to left and separated by dots. A label need only be unique within
its parent domain.
For example, a domain name for a host at the company Example,
Inc. could be ourhost.example.com,
where com is the top level domain to which ourhost.example.com belongs, example is
a subdomain of com, and ourhost is
the name of the host.
For administrative purposes, the name space is partitioned into areas
called zones, each starting at a
node and extending down to the leaf nodes or to nodes where other zones
start. The data for each zone is stored in a name
server, which answers queries about the zone using the DNS
protocol.
The data associated with each domain name is stored in the form of resource
records (RRs). Some
of the supported resource record types are described in the
section called “Types of Resource Records and When to Use Them”.
For more detailed information about the design of the DNS and the DNS
protocol, please refer to the standards documents listed in the
section called “Request for Comments (RFCs)”.
To properly operate a name server, it is important to understand the
difference between a zone and a domain.
As stated previously, a zone is a point of delegation in the DNS tree.
A zone consists of those contiguous parts of the domain tree for which
a name server has complete information and over which it has authority.
It contains all domain names from a certain point downward in the domain
tree except those which are delegated to other zones. A delegation point
is marked by one or more NS records in
the parent zone, which should be matched by equivalent NS records at the
root of the delegated zone.
For instance, consider the example.com domain
which includes names such as host.aaa.example.com and host.bbb.example.com even
though the example.com zone includes only
delegations for the aaa.example.com and bbb.example.com zones.
A zone can map exactly to a single domain, but could also include only
part of a domain, the rest of which could be delegated to other name servers.
Every name in the DNS tree is a domain,
even if it is terminal, that is,
has no subdomains. Every subdomain
is a domain and every domain except the root is also a subdomain. The terminology
is not intuitive and we suggest that you read RFCs 1033, 1034 and 1035
to gain a complete understanding of this difficult and subtle topic.
Though BIND is called a "domain name
server", it deals primarily in terms of zones. The master and slave declarations
in the named.conf file specify zones, not
domains. When you ask some other site if it is willing to be a slave server
for your domain, you are actually
asking for slave service for some collection of zones.
Each zone is served by at least one authoritative
name server, which contains the complete data for the zone.
To make the DNS tolerant of server and network failures, most zones
have two or more authoritative servers, on different networks.
Responses from authoritative servers have the "authoritative answer" (AA)
bit set in the response packets. This makes them easy to identify when
debugging DNS configurations using tools like dig (the
section called “Diagnostic Tools”).
The authoritative server where the master copy of the zone data is
maintained is called the primary master server,
or simply the primary. Typically
it loads the zone contents from some local file edited by humans or perhaps
generated mechanically from some other local file which is edited by
humans. This file is called the zone file or master
file.
In some cases, however, the master file may not be edited by humans
at all, but may instead be the result of dynamic
update operations.
The other authoritative servers, the slave servers
(also known as secondary servers)
load the zone contents from another server using a replication process
known as a zone transfer. Typically
the data are transferred directly from the primary master, but it is
also possible to transfer it from another slave. In other words, a slave
server may itself act as a master to a subordinate slave server.
Usually all of the zone's authoritative servers are listed in NS records
in the parent zone. These NS records constitute a delegation of
the zone from the parent. The authoritative servers are also listed in
the zone file itself, at the top level or apex of
the zone. You can list servers in the zone's top-level NS records that
are not in the parent's NS delegation, but you cannot list servers in
the parent's delegation that are not present at the zone's top level.
A stealth server is a server
that is authoritative for a zone but is not listed in that zone's NS
records. Stealth servers can be used for keeping a local copy of a zone
to speed up access to the zone's records or to make sure that the zone
is available even if all the "official" servers for the zone are inaccessible.
A configuration where the primary master server itself is a stealth
server is often referred to as a "hidden primary"
configuration. One use for this configuration is when the primary master
is behind a firewall and therefore unable to communicate directly with
the outside world.
The resolver libraries provided by most operating systems are stub
resolvers, meaning that they are not capable of performing
the full DNS resolution process by themselves by talking directly to
the authoritative servers. Instead, they rely on a local name server
to perform the resolution on their behalf. Such a server is called
a recursive name server; it
performs recursive lookups for
local clients.
To improve performance, recursive servers cache the results of the lookups
they perform. Since the processes of recursion and caching are intimately
connected, the terms recursive server and caching
server are often used synonymously.
The length of time for which a record may be retained in the cache of
a caching name server is controlled by the Time To Live (TTL) field associated
with each resource record.
Even a caching name server does not necessarily perform the complete
recursive lookup itself. Instead, it can forward some
or all of the queries that it cannot satisfy from its cache to another
caching name server, commonly referred to as a forwarder.
There may be one or more forwarders, and they are queried in turn until
the list is exhausted or an answer is found. Forwarders are typically
used when you do not wish all the servers at a given site to interact
directly with the rest of the Internet servers. A typical scenario would
involve a number of internal DNS servers
and an Internet firewall. Servers unable to pass packets through the
firewall would forward to the server that can do it, and that server
would query the Internet DNS servers
on the internal server's behalf.
The BIND name server can simultaneously
act as a master for some zones, a slave for other zones, and as a caching
(recursive) server for a set of local clients.
However, since the functions of authoritative name service and caching/recursive
name service are logically separate, it is often advantageous to run them
on separate server machines. A server that only provides authoritative
name service (an authoritative-only server)
can run with recursion disabled, improving reliability and security. A
server that is not authoritative for any zones and only provides recursive
service to local clients (a caching-only server)
does not need to be reachable from the Internet at large and can be placed
inside a firewall.
Table of Contents
DNS hardware requirements have traditionally
been quite modest. For many installations, servers that have been pensioned
off from active duty have performed admirably as DNS servers.
The DNSSEC features of BIND 9 may prove
to be quite CPU intensive however, so organizations that make heavy use of
these features may wish to consider larger systems for these applications. BIND 9
is fully multithreaded, allowing full utilization of multiprocessor systems
for installations that need it.
CPU requirements for BIND 9 range from
i486-class machines for serving of static zones without caching, to enterprise-class
machines if you intend to process many dynamic updates and DNSSEC signed
zones, serving many thousands of queries per second.
The memory of the server has to be large enough to fit the cache and zones
loaded off disk. The max-cache-size option
can be used to limit the amount of memory used by the cache, at the expense
of reducing cache hit rates and causing more DNS traffic.
Additionally, if additional section caching (the
section called “Additional Section Caching”) is enabled,
the max-acache-size can be
used to limit the amount of memory used by the mechanism. It is still good
practice to have enough memory to load all zone and cache data into memory — unfortunately,
the best way to determine this for a given installation is to watch the name
server in operation. After a few weeks the server process should reach a
relatively stable size where entries are expiring from the cache as fast
as they are being inserted.
For name server intensive environments, there are two alternative configurations
that may be used. The first is where clients and any second-level internal
name servers query a main name server, which has enough memory to build a
large cache. This approach minimizes the bandwidth used by external name
lookups. The second alternative is to set up second-level internal name servers
to make queries independently. In this configuration, none of the individual
machines needs to have as much memory or CPU power as in the first alternative,
but this has the disadvantage of making many more external queries, as none
of the name servers share their cached data.
ISC BIND 9 compiles and runs on a large
number of Unix-like operating system and on NT-derived versions of Microsoft
Windows such as Windows 2000 and Windows XP. For an up-to-date list of supported
systems, see the README file in the top level directory of the BIND 9 source
distribution.
Table of Contents
In this section we provide some suggested configurations along with guidelines
for their use. We suggest reasonable values for certain option settings.
The following sample configuration is appropriate for a caching-only
name server for use by clients internal to a corporation. All queries from
outside clients are refused using the allow-query option.
Alternatively, the same effect could be achieved using suitable firewall
rules.
// Two corporate subnets we wish to allow queries from.
acl corpnets { 192.168.4.0/24; 192.168.7.0/24; };
options {
directory "/etc/namedb"; // Working directory
allow-query { corpnets; };
};
// Provide a reverse mapping for the loopback address 127.0.0.1
zone "0.0.127.in-addr.arpa" {
type master;
file "localhost.rev";
notify no;
};
This sample configuration is for an authoritative-only server that is
the master server for "example.com"
and a slave for the subdomain "eng.example.com".
options {
directory "/etc/namedb"; // Working directory
allow-query-cache { none; }; // Do not allow access to cache
allow-query { any; }; // This is the default
recursion no; // Do not provide recursive service
};
// Provide a reverse mapping for the loopback address 127.0.0.1
zone "0.0.127.in-addr.arpa" {
type master;
file "localhost.rev";
notify no;
};
// We are the master server for example.com
zone "example.com" {
type master;
file "example.com.db";
// IP addresses of slave servers allowed to transfer example.com
allow-transfer {
192.168.4.14;
192.168.5.53;
};
};
// We are a slave server for eng.example.com
zone "eng.example.com" {
type slave;
file "eng.example.com.bk";
// IP address of eng.example.com master server
masters { 192.168.4.12; };
};
A primitive form of load balancing can be achieved in the DNS by
using multiple A records for one name.
For example, if you have three WWW servers with network addresses of 10.0.0.1,
10.0.0.2 and 10.0.0.3, a set of records such as the following means that
clients will connect to each machine one third of the time:
|
Name |
TTL |
CLASS |
TYPE |
Resource Record (RR) Data |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
When a resolver queries for these records, BIND will
rotate them and respond to the query with the records in a different order.
In the example above, clients will randomly receive records in the order
1, 2, 3; 2, 3, 1; and 3, 1, 2. Most clients will use the first record returned
and discard the rest.
For more detail on ordering responses, check the rrset-order substatement
in the options statement, see RRset
Ordering.
This section describes several indispensable diagnostic, administrative
and monitoring tools available to the system administrator for controlling
and debugging the name server daemon.
The dig, host,
and nslookup programs are
all command line tools for manually querying name servers. They differ
in style and output format.
- dig
-
The domain information groper (dig)
is the most versatile and complete of these lookup tools. It has
two modes: simple interactive mode for a single query, and batch
mode which executes a query for each in a list of several query
lines. All query options are accessible from the command line.dig[@server]domain[query-type]
[query-class] [+query-option]
[-dig-option] [%comment]The usual simple use of dig will take the form
dig @server domain query-type query-class
For more information and a list of available commands and options,
see the dig man page. - host
-
The host utility
emphasizes simplicity and ease of use. By default, it converts
between host names and Internet addresses, but its functionality
can be extended with the use of options.host[-aCdlrTwv] [-cclass]
[-Nndots] [-ttype]
[-Wtimeout] [-Rretries]hostname[server]For more information and a list of available commands and options,
see the host man
page. - nslookup
-
nslookup has two
modes: interactive and non-interactive. Interactive mode allows
the user to query name servers for information about various hosts
and domains or to print a list of hosts in a domain. Non-interactive
mode is used to print just the name and requested information for
a host or domain.nslookup[-option...] [[host-to-find]
| [- [server]]]Interactive mode is entered when no arguments are given (the
default name server will be used) or when the first argument is
a hyphen (`-') and the second argument is the host name or Internet
address of a name server.Non-interactive mode is used when the name or Internet address
of the host to be looked up is given as the first argument. The
optional second argument specifies the host name or address of
a name server.Due to its arcane user interface and frequently inconsistent
behavior, we do not recommend the use of nslookup.
Use dig instead.
Administrative tools play an integral part in the management of a server.
- named-checkconf
-
The named-checkconf program
checks the syntax of anamed.conffile.named-checkconf[-jvz] [-tdirectory]
[filename] - named-checkzone
-
The named-checkzone program
checks a master file for syntax and consistency.named-checkzone[-djqvD] [-cclass]
[-ooutput] [-tdirectory]
[-wdirectory] [-k(ignore|warn|fail)]
[-n(ignore|warn|fail)]
[-W(ignore|warn)]zone[filename] - named-compilezone
-
Similar to named-checkzone, but
it always dumps the zone content to a specified file (typically
in a different format). - rndc
-
The remote name daemon control (rndc)
program allows the system administrator to control the operation
of a name server. If you run rndc without
any options it will display a usage message as follows:rndc[-cconfig]
[-sserver] [-pport]
[-ykey]command[command...]The command is one
of the following:reload-
Reload configuration file and zones.
reload zone [class [view]]-
Reload the given zone.
refresh zone [class [view]]-
Schedule zone maintenance for the given zone.
retransfer zone [class [view]]-
Retransfer the given zone from the master.
freeze
[zone [class [view]]]-
Suspend updates to a dynamic zone. If no zone is specified,
then all zones are suspended. This allows manual edits to
be made to a zone normally updated by dynamic update. It
also causes changes in the journal file to be synced into
the master and the journal file to be removed. All dynamic
update attempts will be refused while the zone is frozen. thaw [zone [class [view]]]-
Enable updates to a frozen dynamic zone. If no zone is
specified, then all frozen zones are enabled. This causes
the server to reload the zone from disk, and re-enables dynamic
updates after the load has completed. After a zone is thawed,
dynamic updates will no longer be refused. notify zone [class [view]]-
Resend NOTIFY messages for the zone.
reconfig-
Reload the configuration file and load new zones, but do
not reload existing zone files even if they have changed.
This is faster than a full reload when
there is a large number of zones because it avoids the need
to examine the modification times of the zones files. stats-
Write server statistics to the statistics file.
querylog-
Toggle query logging. Query logging can also be enabled
by explicitly directing the queries category to
a channel in
the logging section
ofnamed.confor by specifying querylog
yes; in the options section
ofnamed.conf. dumpdb
[-all|-cache|-zone] [view
...]-
Dump the server's caches (default) and/or zones to the
dump file for the specified views. If no view is specified,
all views are dumped. stop [-p]-
Stop the server, making sure any recent changes made through
dynamic update or IXFR are first saved to the master files
of the updated zones. If -p is specified named's process
id is returned. This allows an external process to determine
when named had completed stopping. halt [-p]-
Stop the server immediately. Recent changes made through
dynamic update or IXFR are not saved to the master files,
but will be rolled forward from the journal files when the
server is restarted. If -p is specified named's process id
is returned. This allows an external process to determine
when named had completed halting. trace-
Increment the servers debugging level by one.
trace level-
Sets the server's debugging level to an explicit value.
notrace-
Sets the server's debugging level to 0.
flush-
Flushes the server's cache.
flushnamename-
Flushes the given name from the server's cache.
status-
Display status of the server. Note that the number of zones
includes the internal bind/CH zone
and the default ./IN hint
zone if there is not an explicit root zone configured. recursing-
Dump the list of queries named is currently recursing on.
In BIND 9.2, rndc supports
all the commands of the BIND 8 ndc utility
except ndc start and ndc
restart, which were also not supported in ndc's
channel mode.A configuration file is required, since all communication with
the server is authenticated with digital signatures that rely on
a shared secret, and there is no way to provide that secret other
than with a configuration file. The default location for the rndc configuration
file is/etc/rndc.conf, but an alternate
location can be specified with the-coption.
If the configuration file is not found, rndc will
also look in/etc/rndc.key(or whateversysconfdirwas
defined when the BIND build
was configured). Therndc.keyfile
is generated by running rndc-confgen
-a as described in the section called “controls Statement
Definition and Usage”.The format of the configuration file is similar to that of
named.conf,
but limited to only four statements, the options, key, server and include statements.
These statements are what associate the secret keys to the servers
with which they are meant to be shared. The order of statements
is not significant.The options statement
has three clauses: default-server, default-key,
and default-port. default-server takes
a host name or address argument and represents the server that
will be contacted if no-soption is
provided on the command line. default-key takes
the name of a key as its argument, as defined by a key statement. default-port specifies
the port to which rndc should
connect if no port is given on the command line or in a server statement.The key statement
defines a key to be used by rndc when
authenticating with named.
Its syntax is identical to the key statement
in named.conf. The keywordkeyis
followed by a key name, which must be a valid domain name, though
it need not actually be hierarchical; thus, a string like "rndc_key" is
a valid name. The key statement
has two clauses: algorithm and secret.
While the configuration parser will accept any string as the argument
to algorithm, currently only the string "hmac-md5"
has any meaning. The secret is a base-64 encoded string as specified
in RFC 3548.The server statement
associates a key defined using the key statement
with a server. The keywordserveris
followed by a host name or address. The server statement
has two clauses: key and port.
The key clause specifies
the name of the key to be used when communicating with this server,
and the port clause
can be used to specify the port rndc should
connect to on the server.A sample minimal configuration file is as follows:
key rndc_key { algorithm "hmac-md5"; secret "c3Ryb25nIGVub3VnaCBmb3IgYSBtYW4gYnV0IG1hZGUgZm9yIGEgd29tYW4K"; }; options { default-server 127.0.0.1; default-key rndc_key; };This file, if installed as
/etc/rndc.conf,
would allow the command:$rndc
reloadto connect to 127.0.0.1 port 953 and cause the name server to
reload, if a name server on the local machine were running with
following controls statements:controls { inet 127.0.0.1 allow { localhost; } keys { rndc_key; }; };and it had an identical key statement for
rndc_key.Running the rndc-confgen program
will conveniently create arndc.conffile
for you, and also display the corresponding controls statement
that you need to add tonamed.conf.
Alternatively, you can run rndc-confgen
-a to set up arndc.keyfile
and not modifynamed.confat all.
Certain UNIX signals cause the name server to take specific actions,
as described in the following table. These signals can be sent using the kill command.
|
SIGHUP |
Causes the server to read |
|
SIGTERM |
Causes the server to clean up and exit. |
|
SIGINT |
Causes the server to clean up and exit. |
Table of Contents
DNS NOTIFY is a mechanism that allows
master servers to notify their slave servers of changes to a zone's data.
In response to a NOTIFY from
a master server, the slave will check to see that its version of the zone
is the current version and, if not, initiate a zone transfer.
For more information about DNS NOTIFY,
see the description of the notify option
in the section called “Boolean
Options” and the description of the zone option also-notify in the
section called “Zone Transfers”. The NOTIFY protocol
is specified in RFC 1996.
Note
As a slave zone can also be a master to other slaves, named, by default,
sends NOTIFY messages for every
zone it loads. Specifying notify master-only; will
cause named to only send NOTIFY for
master zones that it loads.
Dynamic Update is a method for adding, replacing or deleting records in
a master server by sending it a special form of DNS messages. The format
and meaning of these messages is specified in RFC 2136.
Dynamic update is enabled by including an allow-update or update-policy clause
in the zone statement.
Updating of secure zones (zones using DNSSEC) follows RFC 3007: RRSIG and
NSEC records affected by updates are automatically regenerated by the server
using an online zone key. Update authorization is based on transaction signatures
and an explicit server policy.
All changes made to a zone using dynamic update are stored in the zone's
journal file. This file is automatically created by the server when the
first dynamic update takes place. The name of the journal file is formed
by appending the extension .jnl to the name
of the corresponding zone file unless specifically overridden. The journal
file is in a binary format and should not be edited manually.
The server will also occasionally write ("dump") the complete contents
of the updated zone to its zone file. This is not done immediately after
each dynamic update, because that would be too slow when a large zone is
updated frequently. Instead, the dump is delayed by up to 15 minutes, allowing
additional updates to take place.
When a server is restarted after a shutdown or crash, it will replay
the journal file to incorporate into the zone any updates that took place
after the last zone dump.
Changes that result from incoming incremental zone transfers are also
journalled in a similar way.
The zone files of dynamic zones cannot normally be edited by hand because
they are not guaranteed to contain the most recent dynamic changes — those
are only in the journal file. The only way to ensure that the zone file
of a dynamic zone is up to date is to run rndc
stop.
If you have to make changes to a dynamic zone manually, the following
procedure will work: Disable dynamic updates to the zone using rndc
freeze zone.
This will also remove the zone's .jnl file
and update the master file. Edit the zone file. Run rndc
thaw zone to
reload the changed zone and re-enable dynamic updates.
The incremental zone transfer (IXFR) protocol is a way for slave servers
to transfer only changed data, instead of having to transfer the entire zone.
The IXFR protocol is specified in RFC 1995. See Proposed
Standards.
When acting as a master, BIND 9 supports
IXFR for those zones where the necessary change history information is available.
These include master zones maintained by dynamic update and slave zones whose
data was obtained by IXFR. For manually maintained master zones, and for
slave zones obtained by performing a full zone transfer (AXFR), IXFR is supported
only if the option ixfr-from-differences is
set to yes.
When acting as a slave, BIND 9 will
attempt to use IXFR unless it is explicitly disabled. For more information
about disabling IXFR, see the description of the request-ixfr clause
of the server statement.
Setting up different views, or visibility, of the DNS space to internal
and external resolvers is usually referred to as a Split
DNS setup. There are several reasons an organization would want
to set up its DNS this way.
One common reason for setting up a DNS system this way is to hide "internal" DNS
information from "external" clients on the Internet. There is some debate
as to whether or not this is actually useful. Internal DNS information leaks
out in many ways (via email headers, for example) and most savvy "attackers" can
find the information they need using other means. However, since listing
addresses of internal servers that external clients cannot possibly reach
can result in connection delays and other annoyances, an organization may
choose to use a Split DNS to present a consistant view of itself to the outside
world.
Another common reason for setting up a Split DNS system is to allow internal
networks that are behind filters or in RFC 1918 space (reserved IP space,
as documented in RFC 1918) to resolve DNS on the Internet. Split DNS can
also be used to allow mail from outside back in to the internal network.
Here is an example of a split DNS setup:
Let's say a company named Example, Inc. (example.com)
has several corporate sites that have an internal network with reserved Internet
Protocol (IP) space and an external demilitarized zone (DMZ), or "outside" section
of a network, that is available to the public.
Example, Inc. wants its internal
clients to be able to resolve external hostnames and to exchange mail with
people on the outside. The company also wants its internal resolvers to have
access to certain internal-only zones that are not available at all outside
of the internal network.
In order to accomplish this, the company will set up two sets of name servers.
One set will be on the inside network (in the reserved IP space) and the
other set will be on bastion hosts, which are
"proxy"
hosts that can talk to both sides of its network, in the DMZ.
The internal servers will be configured to forward all queries, except
queries for site1.internal, site2.internal, site1.example.com,
and site2.example.com, to the servers in the
DMZ. These internal servers will have complete sets of information for site1.example.com, site2.example.com, site1.internal,
and site2.internal.
To protect the site1.internal and site2.internal domains,
the internal name servers must be configured to disallow all queries to these
domains from any external hosts, including the bastion hosts.
The external servers, which are on the bastion hosts, will be configured
to serve the "public" version of the site1 and site2.example.com zones.
This could include things such as the host records for public servers (www.example.com and ftp.example.com),
and mail exchange (MX) records (a.mx.example.com and b.mx.example.com).
In addition, the public site1 and site2.example.com zones
should have special MX records that contain wildcard (`*') records pointing
to the bastion hosts. This is needed because external mail servers do not
have any other way of looking up how to deliver mail to those internal hosts.
With the wildcard records, the mail will be delivered to the bastion host,
which can then forward it on to internal hosts.
Here's an example of a wildcard MX record:
* IN MX 10 external1.example.com.
Now that they accept mail on behalf of anything in the internal network,
the bastion hosts will need to know how to deliver mail to internal hosts.
In order for this to work properly, the resolvers on the bastion hosts will
need to be configured to point to the internal name servers for DNS resolution.
Queries for internal hostnames will be answered by the internal servers,
and queries for external hostnames will be forwarded back out to the DNS
servers on the bastion hosts.
In order for all this to work properly, internal clients will need to be
configured to query only the internal
name servers for DNS queries. This could also be enforced via selective filtering
on the network.
If everything has been set properly, Example,
Inc.'s internal clients will now be able to:
- Look up any hostnames in the
site1andsite2.example.comzones. - Look up any hostnames in the
site1.internalandsite2.internaldomains. - Look up any hostnames on the Internet.
- Exchange mail with both internal and external people.
Hosts on the Internet will be able to:
- Look up any hostnames in the
site1andsite2.example.comzones. - Exchange mail with anyone in the
site1andsite2.example.comzones.
Here is an example configuration for the setup we just described above.
Note that this is only configuration information; for information on how
to configure your zone files, see the
section called “Sample Configurations”.
Internal DNS server config:
acl internals { 172.16.72.0/24; 192.168.1.0/24; };
acl externals { bastion-ips-go-here; };
options {
...
...
forward only;
forwarders { // forward to external servers
bastion-ips-go-here;
};
allow-transfer { none; }; // sample allow-transfer (no one)
allow-query { internals; externals; }; // restrict query access
allow-recursion { internals; }; // restrict recursion
...
...
};
zone "site1.example.com" { // sample master zone
type master;
file "m/site1.example.com";
forwarders { }; // do normal iterative
// resolution (do not forward)
allow-query { internals; externals; };
allow-transfer { internals; };
};
zone "site2.example.com" { // sample slave zone
type slave;
file "s/site2.example.com";
masters { 172.16.72.3; };
forwarders { };
allow-query { internals; externals; };
allow-transfer { internals; };
};
zone "site1.internal" {
type master;
file "m/site1.internal";
forwarders { };
allow-query { internals; };
allow-transfer { internals; }
};
zone "site2.internal" {
type slave;
file "s/site2.internal";
masters { 172.16.72.3; };
forwarders { };
allow-query { internals };
allow-transfer { internals; }
};
External (bastion host) DNS server config:
acl internals { 172.16.72.0/24; 192.168.1.0/24; };
acl externals { bastion-ips-go-here; };
options {
...
...
allow-transfer { none; }; // sample allow-transfer (no one)
allow-query { any; }; // default query access
allow-query-cache { internals; externals; }; // restrict cache access
allow-recursion { internals; externals; }; // restrict recursion
...
...
};
zone "site1.example.com" { // sample slave zone
type master;
file "m/site1.foo.com";
allow-transfer { internals; externals; };
};
zone "site2.example.com" {
type slave;
file "s/site2.foo.com";
masters { another_bastion_host_maybe; };
allow-transfer { internals; externals; }
};
In the resolv.conf (or equivalent) on the
bastion host(s):
search ... nameserver 172.16.72.2 nameserver 172.16.72.3 nameserver 172.16.72.4
This is a short guide to setting up Transaction SIGnatures (TSIG) based
transaction security in BIND. It describes
changes to the configuration file as well as what changes are required for
different features, including the process of creating transaction keys and
using transaction signatures with BIND.
BIND primarily supports TSIG for server
to server communication. This includes zone transfer, notify, and recursive
query messages. Resolvers based on newer versions of BIND 8
have limited support for TSIG.
TSIG can also be useful for dynamic update. A primary server for a dynamic
zone should control access to the dynamic update service, but IP-based access
control is insufficient. The cryptographic access control provided by TSIG
is far superior. The nsupdate program
supports TSIG via the -k and -y command
line options or inline by use of the key.
A shared secret is generated to be shared between host1 and host2.
An arbitrary key name is chosen: "host1-host2.". The key name must be the
same on both hosts.
The following command will generate a 128-bit (16 byte) HMAC-MD5 key
as described above. Longer keys are better, but shorter keys are easier
to read. Note that the maximum key length is 512 bits; keys longer than
that will be digested with MD5 to produce a 128-bit key.
dnssec-keygen -a hmac-md5 -b 128 -n
HOST host1-host2.
The key is in the file Khost1-host2.+157+00000.private.
Nothing directly uses this file, but the base-64 encoded string following "Key:"
can be extracted from the file and used as a shared secret:
Key: La/E5CjG9O+os1jq0a2jdA==
The string "La/E5CjG9O+os1jq0a2jdA==" can
be used as the shared secret.
The shared secret is simply a random sequence of bits, encoded in base-64.
Most ASCII strings are valid base-64 strings (assuming the length is
a multiple of 4 and only valid characters are used), so the shared secret
can be manually generated.
Also, a known string can be run through mmencode or
a similar program to generate base-64 encoded data.
This is beyond the scope of DNS. A secure transport mechanism should
be used. This could be secure FTP, ssh, telephone, etc.
Imagine host1 and host
2 are both servers. The following is added to each server's named.conf file:
key host1-host2. {
algorithm hmac-md5;
secret "La/E5CjG9O+os1jq0a2jdA==";
};
The algorithm, hmac-md5, is the only one supported by BIND.
The secret is the one generated above. Since this is a secret, it is recommended
that either named.conf be non-world readable,
or the key directive be added to a non-world readable file that is included
by named.conf.
At this point, the key is recognized. This means that if the server receives
a message signed by this key, it can verify the signature. If the signature
is successfully verified, the response is signed by the same key.
Since keys are shared between two hosts only, the server must be told
when keys are to be used. The following is added to the named.conf file
for host1, if the IP address of host2 is
10.1.2.3:
server 10.1.2.3 {
keys { host1-host2. ;};
};
Multiple keys may be present, but only the first is used. This directive
does not contain any secrets, so it may be in a world-readable file.
If host1 sends a message that
is a request to that address, the message will be signed with the specified
key. host1 will expect any responses
to signed messages to be signed with the same key.
A similar statement must be present in host2's
configuration file (with host1's
address) for host2 to sign request
messages to host1.
BIND allows IP addresses and ranges
to be specified in ACL definitions and allow-{
query | transfer | update } directives. This has been extended
to allow TSIG keys also. The above key would be denoted key
host1-host2.
An example of an allow-update directive would be:
allow-update { key host1-host2. ;};
This allows dynamic updates to succeed only if the request was signed
by a key named
"host1-host2.".
You may want to read about the more powerful update-policy statement
in the
section called “Dynamic Update Policies”.
The processing of TSIG signed messages can result in several errors.
If a signed message is sent to a non-TSIG aware server, a FORMERR (format
error) will be returned, since the server will not understand the record.
This is a result of misconfiguration, since the server must be explicitly
configured to send a TSIG signed message to a specific server.
If a TSIG aware server receives a message signed by an unknown key, the
response will be unsigned with the TSIG extended error code set to BADKEY.
If a TSIG aware server receives a message with a signature that does not
validate, the response will be unsigned with the TSIG extended error code
set to BADSIG. If a TSIG aware server receives a message with a time outside
of the allowed range, the response will be signed with the TSIG extended
error code set to BADTIME, and the time values will be adjusted so that
the response can be successfully verified. In any of these cases, the message's
rcode is set to NOTAUTH (not authenticated).
TKEY is a mechanism for automatically
generating a shared secret between two hosts. There are several "modes" of TKEY that
specify how the key is generated or assigned. BIND 9
implements only one of these modes, the Diffie-Hellman key exchange. Both
hosts are required to have a Diffie-Hellman KEY record (although this record
is not required to be present in a zone). The TKEY process
must use signed messages, signed either by TSIG or SIG(0). The result of TKEY is
a shared secret that can be used to sign messages with TSIG. TKEY can
also be used to delete shared secrets that it had previously generated.
The TKEY process is initiated
by a client or server by sending a signed TKEY query
(including any appropriate KEYs) to a TKEY-aware server. The server response,
if it indicates success, will contain a TKEY record
and any appropriate keys. After this exchange, both participants have enough
information to determine the shared secret; the exact process depends on
the TKEY mode. When using the
Diffie-Hellman TKEY mode, Diffie-Hellman
keys are exchanged, and the shared secret is derived by both participants.
BIND 9 partially supports DNSSEC SIG(0)
transaction signatures as specified in RFC 2535 and RFC2931. SIG(0) uses
public/private keys to authenticate messages. Access control is performed
in the same manner as TSIG keys; privileges can be granted or denied based
on the key name.
When a SIG(0) signed message is received, it will only be verified if the
key is known and trusted by the server; the server will not attempt to locate
and/or validate the key.
SIG(0) signing of multiple-message TCP streams is not supported.
The only tool shipped with BIND 9 that
generates SIG(0) signed messages is nsupdate.
Cryptographic authentication of DNS information is possible through the
DNS Security (DNSSEC-bis) extensions,
defined in RFC 4033, RFC 4034 and RFC 4035. This section describes the creation
and use of DNSSEC signed zones.
In order to set up a DNSSEC secure zone, there are a series of steps which
must be followed. BIND 9 ships with several
tools that are used in this process, which are explained in more detail below.
In all cases, the -h option prints a full list
of parameters. Note that the DNSSEC tools require the keyset files to be
in the working directory or the directory specified by the -d option,
and that the tools shipped with BIND 9.2.x and earlier are not compatible
with the current ones.
There must also be communication with the administrators of the parent
and/or child zone to transmit keys. A zone's security status must be indicated
by the parent zone for a DNSSEC capable resolver to trust its data. This
is done through the presence or absence of a DS record
at the delegation point.
For other servers to trust data in this zone, they must either be statically
configured with this zone's zone key or the zone key of another zone above
this one in the DNS tree.
The dnssec-keygen program
is used to generate keys.
A secure zone must contain one or more zone keys. The zone keys will
sign all other records in the zone, as well as the zone keys of any secure
delegated zones. Zone keys must have the same name as the zone, a name
type of ZONE, and must be
usable for authentication. It is recommended that zone keys use a cryptographic
algorithm designated as "mandatory to implement" by the IETF; currently
the only one is RSASHA1.
The following command will generate a 768-bit RSASHA1 key for the child.example zone:
dnssec-keygen -a RSASHA1 -b 768 -n ZONE
child.example.
Two output files will be produced: Kchild.example.+005+12345.key and Kchild.example.+005+12345.private (where
12345 is an example of a key tag). The key file names contain the key name
(child.example.), algorithm (3 is DSA, 1
is RSAMD5, 5 is RSASHA1, etc.), and the key tag (12345 in this case). The
private key (in the .private file) is used
to generate signatures, and the public key (in the .key file)
is used for signature verification.
To generate another key with the same properties (but with a different
key tag), repeat the above command.
The public keys should be inserted into the zone file by including the .key files
using $INCLUDE statements.
The dnssec-signzone program
is used to sign a zone.
Any keyset files corresponding to secure
subzones should be present. The zone signer will generate NSEC and RRSIG records
for the zone, as well as DS for the child
zones if '-d' is specified. If '-d' is
not specified, then DS RRsets for the secure child zones need to be added
manually.
The following command signs the zone, assuming it is in a file called zone.child.example.
By default, all zone keys which have an available private key are used
to generate signatures.
dnssec-signzone -o child.example zone.child.example
One output file is produced: zone.child.example.signed.
This file should be referenced by named.conf as
the input file for the zone.
dnssec-signzone will also
produce a keyset and dsset files and optionally a dlvset file. These are
used to provide the parent zone administators with the DNSKEYs (or
their corresponding DS records) that are the
secure entry point to the zone.
To enable named to respond
appropriately to DNS requests from DNSSEC aware clients, dnssec-enable must
be set to yes.
To enable named to validate
answers from other servers both dnssec-enable and dnssec-validate must
be set and some trusted-keys must
be configured into named.conf.
trusted-keys are copies
of DNSKEY RRs for zones that are used to form the first link in the cryptographic
chain of trust. All keys listed in trusted-keys (and
corresponding zones) are deemed to exist and only the listed keys will
be used to validated the DNSKEY RRset that they are from.
trusted-keys are described
in more detail later in this document.
Unlike BIND 8, BIND 9
does not verify signatures on load, so zone keys for authoritative zones
do not need to be specified in the configuration file.
After DNSSEC gets established, a typical DNSSEC configuration will look
something like the following. It has a one or more public keys for the
root. This allows answers from outside the organization to be validated.
It will also have several keys for parts of the namespace the organization
controls. These are here to ensure that named is immune to compromises
in the DNSSEC components of the security of parent zones.
trusted-keys {
/* Root Key */
"." 257 3 3 "BNY4wrWM1nCfJ+CXd0rVXyYmobt7sEEfK3clRbGaTwSJxrGkxJWoZu6I7PzJu/
E9gx4UC1zGAHlXKdE4zYIpRhaBKnvcC2U9mZhkdUpd1Vso/HAdjNe8LmMlnzY3
zy2Xy4klWOADTPzSv9eamj8V18PHGjBLaVtYvk/ln5ZApjYghf+6fElrmLkdaz
MQ2OCnACR817DF4BBa7UR/beDHyp5iWTXWSi6XmoJLbG9Scqc7l70KDqlvXR3M
/lUUVRbkeg1IPJSidmK3ZyCllh4XSKbje/45SKucHgnwU5jefMtq66gKodQj+M
iA21AfUVe7u99WzTLzY3qlxDhxYQQ20FQ97S+LKUTpQcq27R7AT3/V5hRQxScI
Nqwcz4jYqZD2fQdgxbcDTClU0CRBdiieyLMNzXG3";
/* Key for our organization's forward zone */
example.com. 257 3 5 "AwEAAaxPMcR2x0HbQV4WeZB6oEDX+r0QM65KbhTjrW1ZaARmPhEZZe
3Y9ifgEuq7vZ/zGZUdEGNWy+JZzus0lUptwgjGwhUS1558Hb4JKUbb
OTcM8pwXlj0EiX3oDFVmjHO444gLkBO UKUf/mC7HvfwYH/Be22GnC
lrinKJp1Og4ywzO9WglMk7jbfW33gUKvirTHr25GL7STQUzBb5Usxt
8lgnyTUHs1t3JwCY5hKZ6CqFxmAVZP20igTixin/1LcrgX/KMEGd/b
iuvF4qJCyduieHukuY3H4XMAcR+xia2 nIUPvm/oyWR8BW/hWdzOvn
SCThlHf3xiYleDbt/o1OTQ09A0=";
/* Key for our reverse zone. */
2.0.192.IN-ADDRPA.NET. 257 3 5 "AQOnS4xn/IgOUpBPJ3bogzwcxOdNax071L18QqZnQQQA
VVr+iLhGTnNGp3HoWQLUIzKrJVZ3zggy3WwNT6kZo6c0
tszYqbtvchmgQC8CzKojM/W16i6MG/ea fGU3siaOdS0
yOI6BgPsw+YZdzlYMaIJGf4M4dyoKIhzdZyQ2bYQrjyQ
4LB0lC7aOnsMyYKHHYeRv PxjIQXmdqgOJGq+vsevG06
zW+1xgYJh9rCIfnm1GX/KMgxLPG2vXTD/RnLX+D3T3UL
7HJYHJhAZD5L59VvjSPsZJHeDCUyWYrvPZesZDIRvhDD
52SKvbheeTJUm6EhkzytNN2SN96QRk8j/iI8ib";
};
options {
...
dnssec-enable yes;
dnssec-validation yes;
};
Note
None of the keys listed in this example are valid. In particular, the root
key is not valid.
BIND 9 fully supports all currently
defined forms of IPv6 name to address and address to name lookups. It will
also use IPv6 addresses to make queries when running on an IPv6 capable system.
For forward lookups, BIND 9 supports
only AAAA records. RFC 3363 deprecated the use of A6 records, and client-side
support for A6 records was accordingly removed from BIND 9.
However, authoritative BIND 9 name servers
still load zone files containing A6 records correctly, answer queries for
A6 records, and accept zone transfer for a zone containing A6 records.
For IPv6 reverse lookups, BIND 9 supports
the traditional "nibble" format used in the ip6.arpa domain,
as well as the older, deprecated ip6.int domain.
Older versions of BIND 9 supported the "binary
label" (also known as "bitstring") format, but support of binary labels has
been completely removed per RFC 3363. Many applications in BIND 9
do not understand the binary label format at all any more, and will return
an error if given. In particular, an authoritative BIND 9
name server will not load a zone file containing binary labels.
For an overview of the format and structure of IPv6 addresses, see the
section called “IPv6 addresses (AAAA)”.
The IPv6 AAAA record is a parallel to the IPv4 A record, and, unlike
the deprecated A6 record, specifies the entire IPv6 address in a single
record. For example,
$ORIGIN example.com. host 3600 IN AAAA 2001:db8::1
Use of IPv4-in-IPv6 mapped addresses is not recommended. If a host has
an IPv4 address, use an A record, not a AAAA, with ::ffff:192.168.42.1 as
the address.
When looking up an address in nibble format, the address components are
simply reversed, just as in IPv4, and ip6.arpa. is
appended to the resulting name. For example, the following would provide
reverse name lookup for a host with address 2001:db8::1.
$ORIGIN 0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa. 1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0 14400 IN PTR host.example.com.
Table of Contents
Traditionally applications have been linked with a stub resolver library
that sends recursive DNS queries to a local caching name server.
IPv6 once introduced new complexity into the resolution process, such as
following A6 chains and DNAME records, and simultaneous lookup of IPv4 and
IPv6 addresses. Though most of the complexity was then removed, these are
hard or impossible to implement in a traditional stub resolver.
BIND 9 therefore can also provide resolution
services to local clients using a combination of a lightweight resolver library
and a resolver daemon process running on the local host. These communicate
using a simple UDP-based protocol, the "lightweight resolver protocol"
that is distinct from and simpler than the full DNS protocol.
To use the lightweight resolver interface, the system must run the resolver
daemon lwresd or a local name
server configured with a lwres statement.
By default, applications using the lightweight resolver library will make
UDP requests to the IPv4 loopback address (127.0.0.1) on port 921. The address
can be overridden by lwserver lines
in /etc/resolv.conf.
The daemon currently only looks in the DNS, but in the future it may use
other sources such as /etc/hosts, NIS, etc.
The lwresd daemon is essentially
a caching-only name server that responds to requests using the lightweight
resolver protocol rather than the DNS protocol. Because it needs to run on
each host, it is designed to require no or minimal configuration. Unless
configured otherwise, it uses the name servers listed on nameserver lines
in /etc/resolv.conf as forwarders, but is also
capable of doing the resolution autonomously if none are specified.
The lwresd daemon may also
be configured with a named.conf style configuration
file, in /etc/lwresd.conf by default. A name
server may also be configured to act as a lightweight resolver daemon using
the lwres statement in named.conf.
Table of Contents
- Configuration
File Elements - Configuration
File Grammar -
- acl Statement
Grammar - acl Statement
Definition and Usage - controls Statement
Grammar - controls Statement
Definition and Usage - include Statement
Grammar - include Statement
Definition and Usage - key Statement
Grammar - key Statement
Definition and Usage - logging Statement
Grammar - logging Statement
Definition and Usage - lwres Statement
Grammar - lwres Statement
Definition and Usage - masters Statement
Grammar - masters Statement
Definition and Usage - options Statement
Grammar - options Statement
Definition and Usage - server Statement
Grammar - server Statement
Definition and Usage - trusted-keys Statement
Grammar - trusted-keys Statement
Definition and Usage - view Statement
Grammar - view Statement
Definition and Usage - zone Statement
Grammar - zone Statement
Definition and Usage
- acl Statement
- Zone File
BIND 9 configuration is broadly similar
to BIND 8; however, there are a few new
areas of configuration, such as views. BIND 8
configuration files should work with few alterations in BIND 9,
although more complex configurations should be reviewed to check if they can
be more efficiently implemented using the new features found in BIND 9.
BIND 4 configuration files can be converted
to the new format using the shell script contrib/named-bootconf/named-bootconf.sh.
Following is a list of elements used throughout the BIND configuration
file documentation:
|
|
The name of an |
|
|
A list of one or more |
|
|
A named list of one or more |
|
|
A quoted string which will be used as a DNS name, for example " |
|
|
One to four integers valued 0 through 255 separated by dots |
|
|
An IPv4 address with exactly four elements in |
|
|
An IPv6 address, such as 2001:db8::1234. |
|
|
An |
|
|
An IP port |
|
|
An IP network specified as an |
|
|
A |
|
|
A list of one or more |
|
|
A non-negative 32-bit integer (i.e., a number between 0 and |
|
|
A quoted string which will be used as a pathname, such as |
|
|
A number, the word An A The value must be representable as a 64-bit unsigned integer |
|
|
Either |
|
|
One of |
address_match_list= address_match_list_element ; [ address_match_list_element; ... ]address_match_list_element= [ ! ] (ip_address [/length] | key key_id | acl_name | { address_match_list } )
Address match lists are primarily used to determine access control
for various server operations. They are also used in the listen-on and sortlist statements.
The elements which constitute an address match list can be any of the
following:
- an IP address (IPv4 or IPv6)
- an IP prefix (in `/' notation)
- a key ID, as defined by the key statement
- the name of an address match list defined with the acl statement
- a nested address match list enclosed in braces
Elements can be negated with a leading exclamation mark (`!'), and
the match list names "any", "none", "localhost", and
"localnets"
are predefined. More information on those names can be found in the description
of the acl statement.
The addition of the key clause made the name of this syntactic element
something of a misnomer, since security keys can be used to validate
access without regard to a host or network address. Nonetheless, the
term "address match list" is still used throughout the documentation.
When a given IP address or prefix is compared to an address match list,
the list is traversed in order until an element matches. The interpretation
of a match depends on whether the list is being used for access control,
defining listen-on ports, or in a sortlist, and whether the element was
negated.
When used as an access control list, a non-negated match allows access
and a negated match denies access. If there is no match, access is denied.
The clauses allow-notify, allow-query, allow-query-cache, allow-transfer, allow-update, allow-update-forwarding,
and blackhole all use address
match lists. Similarly, the listen-on option will cause the server to
not accept queries on any of the machine's addresses which do not match
the list.
Because of the first-match aspect of the algorithm, an element that
defines a subset of another element in the list should come before the
broader element, regardless of whether either is negated. For example,
in 1.2.3/24; ! 1.2.3.13; the
1.2.3.13 element is completely useless because the algorithm will match
any lookup for 1.2.3.13 to the 1.2.3/24 element. Using !
1.2.3.13; 1.2.3/24 fixes that problem by having 1.2.3.13
blocked by the negation but all other 1.2.3.* hosts fall through.
The BIND 9 comment syntax allows for
comments to appear anywhere that white space may appear in a BIND configuration
file. To appeal to programmers of all kinds, they can be written in the
C, C++, or shell/perl style.
/* This is a BIND comment as in C */
// This is a BIND comment as in C++
# This is a BIND comment as in common UNIX shells and perl
Comments may appear anywhere that white space may appear in a BIND configuration
file.
C-style comments start with the two characters /* (slash, star) and
end with */ (star, slash). Because they are completely delimited with
these characters, they can be used to comment only a portion of a line
or to span multiple lines.
C-style comments cannot be nested. For example, the following is not
valid because the entire comment ends with the first */:
/* This is the start of a comment. This is still part of the comment. /* This is an incorrect attempt at nesting a comment. */ This is no longer in any comment. */
C++-style comments start with the two characters // (slash, slash)
and continue to the end of the physical line. They cannot be continued
across multiple physical lines; to have one logical comment span multiple
lines, each line must use the // pair.
For example:
// This is the start of a comment. The next line
// is a new comment, even though it is logically
// part of the previous comment.
Shell-style (or perl-style, if you prefer) comments start with the
character # (number sign) and continue to
the end of the physical line, as in C++ comments.
For example:
# This is the start of a comment. The next line # is a new comment, even though it is logically # part of the previous comment.
Warning
You cannot use the semicolon (`;') character to start a comment such
as you would in a zone file. The semicolon indicates the end of a configuration
statement.
A BIND 9 configuration consists of statements
and comments. Statements end with a semicolon. Statements and comments are
the only elements that can appear without enclosing braces. Many statements
contain a block of sub-statements, which are also terminated with a semicolon.
The following statements are supported:
|
acl |
defines a named IP address matching list, for access control |
|
controls |
declares control channels to be used by the rndc utility. |
|
include |
includes a file. |
|
key |
specifies key information for use in authentication and authorization |
|
logging |
specifies what the server logs, and where the log messages |
|
lwres |
configures named to |
|
masters |
defines a named masters list for inclusion in stub and slave |
|
options |
controls global server configuration options and sets defaults |
|
server |
sets certain configuration options on a per-server basis. |
|
trusted-keys |
defines trusted DNSSEC keys. |
|
view |
defines a view. |
|
zone |
defines a zone. |
The logging and options statements
may only occur once per configuration.
The acl statement assigns
a symbolic name to an address match list. It gets its name from a primary
use of address match lists: Access Control Lists (ACLs).
Note that an address match list's name must be defined with acl before
it can be used elsewhere; no forward references are allowed.
The following ACLs are built-in:
|
any |
Matches all hosts. |
|
none |
Matches no hosts. |
|
localhost |
Matches the IPv4 and IPv6 addresses of all network interfaces |
|
localnets |
Matches any host on an IPv4 or IPv6 network for which the |
controls { [ inet ( ip_addr | * ) [ port ip_port ] allow {address_match_list} keys {key_list}; ] [ inet ...; ] [ unixpathpermnumberownernumbergroupnumberkeys {key_list}; ] [ unix ...; ] };
The controls statement
declares control channels to be used by system administrators to control
the operation of the name server. These control channels are used by the rndc utility
to send commands to and retrieve non-DNS results from a name server.
An inet control channel
is a TCP socket listening at the specified ip_port on
the specified ip_addr, which
can be an IPv4 or IPv6 address. An ip_addr of * (asterisk)
is interpreted as the IPv4 wildcard address; connections will be accepted
on any of the system's IPv4 addresses. To listen on the IPv6 wildcard address,
use an ip_addr of ::.
If you will only use rndc on
the local host, using the loopback address (127.0.0.1 or ::1)
is recommended for maximum security.
If no port is specified, port 953 is used. The asterisk
"*" cannot be used for ip_port.
The ability to issue commands over the control channel is restricted
by the allow and keys clauses.
Connections to the control channel are permitted based on the address_match_list.
This is for simple IP address based filtering only; any key_id elements
of the address_match_list are
ignored.
A unix control channel
is a UNIX domain socket listening at the specified path in the file system.
Access to the socket is specified by the perm, owner and group clauses.
Note on some platforms (SunOS and Solaris) the permissions (perm)
are applied to the parent directory as the permissions on the socket itself
are ignored.
The primary authorization mechanism of the command channel is the key_list,
which contains a list of key_ids.
Each key_id in the key_list is
authorized to execute commands over the control channel. See Remote
Name Daemon Control application in the
section called “Administrative Tools”) for information
about configuring keys in rndc.
If no controls statement
is present, named will set
up a default control channel listening on the loopback address 127.0.0.1
and its IPv6 counterpart ::1. In this case, and also when the controls statement
is present but does not have a keys clause, named will
attempt to load the command channel key from the file rndc.key in /etc (or
whatever sysconfdir was specified as when BIND was
built). To create a rndc.key file, run rndc-confgen.
-a
The rndc.key feature was created to ease
the transition of systems from BIND 8,
which did not have digital signatures on its command channel messages and
thus did not have a keys clause.
It makes it possible to use an existing BIND 8
configuration file in BIND 9 unchanged,
and still have rndc work
the same way ndc worked in
BIND 8, simply by executing the command rndc-confgen after BIND 9 is installed.
-a
Since the rndc.key feature is only intended
to allow the backward-compatible usage of BIND 8
configuration files, this feature does not have a high degree of configurability.
You cannot easily change the key name or the size of the secret, so you
should make a rndc.conf with your own key
if you wish to change those things. The rndc.key file
also has its permissions set such that only the owner of the file (the
user that named is running
as) can access it. If you desire greater flexibility in allowing other
users to access rndc commands,
then you need to create a rndc.conf file
and make it group readable by a group that contains the users who should
have access.
To disable the command channel, use an empty controls statement: controls
{ };.
The include statement inserts
the specified file at the point where the include statement
is encountered. The include statement
facilitates the administration of configuration files by permitting the
reading or writing of some things but not others. For example, the statement
could include private keys that are readable only by the name server.
The key statement defines
a shared secret key for use with TSIG (see the
section called “TSIG”) or the command channel (see the section called “controls Statement
Definition and Usage”).
The key statement can occur
at the top level of the configuration file or inside a view statement.
Keys defined in top-level key statements
can be used in all views. Keys intended for use in a controls statement
(see the section called “controls Statement
Definition and Usage”) must be defined at the top level.
The key_id, also known as the
key name, is a domain name uniquely identifying the key. It can be used
in a server statement to
cause requests sent to that server to be signed with this key, or in address
match lists to verify that incoming requests have been signed with a key
matching this name, algorithm, and secret.
The algorithm_id is a string
that specifies a security/authentication algorithm. Named supports hmac-md5, hmac-sha1, hmac-sha224, hmac-sha256, hmac-sha384 and hmac-sha512 TSIG
authentication. Truncated hashes are supported by appending the minimum
number of required bits preceeded by a dash, e.g. hmac-sha1-80.
The secret_string is the secret
to be used by the algorithm, and is treated as a base-64 encoded string.
logging { [ channelchannel_name{ ( filepath name[ versions (number| unlimited ) ] [ sizesize spec] | syslogsyslog_facility| stderr | null ); [ severity (critical|error|warning|notice|info|debug[level] |dynamic); ] [ print-categoryyesorno; ] [ print-severityyesorno; ] [ print-timeyesorno; ] }; ] [ categorycategory_name{channel_name; [channel_name; ... ] }; ] ... };
The logging statement configures
a wide variety of logging options for the name server. Its channel phrase
associates output methods, format options and severity levels with a name
that can then be used with the category phrase
to select how various classes of messages are logged.
Only one logging statement
is used to define as many channels and categories as are wanted. If there
is no logging statement,
the logging configuration will be:
logging {
category default { default_syslog; default_debug; };
category unmatched { null; };
};
In BIND 9, the logging configuration
is only established when the entire configuration file has been parsed.
In BIND 8, it was established as soon
as the logging statement
was parsed. When the server is starting up, all logging messages regarding
syntax errors in the configuration file go to the default channels, or
to standard error if the "-g" option was specified.
All log output goes to one or more channels;
you can make as many of them as you want.
Every channel definition must include a destination clause that says
whether messages selected for the channel go to a file, to a particular
syslog facility, to the standard error stream, or are discarded. It can
optionally also limit the message severity level that will be accepted
by the channel (the default is info),
and whether to include a named-generated
time stamp, the category name and/or severity level (the default is not
to include any).
The null destination
clause causes all messages sent to the channel to be discarded; in that
case, other options for the channel are meaningless.
The file destination
clause directs the channel to a disk file. It can include limitations
both on how large the file is allowed to become, and how many versions
of the file will be saved each time the file is opened.
If you use the versions log
file option, then named will
retain that many backup versions of the file by renaming them when opening.
For example, if you choose to keep three old versions of the file lamers.log,
then just before it is opened lamers.log.1 is
renamed to lamers.log.2, lamers.log.0 is
renamed to lamers.log.1, and lamers.log is
renamed to lamers.log.0. You can say versions
unlimited to not limit the number of versions. If a size option
is associated with the log file, then renaming is only done when the
file being opened exceeds the indicated size. No backup versions are
kept by default; any existing log file is simply appended.
The size option for files
is used to limit log growth. If the file ever exceeds the size, then named will
stop writing to the file unless it has a versions option
associated with it. If backup versions are kept, the files are rolled
as described above and a new one begun. If there is no versions option,
no more data will be written to the log until some out-of-band mechanism
removes or truncates the log to less than the maximum size. The default
behavior is not to limit the size of the file.
Example usage of the size and versions options:
channel an_example_channel {
file "example.log" versions 3 size 20m;
print-time yes;
print-category yes;
};
The syslog destination
clause directs the channel to the system log. Its argument is a syslog
facility as described in the syslog man
page. Known facilities are kern, user, mail, daemon, auth, syslog, lpr, news, uucp, cron, authpriv, ftp, local0, local1, local2, local3, local4, local5, local6 and local7,
however not all facilities are supported on all operating systems. How syslog will
handle messages sent to this facility is described in the syslog.conf man
page. If you have a system which uses a very old version of syslog that
only uses two arguments to the openlog() function,
then this clause is silently ignored.
The severity clause works
like syslog's
"priorities", except that they can also be used if you are writing straight
to a file rather than using syslog.
Messages which are not at least of the severity level given will not
be selected for the channel; messages of higher severity levels will
be accepted.
If you are using syslog,
then the syslog.conf priorities
will also determine what eventually passes through. For example, defining
a channel facility and severity as daemon and debug but
only logging daemon.warning via syslog.conf will
cause messages of severity info and notice to
be dropped. If the situation were reversed, with named writing
messages of only warning or
higher, then syslogd would
print all messages it received from the channel.
The stderr destination
clause directs the channel to the server's standard error stream. This
is intended for use when the server is running as a foreground process,
for example when debugging a configuration.
The server can supply extensive debugging information when it is in
debugging mode. If the server's global debug level is greater than zero,
then debugging mode will be active. The global debug level is set either
by starting the named server
with the -d flag followed by a positive integer,
or by running rndc trace.
The global debug level can be set to zero, and debugging mode turned
off, by running rndc notrace.
All debugging messages in the server have a debug level, and higher debug
levels give more detailed output. Channels that specify a specific debug
severity, for example:
channel specific_debug_level {
file "foo";
severity debug 3;
};
will get debugging output of level 3 or less any time the server is
in debugging mode, regardless of the global debugging level. Channels
with dynamic severity use
the server's global debug level to determine what messages to print.
If print-time has been
turned on, then the date and time will be logged. print-time may
be specified for a syslog channel,
but is usually pointless since syslog also
prints the date and time. If print-category is
requested, then the category of the message will be logged as well. Finally,
if print-severity is on,
then the severity level of the message will be logged. The print- options
may be used in any combination, and will always be printed in the following
order: time, category, severity. Here is an example where all three print- options
are on:
28-Feb-2000 15:05:32.863 general: notice:
running
There are four predefined channels that are used for named's
default logging as follows. How they are used is described in the
section called “The category Phrase”.
channel default_syslog {
syslog daemon; // send to syslog's daemon
// facility
severity info; // only send priority info
// and higher
};
channel default_debug {
file "named.run"; // write to named.run in
// the working directory
// Note: stderr is used instead
// of "named.run"
// if the server is started
// with the '-f' option.
severity dynamic; // log at the server's
// current debug level
};
channel default_stderr {
stderr; // writes to stderr
severity info; // only send priority info
// and higher
};
channel null {
null; // toss anything sent to
// this channel
};
The default_debug channel
has the special property that it only produces output when the server's
debug level is nonzero. It normally writes to a file called named.run in
the server's working directory.
For security reasons, when the "-u"
command line option is used, the named.run file
is created only after named has
changed to the new UID, and any debug output generated while named is
starting up and still running as root is discarded. If you need to capture
this output, you must run the server with the "-g"
option and redirect standard error to a file.
Once a channel is defined, it cannot be redefined. Thus you cannot
alter the built-in channels directly, but you can modify the default
logging by pointing categories at channels you have defined.
There are many categories, so you can send the logs you want to see
wherever you want, without seeing logs you don't want. If you don't specify
a list of channels for a category, then log messages in that category
will be sent to the default category
instead. If you don't specify a default category, the following
"default default" is used:
category default { default_syslog; default_debug; };
As an example, let's say you want to log security events to a file,
but you also want keep the default logging behavior. You'd specify the
following:
channel my_security_channel {
file "my_security_file";
severity info;
};
category security {
my_security_channel;
default_syslog;
default_debug;
};To discard all messages in a category, specify the null channel:
category xfer-out { null; };
category notify { null; };
Following are the available categories and brief descriptions of the
types of log information they contain. More categories may be added in
future BIND releases.
|
default |
The default category defines the logging options for those |
|
general |
The catch-all. Many things still aren't classified into |
|
database |
Messages relating to the databases used internally by the |
|
security |
Approval and denial of requests. |
|
config |
Configuration file parsing and processing. |
|
resolver |
DNS resolution, such as the recursive lookups performed |
|
xfer-in |
Zone transfers the server is receiving. |
|
xfer-out |
Zone transfers the server is sending. |
|
notify |
The NOTIFY protocol. |
|
client |
Processing of client requests. |
|
unmatched |
Messages that named was unable to determine the class of |
|
network |
Network operations. |
|
update |
Dynamic updates. |
|
update-security |
Approval and denial of update requests. |
|
queries |
Specify where queries should be logged to. At startup, specifying the category queries will The query log entry reports the client's IP address and port |
|
dispatch |
Dispatching of incoming packets to the server modules where |
|
dnssec |
DNSSEC and TSIG protocol processing. |
|
lame-servers |
Lame servers. These are misconfigurations in remote servers, |
|
delegation-only |
Delegation only. Logs queries that have have been forced |
This is the grammar of the lwres statement
in the named.conf file:
lwres { [ listen-on {ip_addr[portip_port] ; [ip_addr[portip_port] ; ... ] }; ] [ viewview_name; ] [ search {domain_name; [domain_name; ... ] }; ] [ ndotsnumber; ] };
The lwres statement configures
the name server to also act as a lightweight resolver server. (See the
section called “Running a Resolver Daemon”.) There may
be be multiple lwres statements
configuring lightweight resolver servers with different properties.
The listen-on statement
specifies a list of addresses (and ports) that this instance of a lightweight
resolver daemon should accept requests on. If no port is specified, port
921 is used. If this statement is omitted, requests will be accepted on
127.0.0.1, port 921.
The view statement binds
this instance of a lightweight resolver daemon to a view in the DNS namespace,
so that the response will be constructed in the same manner as a normal
DNS query matching this view. If this statement is omitted, the default
view is used, and if there is no default view, an error is triggered.
The search statement is
equivalent to the search statement
in /etc/resolv.conf. It provides a list of
domains which are appended to relative names in queries.
The ndots statement is
equivalent to the ndots statement
in /etc/resolv.conf. It indicates the minimum
number of dots in a relative domain name that should result in an exact
match lookup before search path elements are appended.
mastersname[portip_port] { (masters_list|ip_addr[portip_port] [keykey] ) ; [...] };
masters lists allow for
a common set of masters to be easily used by multiple stub and slave zones.
This is the grammar of the options statement
in the named.conf file:
options {
[ version version_string; ]
[ hostname hostname_string; ]
[ server-id server_id_string; ]
[ directory path_name; ]
[ key-directory path_name; ]
[ named-xfer path_name; ]
[ tkey-domain domainname; ]
[ tkey-dhkey key_name key_tag; ]
[ cache-file path_name; ]
[ dump-file path_name; ]
[ memstatistics-file path_name; ]
[ pid-file path_name; ]
[ statistics-file path_name; ]
[ zone-statistics yes_or_no; ]
[ auth-nxdomain yes_or_no; ]
[ deallocate-on-exit yes_or_no; ]
[ dialup dialup_option; ]
[ fake-iquery yes_or_no; ]
[ fetch-glue yes_or_no; ]
[ flush-zones-on-shutdown yes_or_no; ]
[ has-old-clients yes_or_no; ]
[ host-statistics yes_or_no; ]
[ host-statistics-max number; ]
[ minimal-responses yes_or_no; ]
[ multiple-cnames yes_or_no; ]
[ notify yes_or_no | explicit | master-only; ]
[ recursion yes_or_no; ]
[ rfc2308-type1 yes_or_no; ]
[ use-id-pool yes_or_no; ]
[ maintain-ixfr-base yes_or_no; ]
[ dnssec-enable yes_or_no; ]
[ dnssec-validation yes_or_no; ]
[ dnssec-lookaside domain trust-anchor domain; ]
[ dnssec-must-be-secure domain yes_or_no; ]
[ dnssec-accept-expired yes_or_no; ]
[ forward ( only | first ); ]
[ forwarders { [ ip_addr [port ip_port] ; ... ] }; ]
[ dual-stack-servers [port ip_port] {
( domain_name [port ip_port] |
ip_addr [port ip_port] ) ;
... }; ]
[ check-names ( master | slave | response )
( warn | fail | ignore ); ]
[ check-mx ( warn | fail | ignore ); ]
[ check-wildcard yes_or_no; ]
[ check-integrity yes_or_no; ]
[ check-mx-cname ( warn | fail | ignore ); ]
[ check-srv-cname ( warn | fail | ignore ); ]
[ check-sibling yes_or_no; ]
[ allow-notify { address_match_list }; ]
[ allow-query { address_match_list }; ]
[ allow-query-cache { address_match_list }; ]
[ allow-transfer { address_match_list }; ]
[ allow-recursion { address_match_list }; ]
[ allow-update { address_match_list }; ]
[ allow-update-forwarding { address_match_list }; ]
[ update-check-ksk yes_or_no; ]
[ allow-v6-synthesis { address_match_list }; ]
[ blackhole { address_match_list }; ]
[ avoid-v4-udp-ports { port_list }; ]
[ avoid-v6-udp-ports { port_list }; ]
[ listen-on [ port ip_port ] { address_match_list }; ]
[ listen-on-v6 [ port ip_port ] { address_match_list }; ]
[ query-source ( ( ip4_addr | * )
[ port ( ip_port | * ) ] |
[ address ( ip4_addr | * ) ]
[ port ( ip_port | * ) ] ) ; ]
[ query-source-v6 ( ( ip6_addr | * )
[ port ( ip_port | * ) ] |
[ address ( ip6_addr | * ) ]
[ port ( ip_port | * ) ] ) ; ]
[ max-transfer-time-in number; ]
[ max-transfer-time-out number; ]
[ max-transfer-idle-in number; ]
[ max-transfer-idle-out number; ]
[ tcp-clients number; ]
[ recursive-clients number; ]
[ serial-query-rate number; ]
[ serial-queries number; ]
[ tcp-listen-queue number; ]
[ transfer-format ( one-answer | many-answers ); ]
[ transfers-in number; ]
[ transfers-out number; ]
[ transfers-per-ns number; ]
[ transfer-source (ip4_addr | *) [port ip_port] ; ]
[ transfer-source-v6 (ip6_addr | *) [port ip_port] ; ]
[ alt-transfer-source (ip4_addr | *) [port ip_port] ; ]
[ alt-transfer-source-v6 (ip6_addr | *) [port ip_port] ; ]
[ use-alt-transfer-source yes_or_no; ]
[ notify-source (ip4_addr | *) [port ip_port] ; ]
[ notify-source-v6 (ip6_addr | *) [port ip_port] ; ]
[ also-notify { ip_addr [port ip_port] ; [ ip_addr [port ip_port] ; ... ] }; ]
[ max-ixfr-log-size number; ]
[ max-journal-size size_spec; ]
[ coresize size_spec ; ]
[ datasize size_spec ; ]
[ files size_spec ; ]
[ stacksize size_spec ; ]
[ cleaning-interval number; ]
[ heartbeat-interval number; ]
[ interface-interval number; ]
[ statistics-interval number; ]
[ topology { address_match_list }];
[ sortlist { address_match_list }];
[ rrset-order { order_spec ; [ order_spec ; ... ] ] };
[ lame-ttl number; ]
[ max-ncache-ttl number; ]
[ max-cache-ttl number; ]
[ sig-validity-interval number ; ]
[ min-roots number; ]
[ use-ixfr yes_or_no ; ]
[ provide-ixfr yes_or_no; ]
[ request-ixfr yes_or_no; ]
[ treat-cr-as-space yes_or_no ; ]
[ min-refresh-time number ; ]
[ max-refresh-time number ; ]
[ min-retry-time number ; ]
[ max-retry-time number ; ]
[ port ip_port; ]
[ additional-from-auth yes_or_no ; ]
[ additional-from-cache yes_or_no ; ]
[ random-device path_name ; ]
[ max-cache-size size_spec ; ]
[ match-mapped-addresses yes_or_no; ]
[ preferred-glue ( A | AAAA | NONE ); ]
[ edns-udp-size number; ]
[ max-udp-size number; ]
[ root-delegation-only [ exclude { namelist } ] ; ]
[ querylog yes_or_no ; ]
[ disable-algorithms domain { algorithm; [ algorithm; ] }; ]
[ acache-enable yes_or_no ; ]
[ acache-cleaning-interval number; ]
[ max-acache-size size_spec ; ]
[ clients-per-query number ; ]
[ max-clients-per-query number ; ]
[ masterfile-format (text|raw) ; ]
[ empty-server name ; ]
[ empty-contact name ; ]
[ empty-zones-enable yes_or_no ; ]
[ disable-empty-zone zone_name ; ]
[ zero-no-soa-ttl yes_or_no ; ]
[ zero-no-soa-ttl-cache yes_or_no ; ]
};
The options statement sets
up global options to be used by BIND.
This statement may appear only once in a configuration file. If there is
no options statement, an
options block with each option set to its default will be used.
- directory
-
The working directory of the server. Any non-absolute pathnames
in the configuration file will be taken as relative to this directory.
The default location for most server output files (e.g.named.run)
is this directory. If a directory is not specified, the working directory
defaults to `.', the directory from
which the server was started. The directory specified should be an
absolute path. - key-directory
-
When performing dynamic update of secure zones, the directory where
the public and private key files should be found, if different than
the current working directory. The directory specified must be an
absolute path. - named-xfer
-
This option is obsolete. It
was used in BIND 8 to specify
the pathname to the named-xfer program.
In BIND 9, no separate named-xfer program
is needed; its functionality is built into the name server. - tkey-domain
-
The domain appended to the names of all shared keys generated with TKEY.
When a client requests a TKEY exchange,
it may or may not specify the desired name for the key. If present,
the name of the shared key will be "client" +
specified part
"tkey-domain". Otherwise, the name of
the shared key will be "random hex digits" + "tkey-domain".
In most cases, the domainname should
be the server's domain name. - tkey-dhkey
-
The Diffie-Hellman key used by the server to generate shared keys
with clients using the Diffie-Hellman mode of TKEY.
The server must be able to load the public and private keys from
files in the working directory. In most cases, the keyname should
be the server's host name. - cache-file
-
This is for testing only. Do not use.
- dump-file
-
The pathname of the file the server dumps the database to when
instructed to do so with rndc dumpdb.
If not specified, the default isnamed_dump.db. - memstatistics-file
-
The pathname of the file the server writes memory usage statistics
to on exit. If not specified, the default isnamed.memstats. - pid-file
-
The pathname of the file the server writes its process ID in. If
not specified, the default is/var/run/named.pid.
The pid-file is used by programs that want to send signals to the
running name server. Specifying pid-file
none disables the use of a PID file — no file
will be written and any existing one will be removed. Note that none is
a keyword, not a file name, and therefore is not enclosed in double
quotes. - statistics-file
-
The pathname of the file the server appends statistics to when
instructed to do so using rndc stats.
If not specified, the default isnamed.statsin
the server's current directory. The format of the file is described
in the section called “The
Statistics File”. - port
-
The UDP/TCP port number the server uses for receiving and sending
DNS protocol traffic. The default is 53. This option is mainly intended
for server testing; a server using a port other than 53 will not
be able to communicate with the global DNS. - random-device
-
The source of entropy to be used by the server. Entropy is primarily
needed for DNSSEC operations, such as TKEY transactions and dynamic
update of signed zones. This options specifies the device (or file)
from which to read entropy. If this is a file, operations requiring
entropy will fail when the file has been exhausted. If not specified,
the default value is/dev/random(or
equivalent) when present, and none otherwise. The random-device option
takes effect during the initial configuration load at server startup
time and is ignored on subsequent reloads. - preferred-glue
-
If specified, the listed type (A or AAAA) will be emitted before
other glue in the additional section of a query response. The default
is not to prefer any type (NONE). - root-delegation-only
-
Turn on enforcement of delegation-only in TLDs (top level domains)
and root zones with an optional exclude list.Note some TLDs are not delegation only (e.g. "DE", "LV", "US"
and "MUSEUM").options { root-delegation-only exclude { "de"; "lv"; "us"; "museum"; }; }; - disable-algorithms
-
Disable the specified DNSSEC algorithms at and below the specified
name. Multiple disable-algorithms statements
are allowed. Only the most specific will be applied. - dnssec-lookaside
-
When set, dnssec-lookaside provides
the validator with an alternate method to validate DNSKEY records
at the top of a zone. When a DNSKEY is at or below a domain specified
by the deepest dnssec-lookaside,
and the normal dnssec validation has left the key untrusted, the
trust-anchor will be append to the key name and a DLV record will
be looked up to see if it can validate the key. If the DLV record
validates a DNSKEY (similarly to the way a DS record does) the DNSKEY
RRset is deemed to be trusted. - dnssec-must-be-secure
-
Specify hierarchies which must be or may not be secure (signed
and validated). Ifyes,
then named will only accept answers if they are secure. Ifno,
then normal dnssec validation applies allowing for insecure answers
to be accepted. The specified domain must be under a trusted-key or dnssec-lookaside must
be active.
- auth-nxdomain
-
If
yes, then
the AA bit is always
set on NXDOMAIN responses, even if the server is not actually authoritative.
The default isno;
this is a change from BIND 8.
If you are using very old DNS software, you may need to set it
toyes. - deallocate-on-exit
-
This option was used in BIND 8
to enable checking for memory leaks on exit. BIND 9
ignores the option and always performs the checks. - dialup
-
If
yes, then
the server treats all zones as if they are doing zone transfers
across a dial-on-demand dialup link, which can be brought up by
traffic originating from this server. This has different effects
according to zone type and concentrates the zone maintenance so
that it all happens in a short interval, once every heartbeat-interval and
hopefully during the one call. It also suppresses some of the normal
zone maintenance traffic. The default isno.The dialup option
may also be specified in the view and zone statements,
in which case it overrides the global dialup option.If the zone is a master zone, then the server will send out a
NOTIFY request to all the slaves (default). This should trigger
the zone serial number check in the slave (providing it supports
NOTIFY) allowing the slave to verify the zone while the connection
is active. The set of servers to which NOTIFY is sent can be controlled
by notify and also-notify.If the zone is a slave or stub zone, then the server will suppress
the regular
"zone up to date" (refresh) queries and only perform them when
the heartbeat-interval expires
in addition to sending NOTIFY requests.Finer control can be achieved by using
notifywhich
only sends NOTIFY messages,notify-passivewhich
sends NOTIFY messages and suppresses the normal refresh queries,refreshwhich
suppresses normal refresh processing and sends refresh queries
when the heartbeat-interval expires,
andpassivewhich
just disables normal refresh processing.
dialup mode
normal refresh
heart-beat refresh
heart-beat notify
no (default)
yes
no
no
yes
no
yes
yes
notify
yes
no
yes
refresh
no
yes
no
passive
no
no
no
notify-passive
no
no
yes
Note that normal NOTIFY processing is not affected by dialup.
- fake-iquery
-
In BIND 8, this option enabled
simulating the obsolete DNS query type IQUERY. BIND 9
never does IQUERY simulation. - fetch-glue
-
This option is obsolete. In BIND 8,
fetch-gluecaused the server to attempt to fetch glue
yes
resource records it didn't have when constructing the additional
data section of a response. This is now considered a bad idea
and BIND 9 never does it. - flush-zones-on-shutdown
-
When the nameserver exits due receiving SIGTERM, flush or do
not flush any pending zone writes. The default is flush-zones-on-shutdownno. - has-old-clients
-
This option was incorrectly implemented in BIND 8,
and is ignored by BIND 9. To
achieve the intended effect of has-old-clientsyes,
specify the two separate options auth-nxdomainyesand rfc2308-type1noinstead. - host-statistics
-
In BIND 8, this enables keeping of statistics for every host
that the name server interacts with. Not implemented in BIND 9. - maintain-ixfr-base
-
This option is obsolete.
It was used in BIND 8 to determine
whether a transaction log was kept for Incremental Zone Transfer. BIND 9
maintains a transaction log whenever possible. If you need to disable
outgoing incremental zone transfers, use provide-ixfrno. - minimal-responses
-
If
yes, then
when generating responses the server will only add records to the
authority and additional data sections when they are required (e.g.
delegations, negative responses). This may improve the performance
of the server. The default isno. - multiple-cnames
-
This option was used in BIND 8
to allow a domain name to have multiple CNAME records in violation
of the DNS standards. BIND 9.2
onwards always strictly enforces the CNAME rules both in master
files and dynamic updates. - notify
-
If
yes(the default),
DNS NOTIFY messages are sent when a zone the server is authoritative
for changes, see the section called “Notify”.
The messages are sent to the servers listed in the zone's NS records
(except the master server identified in the SOA MNAME field), and
to any servers listed in the also-notify option.If
master-only,
notifies are only sent for master zones. Ifexplicit,
notifies are sent only to servers explicitly listed using also-notify.
Ifno, no notifies
are sent.The notify option
may also be specified in the zone statement,
in which case it overrides the options
notify statement. It would only be necessary to
turn off this option if it caused slaves to crash. - recursion
-
If
yes, and a
DNS query requests recursion, then the server will attempt to do
all the work required to answer the query. If recursion is off
and the server does not already know the answer, it will return
a referral response. The default isyes.
Note that setting recursion no does
not prevent clients from getting data from the server's cache;
it only prevents new data from being cached as an effect of client
queries. Caching may still occur as an effect the server's internal
operation, such as NOTIFY address lookups. See also fetch-glue above. - rfc2308-type1
-
Setting this to
yeswill
cause the server to send NS records along with the SOA record for
negative answers. The default isno.Note
Not yet implemented in BIND 9.
- use-id-pool
-
This option is obsolete. BIND 9
always allocates query IDs from a pool. - zone-statistics
-
If
yes, the server
will collect statistical data on all zones (unless specifically
turned off on a per-zone basis by specifying zone-statistics
no in the zone statement).
These statistics may be accessed using rndc
stats, which will dump them to the file listed
in the statistics-file.
See also the section
called “The Statistics File”. - use-ixfr
-
This option is obsolete.
If you need to disable IXFR to a particular server or servers see
the information on the provide-ixfr option
in the section called “server Statement
Definition and Usage”. See also the
section called “Incremental Zone Transfers (IXFR)”. - provide-ixfr
-
See the description of provide-ixfr in the section called “server Statement
Definition and Usage”. - request-ixfr
-
See the description of request-ixfr in the section called “server Statement
Definition and Usage”. - treat-cr-as-space
-
This option was used in BIND 8
to make the server treat carriage return ("\r")
characters the same way as a space or tab character, to facilitate
loading of zone files on a UNIX system that were generated on an
NT or DOS machine. In BIND 9,
both UNIX "\n"
and NT/DOS "\r\n" newlines
are always accepted, and the option is ignored. - additional-from-auth, additional-from-cache
-
These options control the behavior of an authoritative server
when answering queries which have additional data, or when following
CNAME and DNAME chains.When both of these options are set to
yes(the
default) and a query is being answered from authoritative data
(a zone configured into the server), the additional data section
of the reply will be filled in using data from other authoritative
zones and from the cache. In some situations this is undesirable,
such as when there is concern over the correctness of the cache,
or in servers where slave zones may be added and modified by untrusted
third parties. Also, avoiding the search for this additional data
will speed up server operations at the possible expense of additional
queries to resolve what would otherwise be provided in the additional
section.For example, if a query asks for an MX record for host
foo.example.com,
and the record found is "MX 10 mail.example.net",
normally the address records (A and AAAA) formail.example.netwill
be provided as well, if known, even though they are not in the
example.com zone. Setting these options to no disables
this behavior and makes the server only search for additional data
in the zone it answers from.These options are intended for use in authoritative-only servers,
or in authoritative-only views. Attempts to set them to no without
also specifying recursion no will
cause the server to ignore the options and log a warning message.Specifying additional-from-cache
no actually disables the use of the cache not
only for additional data lookups but also when looking up the
answer. This is usually the desired behavior in an authoritative-only
server where the correctness of the cached data is an issue.When a name server is non-recursively queried for a name that
is not below the apex of any served zone, it normally answers with
an
"upwards referral" to the root servers or the servers of some other
known parent of the query name. Since the data in an upwards referral
comes from the cache, the server will not be able to provide upwards
referrals when additional-from-cache
no has been specified. Instead, it will respond
to such queries with REFUSED. This should not cause any problems
since upwards referrals are not required for the resolution process. - match-mapped-addresses
-
If
yes, then
an IPv4-mapped IPv6 address will match any address match list entries
that match the corresponding IPv4 address. Enabling this option
is sometimes useful on IPv6-enabled Linux systems, to work around
a kernel quirk that causes IPv4 TCP connections such as zone transfers
to be accepted on an IPv6 socket using mapped addresses, causing
address match lists designed for IPv4 to fail to match. The use
of this option for any other purpose is discouraged. - ixfr-from-differences
-
When
yesand
the server loads a new version of a master zone from its zone file
or receives a new version of a slave file by a non-incremental
zone transfer, it will compare the new version to the previous
one and calculate a set of differences. The differences are then
logged in the zone's journal file such that the changes can be
transmitted to downstream slaves as an incremental zone transfer.By allowing incremental zone transfers to be used for non-dynamic
zones, this option saves bandwidth at the expense of increased
CPU and memory consumption at the master. In particular, if the
new version of a zone is completely different from the previous
one, the set of differences will be of a size comparable to the
combined size of the old and new zone version, and the server will
need to temporarily allocate memory to hold this complete difference
set.ixfr-from-differences also
accepts master and slave at
the view and options levels which causes ixfr-from-differences to
apply to all master or slave zones
respectively. - multi-master
-
This should be set when you have multiple masters for a zone
and the addresses refer to different machines. Ifyes,
named will not log when the serial number on the master is less
than what named currently has. The default isno. - dnssec-enable
-
Enable DNSSEC support in named. Unless set to
yes,
named behaves as if it does not support DNSSEC. The default isyes. - dnssec-validation
-
Enable DNSSEC validation in named. Note dnssec-enable also
needs to be set toyesto
be effective. The default isno. - dnssec-accept-expired
-
Accept expired signatures when verifying DNSSEC signatures. The
default isno. - querylog
-
Specify whether query logging should be started when named starts.
If querylog is not
specified, then the query logging is determined by the presence
of the logging category queries. - check-names
-
This option is used to restrict the character set and syntax
of certain domain names in master files and/or DNS responses received
from the network. The default varies according to usage area. For master zones
the default is fail.
For slave zones the
default is warn.
For answers received from the network (response)
the default is ignore.The rules for legal hostnames and mail domains are derived from
RFC 952 and RFC 821 as modified by RFC 1123.check-names applies
to the owner names of A, AAA and MX records. It also applies to
the domain names in the RDATA of NS, SOA and MX records. It also
applies to the RDATA of PTR records where the owner name indicated
that it is a reverse lookup of a hostname (the owner name ends
in IN-ADDR.ARPA, IP6.ARPA or IP6.INT). - check-mx
-
Check whether the MX record appears to refer to a IP address.
The default is to warn.
Other possible values are fail and ignore. - check-wildcard
-
This option is used to check for non-terminal wildcards. The
use of non-terminal wildcards is almost always as a result of a
failure to understand the wildcard matching algorithm (RFC 1034).
This option affects master zones. The default (yes)
is to check for non-terminal wildcards and issue a warning. - check-integrity
-
Perform post load zone integrity checks on master zones. This
checks that MX and SRV records refer to address (A or AAAA) records
and that glue address records exist for delegated zones. For MX
and SRV records only in-zone hostnames are checked (for out-of-zone
hostnames use named-checkzone). For NS records only names below
top of zone are checked (for out-of-zone names and glue consistancy
checks use named-checkzone). The default is yes. - check-mx-cname
-
If check-integrity is
set then fail, warn or ignore MX records that refer to CNAMES.
The default is to warn. - check-srv-cname
-
If check-integrity is
set then fail, warn or ignore SRV records that refer to CNAMES.
The default is to warn. - check-sibling
-
When performing integrity checks, also check that sibling glue
exists. The default is yes. - zero-no-soa-ttl
-
When returning authoritative negative responses to SOA queries
set the TTL of the SOA recored returned in the authority section
to zero. The default is yes. - zero-no-soa-ttl-cache
-
When caching a negative response to a SOA query set the TTL to
zero. The default is no. - update-check-ksk
-
When regenerating the RRSIGs following a UPDATE request to a
secure zone, check the KSK flag on the DNSKEY RR to determine if
this key should be used to generate the RRSIG. This flag is ignored
if there are not DNSKEY RRs both with and without a KSK. The default
is yes.
The forwarding facility can be used to create a large site-wide cache
on a few servers, reducing traffic over links to external name servers.
It can also be used to allow queries by servers that do not have direct
access to the Internet, but wish to look up exterior names anyway. Forwarding
occurs only on those queries for which the server is not authoritative
and does not have the answer in its cache.
- forward
-
This option is only meaningful if the forwarders list is not
empty. A value offirst, the default,
causes the server to query the forwarders first — and if
that doesn't answer the question, the server will then look for
the answer itself. Ifonlyis specified,
the server will only query the forwarders. - forwarders
-
Specifies the IP addresses to be used for forwarding. The default
is the empty list (no forwarding).
Forwarding can also be configured on a per-domain basis, allowing for
the global forwarding options to be overridden in a variety of ways.
You can set particular domains to use different forwarders, or have a
different forward only/first behavior,
or not forward at all, see the section called “zone Statement
Grammar”.
Dual-stack servers are used as servers of last resort to work around
problems in reachability due the lack of support for either IPv4 or IPv6
on the host machine.
- dual-stack-servers
-
Specifies host names or addresses of machines with access to
both IPv4 and IPv6 transports. If a hostname is used, the server
must be able to resolve the name using only the transport it has.
If the machine is dual stacked, then the dual-stack-servers have
no effect unless access to a transport has been disabled on the
command line (e.g. named -4).
Access to the server can be restricted based on the IP address of the
requesting system. See the
section called “Address Match Lists” for details on how
to specify IP address lists.
- allow-notify
-
Specifies which hosts are allowed to notify this server, a slave,
of zone changes in addition to the zone masters. allow-notify may
also be specified in the zone statement,
in which case it overrides the options
allow-notify statement. It is only meaningful for
a slave zone. If not specified, the default is to process notify
messages only from a zone's master. - allow-query
-
Specifies which hosts are allowed to ask ordinary DNS questions. allow-query may
also be specified in the zone statement,
in which case it overrides the options
allow-query statement. If not specified, the default
is to allow queries from all hosts.Note
allow-query-cache is
now used to specify access to the cache. - allow-query-cache
-
Specifies which hosts are allowed to get answers from the cache.
If allow-query-cache is
not set then allow-recursion is
used if set, otherwise allow-query is
used if set, otherwise the default (localnets; localhost;)
is used. - allow-recursion
-
Specifies which hosts are allowed to make recursive queries through
this server. If allow-recursion is
not set then allow-query-cache is
used if set, otherwise allow-query is
used if set, otherwise the default (localnets; localhost;)
is used. - allow-update
-
Specifies which hosts are allowed to submit Dynamic DNS updates
for master zones. The default is to deny updates from all hosts.
Note that allowing updates based on the requestor's IP address
is insecure; see the
section called “Dynamic Update Security” for details. - allow-update-forwarding
-
Specifies which hosts are allowed to submit Dynamic DNS updates
to slave zones to be forwarded to the master. The default is{, which means that no update forwarding
none; }
will be performed. To enable update forwarding, specifyallow-update-forwarding. Specifying values other than
{ any; };{or
none; }{ any;is usually counterproductive, since the responsibility
}
for update access control should rest with the master server, not
the slaves.Note that enabling the update forwarding feature on a slave server
may expose master servers relying on insecure IP address based
access control to attacks; see the
section called “Dynamic Update Security” for more
details. - allow-v6-synthesis
-
This option was introduced for the smooth transition from AAAA
to A6 and from "nibble labels" to binary labels. However, since
both A6 and binary labels were then deprecated, this option was
also deprecated. It is now ignored with some warning messages. - allow-transfer
-
Specifies which hosts are allowed to receive zone transfers from
the server. allow-transfer may
also be specified in the zone statement,
in which case it overrides the options
allow-transfer statement. If not specified, the
default is to allow transfers to all hosts. - blackhole
-
Specifies a list of addresses that the server will not accept
queries from or use to resolve a query. Queries from these addresses
will not be responded to. The default isnone.
The interfaces and ports that the server will answer queries from may
be specified using the listen-on option. listen-on takes
an optional port, and an address_match_list.
The server will listen on all interfaces allowed by the address match
list. If a port is not specified, port 53 will be used.
Multiple listen-on statements
are allowed. For example,
listen-on { 5.6.7.8; };
listen-on port 1234 { !1.2.3.4; 1.2/16; };
will enable the name server on port 53 for the IP address 5.6.7.8,
and on port 1234 of an address on the machine in net 1.2 that is not
1.2.3.4.
If no listen-on is specified,
the server will listen on port 53 on all interfaces.
The listen-on-v6 option
is used to specify the interfaces and the ports on which the server will
listen for incoming queries sent using IPv6.
When
{ any; } is specified as the address_match_list for
the listen-on-v6 option,
the server does not bind a separate socket to each IPv6 interface address
as it does for IPv4 if the operating system has enough API support for
IPv6 (specifically if it conforms to RFC 3493 and RFC 3542). Instead,
it listens on the IPv6 wildcard address. If the system only has incomplete
API support for IPv6, however, the behavior is the same as that for IPv4.
A list of particular IPv6 addresses can also be specified, in which
case the server listens on a separate socket for each specified address,
regardless of whether the desired API is supported by the system.
Multiple listen-on-v6 options
can be used. For example,
listen-on-v6 { any; };
listen-on-v6 port 1234 { !2001:db8::/32; any; };
will enable the name server on port 53 for any IPv6 addresses (with
a single wildcard socket), and on port 1234 of IPv6 addresses that is
not in the prefix 2001:db8::/32 (with separate sockets for each matched
address.)
To make the server not listen on any IPv6 address, use
listen-on-v6 { none; };
If no listen-on-v6 option
is specified, the server will not listen on any IPv6 address.
If the server doesn't know the answer to a question, it will query
other name servers. query-source specifies
the address and port used for such queries. For queries sent over IPv6,
there is a separate query-source-v6 option.
If address is * (asterisk)
or is omitted, a wildcard IP address (INADDR_ANY)
will be used. If port is * or
is omitted, a random unprivileged port will be used. The avoid-v4-udp-ports and avoid-v6-udp-ports options
can be used to prevent named from selecting certain ports. The defaults
are:
query-source address * port *; query-source-v6 address * port *;
Note
The address specified in the query-source option
is used for both UDP and TCP queries, but the port applies only to
UDP queries. TCP queries always use a random unprivileged port.
Note
Solaris 2.5.1 and earlier does not support setting the source address
for TCP sockets.
Note
See also transfer-source and notify-source.
BIND has mechanisms in place to
facilitate zone transfers and set limits on the amount of load that transfers
place on the system. The following options apply to zone transfers.
- also-notify
-
Defines a global list of IP addresses of name servers that are
also sent NOTIFY messages whenever a fresh copy of the zone is
loaded, in addition to the servers listed in the zone's NS records.
This helps to ensure that copies of the zones will quickly converge
on stealth servers. If an also-notify list
is given in a zone statement,
it will override the options also-notify statement.
When a zone notify statement
is set to no, the
IP addresses in the global also-notify list
will not be sent NOTIFY messages for that zone. The default is
the empty list (no global notification list). - max-transfer-time-in
-
Inbound zone transfers running longer than this many minutes
will be terminated. The default is 120 minutes (2 hours). The maximum
value is 28 days (40320 minutes). - max-transfer-idle-in
-
Inbound zone transfers making no progress in this many minutes
will be terminated. The default is 60 minutes (1 hour). The maximum
value is 28 days (40320 minutes). - max-transfer-time-out
-
Outbound zone transfers running longer than this many minutes
will be terminated. The default is 120 minutes (2 hours). The maximum
value is 28 days (40320 minutes). - max-transfer-idle-out
-
Outbound zone transfers making no progress in this many minutes
will be terminated. The default is 60 minutes (1 hour). The maximum
value is 28 days (40320 minutes). - serial-query-rate
-
Slave servers will periodically query master servers to find
out if zone serial numbers have changed. Each such query uses a
minute amount of the slave server's network bandwidth. To limit
the amount of bandwidth used, BIND 9 limits the rate at which queries
are sent. The value of the serial-query-rate option,
an integer, is the maximum number of queries sent per second. The
default is 20. - serial-queries
-
In BIND 8, the serial-queries option
set the maximum number of concurrent serial number queries allowed
to be outstanding at any given time. BIND 9 does not limit the
number of outstanding serial queries and ignores the serial-queries option.
Instead, it limits the rate at which the queries are sent as defined
using the serial-query-rate option. - transfer-format
-
Zone transfers can be sent using two different formats, one-answer and many-answers.
The transfer-format option
is used on the master server to determine which format it sends. one-answer uses
one DNS message per resource record transferred. many-answers packs
as many resource records as possible into a message. many-answers is
more efficient, but is only supported by relatively new slave servers,
such as BIND 9, BIND 8.x
and BIND 4.9.5 onwards. The many-answers format
is also supported by recent Microsoft Windows nameservers. The
default is many-answers. transfer-format may
be overridden on a per-server basis by using the server statement. - transfers-in
-
The maximum number of inbound zone transfers that can be running
concurrently. The default value is10.
Increasing transfers-in may
speed up the convergence of slave zones, but it also may increase
the load on the local system. - transfers-out
-
The maximum number of outbound zone transfers that can be running
concurrently. Zone transfer requests in excess of the limit will
be refused. The default value is10. - transfers-per-ns
-
The maximum number of inbound zone transfers that can be concurrently
transferring from a given remote name server. The default value
is2. Increasing transfers-per-ns may
speed up the convergence of slave zones, but it also may increase
the load on the remote name server. transfers-per-ns may
be overridden on a per-server basis by using the transfers phrase
of the server statement. - transfer-source
-
transfer-source determines
which local address will be bound to IPv4 TCP connections used
to fetch zones transferred inbound by the server. It also determines
the source IPv4 address, and optionally the UDP port, used for
the refresh queries and forwarded dynamic updates. If not set,
it defaults to a system controlled value which will usually be
the address of the interface "closest to" the remote end. This
address must appear in the remote end's allow-transfer option
for the zone being transferred, if one is specified. This statement
sets the transfer-source for
all zones, but can be overridden on a per-view or per-zone basis
by including a transfer-source statement
within the view or zone block
in the configuration file.Note
Solaris 2.5.1 and earlier does not support setting the source
address for TCP sockets. - transfer-source-v6
-
The same as transfer-source,
except zone transfers are performed using IPv6. - alt-transfer-source
-
An alternate transfer source if the one listed in transfer-source fails
and use-alt-transfer-source is
set.Note
If you do not wish the alternate transfer source to be used, you
should set use-alt-transfer-source appropriately
and you should not depend upon getting a answer back to the first
refresh query. - alt-transfer-source-v6
-
An alternate transfer source if the one listed in transfer-source-v6 fails
and use-alt-transfer-source is
set. - use-alt-transfer-source
-
Use the alternate transfer sources or not. If views are specified
this defaults to no otherwise
it defaults to yes (for
BIND 8 compatibility). - notify-source
-
notify-source determines
which local source address, and optionally UDP port, will be used
to send NOTIFY messages. This address must appear in the slave
server's masters zone
clause or in an allow-notify clause.
This statement sets the notify-source for
all zones, but can be overridden on a per-zone or per-view basis
by including a notify-source statement
within the zone or view block
in the configuration file.Note
Solaris 2.5.1 and earlier does not support setting the source
address for TCP sockets. - notify-source-v6
-
Like notify-source,
but applies to notify messages sent to IPv6 addresses.
avoid-v4-udp-ports and avoid-v6-udp-ports specify
a list of IPv4 and IPv6 UDP ports that will not be used as system assigned
source ports for UDP sockets. These lists prevent named from choosing
as its random source port a port that is blocked by your firewall. If
a query went out with such a source port, the answer would not get by
the firewall and the name server would have to query again.
The server's usage of many system resources can be limited. Scaled
values are allowed when specifying resource limits. For example, 1G can
be used instead of 1073741824 to
specify a limit of one gigabyte. unlimited requests
unlimited use, or the maximum available amount. default uses
the limit that was in force when the server was started. See the description
of size_spec in the
section called “Configuration File Elements”.
The following options set operating system resource limits for the
name server process. Some operating systems don't support some or any
of the limits. On such systems, a warning will be issued if the unsupported
limit is used.
- coresize
-
The maximum size of a core dump. The default is
default. - datasize
-
The maximum amount of data memory the server may use. The default
isdefault. This is a hard limit on
server memory usage. If the server attempts to allocate memory
in excess of this limit, the allocation will fail, which may in
turn leave the server unable to perform DNS service. Therefore,
this option is rarely useful as a way of limiting the amount of
memory used by the server, but it can be used to raise an operating
system data size limit that is too small by default. If you wish
to limit the amount of memory used by the server, use the max-cache-size and recursive-clients options
instead. - files
-
The maximum number of files the server may have open concurrently.
The default isunlimited. - stacksize
-
The maximum amount of stack memory the server may use. The default
isdefault.
The following options set limits on the server's resource consumption
that are enforced internally by the server rather than the operating
system.
- max-ixfr-log-size
-
This option is obsolete; it is accepted and ignored for BIND
8 compatibility. The option max-journal-size performs
a similar function in BIND 9. - max-journal-size
-
Sets a maximum size for each journal file (see the
section called “The journal file”). When the
journal file approaches the specified size, some of the oldest
transactions in the journal will be automatically removed. The
default isunlimited. - host-statistics-max
-
In BIND 8, specifies the maximum number of host statistics entries
to be kept. Not implemented in BIND 9. - recursive-clients
-
The maximum number of simultaneous recursive lookups the server
will perform on behalf of clients. The default is1000.
Because each recursing client uses a fair bit of memory, on the
order of 20 kilobytes, the value of the recursive-clients option
may have to be decreased on hosts with limited memory. - tcp-clients
-
The maximum number of simultaneous client TCP connections that
the server will accept. The default is100. - max-cache-size
-
The maximum amount of memory to use for the server's cache, in
bytes. When the amount of data in the cache reaches this limit,
the server will cause records to expire prematurely so that the
limit is not exceeded. In a server with multiple views, the limit
applies separately to the cache of each view. The default isunlimited,
meaning that records are purged from the cache only when their
TTLs expire. - tcp-listen-queue
-
The listen queue depth. The default and minimum is 3. If the
kernel supports the accept filter "dataready" this also controls
how many TCP connections that will be queued in kernel space waiting
for some data before being passed to accept. Values less than 3
will be silently raised.
- cleaning-interval
-
The server will remove expired resource records from the cache
every cleaning-interval minutes.
The default is 60 minutes. The maximum value is 28 days (40320
minutes). If set to 0, no periodic cleaning will occur. - heartbeat-interval
-
The server will perform zone maintenance tasks for all zones
marked as dialup whenever
this interval expires. The default is 60 minutes. Reasonable values
are up to 1 day (1440 minutes). The maximum value is 28 days (40320
minutes). If set to 0, no zone maintenance for these zones will
occur. - interface-interval
-
The server will scan the network interface list every interface-interval minutes.
The default is 60 minutes. The maximum value is 28 days (40320
minutes). If set to 0, interface scanning will only occur when
the configuration file is loaded. After the scan, the server will
begin listening for queries on any newly discovered interfaces
(provided they are allowed by the listen-on configuration),
and will stop listening on interfaces that have gone away. - statistics-interval
-
Name server statistics will be logged every statistics-interval minutes.
The default is 60. The maximum value is 28 days (40320 minutes).
If set to 0, no statistics will be logged.Note
Not yet implemented in BIND9.
All other things being equal, when the server chooses a name server
to query from a list of name servers, it prefers the one that is topologically
closest to itself. The topology statement
takes an address_match_list and
interprets it in a special way. Each top-level list element is assigned
a distance. Non-negated elements get a distance based on their position
in the list, where the closer the match is to the start of the list,
the shorter the distance is between it and the server. A negated match
will be assigned the maximum distance from the server. If there is no
match, the address will get a distance which is further than any non-negated
list element, and closer than any negated element. For example,
topology {
10/8;
!1.2.3/24;
{ 1.2/16; 3/8; };
}; will prefer servers on network 10 the most, followed by hosts on network
1.2.0.0 (netmask 255.255.0.0) and network 3, with the exception of hosts
on network 1.2.3 (netmask 255.255.255.0), which is preferred least of
all.
The default topology is
topology { localhost; localnets; };
Note
The topology option
is not implemented in BIND 9.
The response to a DNS query may consist of multiple resource records
(RRs) forming a resource records set (RRset). The name server will normally
return the RRs within the RRset in an indeterminate order (but see the rrset-order statement
in the section called “RRset
Ordering”). The client resolver code should rearrange the RRs
as appropriate, that is, using any addresses on the local net in preference
to other addresses. However, not all resolvers can do this or are correctly
configured. When a client is using a local server, the sorting can be
performed in the server, based on the client's address. This only requires
configuring the name servers, not all the clients.
The sortlist statement
(see below) takes an address_match_list and
interprets it even more specifically than the topology statement
does (the section called “Topology”).
Each top level statement in the sortlist must
itself be an explicit address_match_list with
one or two elements. The first element (which may be an IP address, an
IP prefix, an ACL name or a nested address_match_list)
of each top level list is checked against the source address of the query
until a match is found.
Once the source address of the query has been matched, if the top level
statement contains only one element, the actual primitive element that
matched the source address is used to select the address in the response
to move to the beginning of the response. If the statement is a list
of two elements, then the second element is treated the same as the address_match_list in
a topology statement. Each
top level element is assigned a distance and the address in the response
with the minimum distance is moved to the beginning of the response.
In the following example, any queries received from any of the addresses
of the host itself will get responses preferring addresses on any of
the locally connected networks. Next most preferred are addresses on
the 192.168.1/24 network, and after that either the 192.168.2/24 or 192.168.3/24
network with no preference shown between these two networks. Queries
received from a host on the 192.168.1/24 network will prefer other addresses
on that network to the 192.168.2/24 and 192.168.3/24 networks. Queries
received from a host on the 192.168.4/24 or the 192.168.5/24 network
will only prefer other addresses on their directly connected networks.
sortlist {
{ localhost; // IF the local host
{ localnets; // THEN first fit on the
192.168.1/24; // following nets
{ 192.168.2/24; 192.168.3/24; }; }; };
{ 192.168.1/24; // IF on class C 192.168.1
{ 192.168.1/24; // THEN use .1, or .2 or .3
{ 192.168.2/24; 192.168.3/24; }; }; };
{ 192.168.2/24; // IF on class C 192.168.2
{ 192.168.2/24; // THEN use .2, or .1 or .3
{ 192.168.1/24; 192.168.3/24; }; }; };
{ 192.168.3/24; // IF on class C 192.168.3
{ 192.168.3/24; // THEN use .3, or .1 or .2
{ 192.168.1/24; 192.168.2/24; }; }; };
{ { 192.168.4/24; 192.168.5/24; }; // if .4 or .5, prefer that net
};
}; The following example will give reasonable behavior for the local host
and hosts on directly connected networks. It is similar to the behavior
of the address sort in BIND 4.9.x.
Responses sent to queries from the local host will favor any of the directly
connected networks. Responses sent to queries from any other hosts on
a directly connected network will prefer addresses on that same network.
Responses to other queries will not be sorted.
sortlist {
{ localhost; localnets; };
{ localnets; };
};
When multiple records are returned in an answer it may be useful to
configure the order of the records placed into the response. The rrset-order statement
permits configuration of the ordering of the records in a multiple record
response. See also the sortlist statement, the
section called “The sortlist Statement”.
An order_spec is defined
as follows:
[class class_name]
[type type_name]
[name "domain_name"]
order ordering
If no class is specified, the default is ANY.
If no type is specified, the default is ANY.
If no name is specified, the default is "*" (asterisk).
The legal values for ordering are:
|
fixed |
Records are returned in the order they are defined in the |
|
random |
Records are returned in some random order. |
|
cyclic |
Records are returned in a round-robin order. |
For example:
rrset-order {
class IN type A name "host.example.com" order random;
order cyclic;
};
will cause any responses for type A records in class IN that have "host.example.com" as
a suffix, to always be returned in random order. All other records are
returned in cyclic order.
If multiple rrset-order statements
appear, they are not combined — the last one applies.
Note
The rrset-order statement
is not yet fully implemented in BIND 9.
BIND 9 currently does not fully support "fixed" ordering.
- lame-ttl
-
Sets the number of seconds to cache a lame server indication.
0 disables caching. (This is NOT recommended.)
The default is600(10 minutes) and
the maximum value is1800(30 minutes). - max-ncache-ttl
-
To reduce network traffic and increase performance, the server
stores negative answers. max-ncache-ttl is
used to set a maximum retention time for these answers in the server
in seconds. The default max-ncache-ttl is10800seconds
(3 hours). max-ncache-ttl cannot
exceed 7 days and will be silently truncated to 7 days if set to
a greater value. - max-cache-ttl
-
Sets the maximum time for which the server will cache ordinary
(positive) answers. The default is one week (7 days). - min-roots
-
The minimum number of root servers that is required for a request
for the root servers to be accepted. The default is2.Note
Not implemented in BIND 9.
- sig-validity-interval
-
Specifies the number of days into the future when DNSSEC signatures
automatically generated as a result of dynamic updates (the
section called “Dynamic Update”) will expire. The
default is30days. The maximum value
is 10 years (3660 days). The signature inception time is unconditionally
set to one hour before the current time to allow for a limited
amount of clock skew. - min-refresh-time, max-refresh-time, min-retry-time, max-retry-time
-
These options control the server's behavior on refreshing a zone
(querying for SOA changes) or retrying failed transfers. Usually
the SOA values for the zone are used, but these values are set
by the master, giving slave server administrators little control
over their contents.These options allow the administrator to set a minimum and maximum
refresh and retry time either per-zone, per-view, or globally.
These options are valid for slave and stub zones, and clamp the
SOA refresh and retry times to the specified values. - edns-udp-size
-
Sets the advertised EDNS UDP buffer size in bytes. Valid values
are 512 to 4096 (values outside this range will be silently adjusted).
The default value is 4096. The usual reason for setting edns-udp-size
to a non-default value it to get UDP answers to pass through broken
firewalls that block fragmented packets and/or block UDP packets
that are greater than 512 bytes. - max-udp-size
-
Sets the maximum EDNS UDP message size named will send in bytes.
Valid values are 512 to 4096 (values outside this range will be
silently adjusted). The default value is 4096. The usual reason
for setting max-udp-size to a non-default value is to get UDP answers
to pass through broken firewalls that block fragmented packets
and/or block UDP packets that are greater than 512 bytes. - masterfile-format
-
Specifies the file format of zone files (see the
section called “Additional File Formats”). The
default value istext, which is
the standard textual representation. Files in other formats thantextare
typically expected to be generated by the named-compilezone tool.
Note that when a zone file in a different format thantextis
loaded, named may
omit some of the checks which would be performed for a file in
thetextformat. In particular, check-names checks
do not apply for therawformat.
This means a zone file in therawformat
must be generated with the same check level as that specified
in the named configuration
file. This statement sets the masterfile-format for
all zones, but can be overridden on a per-zone or per-view basis
by including a masterfile-format statement
within the zone or view block
in the configuration file. - clients-per-query, max-clients-per-query
-
These set the initial value (minimum) and maximum number of recursive
simultanious clients for any given query (<qname,qtype,qclass>)
that the server will accept before dropping additional clients.
named will attempt to self tune this value and changes will be
logged. The default values are 10 and 100.This value should reflect how many queries come in for a given
name in the time it takes to resolve that name. If the number of
queries exceed this value, named will assume that it is dealing
with a non-responsive zone and will drop additional queries. If
it gets a response after dropping queries, it will raise the estimate.
The estimate will then be lowered in 20 minutes if it has remained
unchanged.If clients-per-query is
set to zero, then there is no limit on the number of clients per
query and no queries will be dropped.If max-clients-per-query is
set to zero, then there is no upper bound other than imposed by recursive-clients.
The server provides some helpful diagnostic information through a number
of built-in zones under the pseudo-top-level-domain bind in
the CHAOS class. These
zones are part of a built-in view (see the
section called “view Statement
Grammar”) of class CHAOS which
is separate from the default view of class IN;
therefore, any global server options such as allow-query do
not apply the these zones. If you feel the need to disable these zones,
use the options below, or hide the built-in CHAOS view
by defining an explicit view of class CHAOS that
matches all clients.
- version
-
The version the server should report via a query of the name
version.bindwith
type TXT, class CHAOS.
The default is the real version number of this server. Specifying version
none disables processing of the queries. - hostname
-
The hostname the server should report via a query of the name
hostname.bindwith
type TXT, class CHAOS.
This defaults to the hostname of the machine hosting the name server
as found by the gethostname() function. The primary purpose of
such queries is to identify which of a group of anycast servers
is actually answering your queries. Specifying hostname
none; disables processing of the queries. - server-id
-
The ID of the server should report via a query of the name
ID.SERVERwith
type TXT, class CHAOS.
The primary purpose of such queries is to identify which of a group
of anycast servers is actually answering your queries. Specifying server-id
none; disables processing of the queries. Specifying server-id
hostname; will cause named to use the hostname
as found by the gethostname() function. The default server-id is none.
Named has some built-in empty zones (SOA and NS records only). These
are for zones that should normally be answered locally and which queries
should not be sent to the Internet's root servers. The offical servers
which cover these namespaces return NXDOMAIN responses to these queries.
In particular, these cover the reverse namespace for addresses from RFC
1918 and RFC 3330. They also include the reverse namespace for IPv6 local
address (locally assigned), IPv6 link local addresses, the IPv6 loopback
address and the IPv6 unknown addresss.
Named will attempt to determine if a built in zone already exists or
is active (covered by a forward-only forwarding declaration) and will
not not create a empty zone in that case.
The current list of empty zones is:
- 10.IN-ADDR.ARPA
- 127.IN-ADDR.ARPA
- 254.169.IN-ADDR.ARPA
- 16.172.IN-ADDR.ARPA
- 17.172.IN-ADDR.ARPA
- 18.172.IN-ADDR.ARPA
- 19.172.IN-ADDR.ARPA
- 20.172.IN-ADDR.ARPA
- 21.172.IN-ADDR.ARPA
- 22.172.IN-ADDR.ARPA
- 23.172.IN-ADDR.ARPA
- 24.172.IN-ADDR.ARPA
- 25.172.IN-ADDR.ARPA
- 26.172.IN-ADDR.ARPA
- 27.172.IN-ADDR.ARPA
- 28.172.IN-ADDR.ARPA
- 29.172.IN-ADDR.ARPA
- 30.172.IN-ADDR.ARPA
- 31.172.IN-ADDR.ARPA
- 168.192.IN-ADDR.ARPA
- 2.0.192.IN-ADDR.ARPA
- 0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.IP6.ARPA
- 1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.IP6.ARPA
- D.F.IP6.ARPA
- 8.E.F.IP6.ARPA
- 9.E.F.IP6.ARPA
- A.E.F.IP6.ARPA
- B.E.F.IP6.ARPA
Empty zones are settable at the view level and only apply to views
of class IN. Disabled empty zones are only inherited from options if
there are no disabled empty zones specified at the view level. To override
the options list of disabled zones, you can disable the root zone at
the view level, for example:
disable-empty-zone ".";
If you are using the address ranges covered here, you should already
have reverse zones covering the addresses you use. In practice this appears
to not be the case with many queries being made to the infrustructure
servers for names in these spaces. So many in fact that sacrificial servers
were needed to be deployed to channel the query load away from the infrustructure
servers.
Note
The real parent servers for these zones should disable all empty zone
under the parent zone they serve. For the real root servers, this is
all built in empty zones. This will enable them to return referrals to
deeper in the tree.
- empty-server
-
Specify what server name will appear in the returned SOA record
for empty zones. If none is specified, then the zone's name will
be used. - empty-contact
-
Specify what contact name will appear in the returned SOA record
for empty zones. If none is specified, then
"." will be used. - empty-zones-enable
-
Enable or disable all empty zones. By default they are enabled.
- disable-empty-zone
-
Disable individual empty zones. By default none are disabled.
This option can be specified multiple times.
The statistics file generated by BIND 9
is similar, but not identical, to that generated by BIND 8.
The statistics dump begins with a line, like:
+++ Statistics Dump +++ (973798949)
The number in parentheses is a standard Unix-style timestamp, measured
as seconds since January 1, 1970. Following that line are a series of
lines containing a counter type, the value of the counter, optionally
a zone name, and optionally a view name. The lines without view and zone
listed are global statistics for the entire server. Lines with a zone
and view name for the given view and zone (the view name is omitted for
the default view).
The statistics dump ends with the line where the number is identical
to the number in the beginning line; for example:
--- Statistics Dump --- (973798949)
The following statistics counters are maintained:
|
success |
The number of successful queries made to the server or |
|
referral |
The number of queries which resulted in referral responses. |
|
nxrrset |
The number of queries which resulted in NOERROR responses |
|
nxdomain |
The number of queries which resulted in NXDOMAIN responses. |
|
failure |
The number of queries which resulted in a failure response |
|
recursion |
The number of queries which caused the server to perform |
Each query received by the server will cause exactly one of success, referral, nxrrset, nxdomain,
or failure to be incremented,
and may additionally cause the recursion counter
to be incremented.
The additional section cache, also called acache,
is an internal cache to improve the response performance of BIND 9. When
additional section caching is enabled, BIND 9 will cache an internal
short-cut to the additional section content for each answer RR. Note
that acache is an internal
caching mechanism of BIND 9, and is not related to the DNS caching server
function.
Additional section caching does not change the response content (except
the RRsets ordering of the additional section, see below), but can improve
the response performance significantly. It is particularly effective
when BIND 9 acts as an authoritative server for a zone that has many
delegations with many glue RRs.
In order to obtain the maximum performance improvement from additional
section caching, setting additional-from-cache to no is
recommended, since the current implementation of acache does
not short-cut of additional section information from the DNS cache data.
One obvious disadvantage of acache is
that it requires much more memory for the internal cached data. Thus,
if the response performance does not matter and memory consumption is
much more critical, the acache mechanism
can be disabled by setting acache-enable to no.
It is also possible to specify the upper limit of memory consumption
for acache by using max-acache-size.
Additional section caching also has a minor effect on the RRset ordering
in the additional section. Without acache, cyclic order
is effective for the additional section as well as the answer and authority
sections. However, additional section caching fixes the ordering when
it first caches an RRset for the additional section, and the same ordering
will be kept in succeeding responses, regardless of the setting of rrset-order.
The effect of this should be minor, however, since an RRset in the additional
section typically only contains a small number of RRs (and in many cases
it only contains a single RR), in which case the ordering does not matter
much.
The following is a summary of options related to acache.
- acache-enable
-
If yes, additional
section caching is enabled. The default value is no. - acache-cleaning-interval
-
The server will remove stale cache entries, based on an LRU based
algorithm, every acache-cleaning-interval minutes.
The default is 60 minutes. If set to 0, no periodic cleaning will
occur. - max-acache-size
-
The maximum amount of memory in bytes to use for the server's
acache. When the amount of data in the acache reaches this limit,
the server will clean more aggressively so that the limit is not
exceeded. In a server with multiple views, the limit applies separately
to the acache of each view. The default isunlimited,
meaning that entries are purged from the acache only at the periodic
cleaning time.
serverip_addr[/prefixlen]{ [ bogusyes_or_no; ] [ provide-ixfryes_or_no; ] [ request-ixfryes_or_no; ] [ ednsyes_or_no; ] [ edns-udp-sizenumber; ] [ max-udp-sizenumber; ] [ transfersnumber; ] [ transfer-format( one-answer | many-answers ); ]] [ keys{ string ; [ string ; [...]] }; ] [ transfer-source (ip4_addr|*) [portip_port] ; ] [ transfer-source-v6 (ip6_addr|*) [portip_port] ; ] [ notify-source (ip4_addr|*) [portip_port] ; ] [ notify-source-v6 (ip6_addr|*) [portip_port] ; ] [ query-source [ address (ip_addr|*) ] [ port (ip_port|*) ]; ] [ query-source-v6 [ address (ip_addr|*) ] [ port (ip_port|*) ]; ] };
The server statement defines
characteristics to be associated with a remote name server. If a prefix
length is specified, then a range of servers is covered. Only the most
specific server clause applies regardless of the order in named.conf.
The server statement can
occur at the top level of the configuration file or inside a view statement.
If a view statement contains
one or more server statements,
only those apply to the view and any top-level ones are ignored. If a view
contains no server statements,
any top-level server statements
are used as defaults.
If you discover that a remote server is giving out bad data, marking
it as bogus will prevent further queries to it. The default value of bogus is no.
The provide-ixfr clause
determines whether the local server, acting as master, will respond with
an incremental zone transfer when the given remote server, a slave, requests
it. If set to yes, incremental
transfer will be provided whenever possible. If set to no,
all transfers to the remote server will be non-incremental. If not set,
the value of the provide-ixfr option
in the view or global options block is used as a default.
The request-ixfr clause
determines whether the local server, acting as a slave, will request incremental
zone transfers from the given remote server, a master. If not set, the
value of the request-ixfr option
in the view or global options block is used as a default.
IXFR requests to servers that do not support IXFR will automatically
fall back to AXFR. Therefore, there is no need to manually list which servers
support IXFR and which ones do not; the global default of yes should
always work. The purpose of the provide-ixfr and request-ixfr clauses
is to make it possible to disable the use of IXFR even when both master
and slave claim to support it, for example if one of the servers is buggy
and crashes or corrupts data when IXFR is used.
The edns clause determines
whether the local server will attempt to use EDNS when communicating with
the remote server. The default is yes.
The edns-udp-size option
sets the EDNS UDP size that is advertised by named when querying the remote
server. Valid values are 512 to 4096 bytes (values outside this range will
be silently adjusted). This option is useful when you wish to advertises
a different value to this server than the value you advertise globally,
for example, when there is a firewall at the remote site that is blocking
large replies.
The max-udp-size option
sets the maximum EDNS UDP message size named will send. Valid values are
512 to 4096 bytes (values outside this range will be silently adjusted).
This option is useful when you know that there is a firewall that is blocking
large replies from named.
The server supports two zone transfer methods. The first, one-answer,
uses one DNS message per resource record transferred. many-answers packs
as many resource records as possible into a message. many-answers is
more efficient, but is only known to be understood by BIND 9, BIND 8.x,
and patched versions of BIND 4.9.5.
You can specify which method to use for a server with the transfer-format option.
If transfer-format is not
specified, the transfer-format specified
by the options statement
will be used.
transfers is used to limit
the number of concurrent inbound zone transfers from the specified server.
If no transfers clause is
specified, the limit is set according to the transfers-per-ns option.
The keys clause identifies
a key_id defined by the key statement,
to be used for transaction security (TSIG, the
section called “TSIG”) when talking to the remote server.
When a request is sent to the remote server, a request signature will be
generated using the key specified here and appended to the message. A request
originating from the remote server is not required to be signed by this
key.
Although the grammar of the keys clause
allows for multiple keys, only a single key per server is currently supported.
The transfer-source and transfer-source-v6 clauses
specify the IPv4 and IPv6 source address to be used for zone transfer with
the remote server, respectively. For an IPv4 remote server, only transfer-source can
be specified. Similarly, for an IPv6 remote server, only transfer-source-v6 can
be specified. For more details, see the description of transfer-source and transfer-source-v6 in the
section called “Zone Transfers”.
The notify-source and notify-source-v6 clauses
specify the IPv4 and IPv6 source address to be used for notify messages
sent to remote servers, respectively. For an IPv4 remote server, only notify-source can
be specified. Similarly, for an IPv6 remote server, only notify-source-v6 can
be specified.
The query-source and query-source-v6 clauses
specify the IPv4 and IPv6 source address to be used for queries sent to
remote servers, respectively. For an IPv4 remote server, only query-source can
be specified. Similarly, for an IPv6 remote server, only query-source-v6 can
be specified.
trusted-keys {
string number number number string ;
[ string number number number string ; [...]]
};
The trusted-keys statement
defines DNSSEC security roots. DNSSEC is described in the
section called “DNSSEC”. A security root is defined when
the public key for a non-authoritative zone is known, but cannot be securely
obtained through DNS, either because it is the DNS root zone or because
its parent zone is unsigned. Once a key has been configured as a trusted
key, it is treated as if it had been validated and proven secure. The resolver
attempts DNSSEC validation on all DNS data in subdomains of a security
root.
All keys (and corresponding zones) listed in trusted-keys are
deemed to exist regardless of what parent zones say. Similarly for all
keys listed in trusted-keys only
those keys are used to validate the DNSKEY RRset. The parent's DS RRset
will not be used.
The trusted-keys statement
can contain multiple key entries, each consisting of the key's domain name,
flags, protocol, algorithm, and the Base-64 representation of the key data.
viewview_name[class] { match-clients {address_match_list}; match-destinations {address_match_list}; match-recursive-onlyyes_or_no; [view_option; ...] [zone_statement; ...] };
The view statement is a
powerful feature of BIND 9 that lets
a name server answer a DNS query differently depending on who is asking.
It is particularly useful for implementing split DNS setups without having
to run multiple servers.
Each view statement defines
a view of the DNS namespace that will be seen by a subset of clients. A
client matches a view if its source IP address matches the address_match_list of
the view's match-clients clause
and its destination IP address matches the address_match_list of
the view's match-destinations clause.
If not specified, both match-clients and match-destinations default
to matching all addresses. In addition to checking IP addresses match-clients and match-destinations can
also take keys which provide
an mechanism for the client to select the view. A view can also be specified
as match-recursive-only,
which means that only recursive requests from matching clients will match
that view. The order of the view statements
is significant —
a client request will be resolved in the context of the first view that
it matches.
Zones defined within a view statement
will be only be accessible to clients that match the view.
By defining a zone of the same name in multiple views, different zone data
can be given to different clients, for example,
"internal"
and "external" clients in a split DNS setup.
Many of the options given in the options statement
can also be used within a view statement,
and then apply only when resolving queries with that view. When no view-specific
value is given, the value in the options statement
is used as a default. Also, zone options can have default values specified
in the view statement; these
view-specific defaults take precedence over those in the options statement.
Views are class specific. If no class is given, class IN is assumed.
Note that all non-IN views must contain a hint zone, since only the IN
class has compiled-in default hints.
If there are no view statements
in the config file, a default view that matches any client is automatically
created in class IN. Any zone statements
specified on the top level of the configuration file are considered to
be part of this default view, and the options statement
will apply to the default view. If any explicit view statements
are present, all zone statements
must occur inside view statements.
Here is an example of a typical split DNS setup implemented using view statements:
view "internal" {
// This should match our internal networks.
match-clients { 10.0.0.0/8; };
// Provide recursive service to internal clients only.
recursion yes;
// Provide a complete view of the example.com zone
// including addresses of internal hosts.
zone "example.com" {
type master;
file "example-internal.db";
};
};
view "external" {
// Match all clients not matched by the previous view.
match-clients { any; };
// Refuse recursive service to external clients.
recursion no;
// Provide a restricted view of the example.com zone
// containing only publicly accessible hosts.
zone "example.com" {
type master;
file "example-external.db";
};
};
zonezone_name[class] { type master; [ allow-query {address_match_list}; ] [ allow-transfer {address_match_list}; ] [ allow-update {address_match_list}; ] [ update-policy {update_policy_rule[...] }; ] [ also-notify {ip_addr[portip_port] ; [ip_addr[portip_port] ; ... ] }; ] [ check-names (warn|fail|ignore) ; ] [ check-mx (warn|fail|ignore) ; ] [ check-wildcardyes_or_no; ] [ check-integrityyes_or_no; ] [ dialupdialup_option; ] [ filestring; ] [ masterfile-format (text|raw) ; ] [ journalstring; ] [ forward (only|first) ; ] [ forwarders { [ip_addr[portip_port] ; ... ] }; ] [ ixfr-basestring; ] [ ixfr-tmp-filestring; ] [ maintain-ixfr-baseyes_or_no; ] [ max-ixfr-log-sizenumber; ] [ max-transfer-idle-outnumber; ] [ max-transfer-time-outnumber; ] [ notifyyes_or_no|explicit|master-only; ] [ pubkeynumbernumbernumberstring; ] [ notify-source (ip4_addr|*) [portip_port] ; ] [ notify-source-v6 (ip6_addr|*) [portip_port] ; ] [ zone-statisticsyes_or_no; ] [ sig-validity-intervalnumber; ] [ databasestring; ] [ min-refresh-timenumber; ] [ max-refresh-timenumber; ] [ min-retry-timenumber; ] [ max-retry-timenumber; ] [ key-directorypath_name; ] [ zero-no-soa-ttlyes_or_no; ] }; zonezone_name[class] { type slave; [ allow-notify {address_match_list}; ] [ allow-query {address_match_list}; ] [ allow-transfer {address_match_list}; ] [ allow-update-forwarding {address_match_list}; ] [ update-check-kskyes_or_no; ] [ also-notify {ip_addr[portip_port] ; [ip_addr[portip_port] ; ... ] }; ] [ check-names (warn|fail|ignore) ; ] [ dialupdialup_option; ] [ filestring; ] [ masterfile-format (text|raw) ; ] [ journalstring; ] [ forward (only|first) ; ] [ forwarders { [ip_addr[portip_port] ; ... ] }; ] [ ixfr-basestring; ] [ ixfr-tmp-filestring; ] [ maintain-ixfr-baseyes_or_no; ] [ masters [portip_port] { (masters_list|ip_addr[portip_port] [keykey] ) ; [...] }; ] [ max-ixfr-log-sizenumber; ] [ max-transfer-idle-innumber; ] [ max-transfer-idle-outnumber; ] [ max-transfer-time-innumber; ] [ max-transfer-time-outnumber; ] [ notifyyes_or_no|explicit|master-only; ] [ pubkeynumbernumbernumberstring; ] [ transfer-source (ip4_addr|*) [portip_port] ; ] [ transfer-source-v6 (ip6_addr|*) [portip_port] ; ] [ alt-transfer-source (ip4_addr|*) [portip_port] ; ] [ alt-transfer-source-v6 (ip6_addr|*) [portip_port] ; ] [ use-alt-transfer-sourceyes_or_no; ] [ notify-source (ip4_addr|*) [portip_port] ; ] [ notify-source-v6 (ip6_addr|*) [portip_port] ; ] [ zone-statisticsyes_or_no; ] [ databasestring; ] [ min-refresh-timenumber; ] [ max-refresh-timenumber; ] [ min-retry-timenumber; ] [ max-retry-timenumber; ] [ multi-masteryes_or_no; ] [ zero-no-soa-ttlyes_or_no; ] }; zonezone_name[class] { type hint; filestring; [ delegation-onlyyes_or_no; ] [ check-names (warn|fail|ignore) ; // Not Implemented. ] }; zonezone_name[class] { type stub; [ allow-query {address_match_list}; ] [ check-names (warn|fail|ignore) ; ] [ dialupdialup_option; ] [ delegation-onlyyes_or_no; ] [ filestring; ] [ masterfile-format (text|raw) ; ] [ forward (only|first) ; ] [ forwarders { [ip_addr[portip_port] ; ... ] }; ] [ masters [portip_port] { (masters_list|ip_addr[portip_port] [keykey] ) ; [...] }; ] [ max-transfer-idle-innumber; ] [ max-transfer-time-innumber; ] [ pubkeynumbernumbernumberstring; ] [ transfer-source (ip4_addr|*) [portip_port] ; ] [ transfer-source-v6 (ip6_addr|*) [portip_port] ; ] [ alt-transfer-source (ip4_addr|*) [portip_port] ; ] [ alt-transfer-source-v6 (ip6_addr|*) [portip_port] ; ] [ use-alt-transfer-sourceyes_or_no; ] [ zone-statisticsyes_or_no; ] [ databasestring; ] [ min-refresh-timenumber; ] [ max-refresh-timenumber; ] [ min-retry-timenumber; ] [ max-retry-timenumber; ] [ multi-masteryes_or_no; ] }; zonezone_name[class] { type forward; [ forward (only|first) ; ] [ forwarders { [ip_addr[portip_port] ; ... ] }; ] [ delegation-onlyyes_or_no; ] }; zonezone_name[class] { type delegation-only; };
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The server has a master copy of the data for the zone and |
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A slave zone is a replica of a master zone. The masters list |
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A stub zone is similar to a slave zone, except that it Stub zones can be used to eliminate the need for glue NS Stub zones can also be used as a way of forcing the resolution |
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A "forward zone" is a way to configure forwarding on a |
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The initial set of root name servers is specified using |
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This is used to enforce the delegation-only status of infrastructure |
The zone's name may optionally be followed by a class. If a class is
not specified, class IN (for Internet),
is assumed. This is correct for the vast majority of cases.
The hesiod class is named for an information
service from MIT's Project Athena. It is used to share information about
various systems databases, such as users, groups, printers and so on.
The keyword HS is a synonym for hesiod.
Another MIT development is CHAOSnet, a LAN protocol created in the
mid-1970s. Zone data for it can be specified with the CHAOS class.
- allow-notify
-
See the description of allow-notify in the
section called “Access Control”. - allow-query
-
See the description of allow-query in the
section called “Access Control”. - allow-transfer
-
See the description of allow-transfer in the
section called “Access Control”. - allow-update
-
See the description of allow-update in the
section called “Access Control”. - update-policy
-
Specifies a "Simple Secure Update" policy. See the
section called “Dynamic Update Policies”. - allow-update-forwarding
-
See the description of allow-update-forwarding in the
section called “Access Control”. - also-notify
-
Only meaningful if notify is
active for this zone. The set of machines that will receive aDNSmessage for this zone is made up of all the listed
NOTIFY
name servers (other than the primary master) for the zone plus
any IP addresses specified with also-notify.
A port may be specified with each also-notify address
to send the notify messages to a port other than the default of
53. also-notify is
not meaningful for stub zones. The default is the empty list. - check-names
-
This option is used to restrict the character set and syntax
of certain domain names in master files and/or DNS responses received
from the network. The default varies according to zone type. For master zones
the default is fail.
For slave zones the
default is warn. - check-mx
-
See the description of check-mx in the
section called “Boolean Options”. - check-wildcard
-
See the description of check-wildcard in the
section called “Boolean Options”. - check-integrity
-
See the description of check-integrity in the
section called “Boolean Options”. - check-sibling
-
See the description of check-sibling in the
section called “Boolean Options”. - zero-no-soa-ttl
-
See the description of zero-no-soa-ttl in the
section called “Boolean Options”. - update-check-ksk
-
See the description of update-check-ksk in the
section called “Boolean Options”. - database
-
Specify the type of database to be used for storing the zone
data. The string following the database keyword
is interpreted as a list of whitespace-delimited words. The first
word identifies the database type, and any subsequent words are
passed as arguments to the database to be interpreted in a way
specific to the database type.The default is
"rbt",
BIND 9's native in-memory red-black-tree database. This database
does not take arguments.Other values are possible if additional database drivers have
been linked into the server. Some sample drivers are included with
the distribution but none are linked in by default. - dialup
-
See the description of dialup in the
section called “Boolean Options”. - delegation-only
-
The flag only applies to hint and stub zones. If set to
yes,
then the zone will also be treated as if it is also a delegation-only
type zone. - forward
-
Only meaningful if the zone has a forwarders list. The only value
causes the lookup to fail after trying the forwarders and getting
no answer, while first would
allow a normal lookup to be tried. - forwarders
-
Used to override the list of global forwarders. If it is not
specified in a zone of type forward,
no forwarding is done for the zone and the global options are not
used. - ixfr-base
-
Was used in BIND 8 to specify
the name of the transaction log (journal) file for dynamic update
and IXFR. BIND 9 ignores the
option and constructs the name of the journal file by appending ".jnl"
to the name of the zone file. - ixfr-tmp-file
-
Was an undocumented option in BIND 8.
Ignored in BIND 9. - journal
-
Allow the default journal's file name to be overridden. The default
is the zone's file with ".jnl" appended.
This is applicable to master and slave zones. - max-transfer-time-in
-
See the description of max-transfer-time-in in the
section called “Zone Transfers”. - max-transfer-idle-in
-
See the description of max-transfer-idle-in in the
section called “Zone Transfers”. - max-transfer-time-out
-
See the description of max-transfer-time-out in the
section called “Zone Transfers”. - max-transfer-idle-out
-
See the description of max-transfer-idle-out in the
section called “Zone Transfers”. - notify
-
See the description of notify in the
section called “Boolean Options”. - pubkey
-
In BIND 8, this option was
intended for specifying a public zone key for verification of signatures
in DNSSEC signed zones when they are loaded from disk. BIND 9
does not verify signatures on load and ignores the option. - zone-statistics
-
If
yes, the server
will keep statistical information for this zone, which can be dumped
to the statistics-file defined
in the server options. - sig-validity-interval
-
See the description of sig-validity-interval in the
section called “Tuning”. - transfer-source
-
See the description of transfer-source in the
section called “Zone Transfers”. - transfer-source-v6
-
See the description of transfer-source-v6 in the
section called “Zone Transfers”. - alt-transfer-source
-
See the description of alt-transfer-source in the
section called “Zone Transfers”. - alt-transfer-source-v6
-
See the description of alt-transfer-source-v6 in the
section called “Zone Transfers”. - use-alt-transfer-source
-
See the description of use-alt-transfer-source in the
section called “Zone Transfers”. - notify-source
-
See the description of notify-source in the
section called “Zone Transfers”. - notify-source-v6
-
See the description of notify-source-v6 in the
section called “Zone Transfers”. - min-refresh-time, max-refresh-time, min-retry-time, max-retry-time
-
See the description in the section
called “Tuning”. - ixfr-from-differences
-
See the description of ixfr-from-differences in the
section called “Boolean Options”. - key-directory
-
See the description of key-directory in the section called “options Statement
Definition and Usage”. - multi-master
-
See the description of multi-master in the
section called “Boolean Options”. - masterfile-format
-
See the description of masterfile-format in the
section called “Tuning”.
BIND 9 supports two alternative
methods of granting clients the right to perform dynamic updates to a
zone, configured by the allow-update and update-policy option,
respectively.
The allow-update clause
works the same way as in previous versions of BIND.
It grants given clients the permission to update any record of any name
in the zone.
The update-policy clause
is new in BIND 9 and allows more fine-grained
control over what updates are allowed. A set of rules is specified, where
each rule either grants or denies permissions for one or more names to
be updated by one or more identities. If the dynamic update request message
is signed (that is, it includes either a TSIG or SIG(0) record), the
identity of the signer can be determined.
Rules are specified in the update-policy zone
option, and are only meaningful for master zones. When the update-policy statement
is present, it is a configuration error for the allow-update statement
to be present. The update-policy statement
only examines the signer of a message; the source address is not relevant.
This is how a rule definition looks:
( grant | deny )identitynametypename[types]
Each rule grants or denies privileges. Once a message has successfully
matched a rule, the operation is immediately granted or denied and no
further rules are examined. A rule is matched when the signer matches
the identity field, the name matches the name field in accordance with
the nametype field, and the type matches the types specified in the type
field.
The identity field specifies a name or a wildcard name. Normally, this
is the name of the TSIG or SIG(0) key used to sign the update request.
When a TKEY exchange has been used to create a shared secret, the identity
of the shared secret is the same as the identity of the key used to authenticate
the TKEY exchange. When the identity field
specifies a wildcard name, it is subject to DNS wildcard expansion, so
the rule will apply to multiple identities. The identity field
must contain a fully qualified domain name.
The nametype field has 6
values: name, subdomain, wildcard, self, selfsub,
and selfwild.
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Exact-match semantics. This rule matches when the name |
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This rule matches when the name being updated is a subdomain |
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The |
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This rule matches when the name being updated matches the |
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This rule is similar to |
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This rule is similar to |
In all cases, the name field
must specify a fully qualified domain name.
If no types are explicitly specified, this rule matches all types except
RRSIG, NS, SOA, and NSEC. Types may be specified by name, including
"ANY" (ANY matches all types except NSEC, which can never be updated).
Note that when an attempt is made to delete all records associated with
a name, the rules are checked for each existing record type.
This section, largely borrowed from RFC 1034, describes the concept of
a Resource Record (RR) and explains when each is used. Since the publication
of RFC 1034, several new RRs have been identified and implemented in the
DNS. These are also included.
A domain name identifies a node. Each node has a set of resource information,
which may be empty. The set of resource information associated with a
particular name is composed of separate RRs. The order of RRs in a set
is not significant and need not be preserved by name servers, resolvers,
or other parts of the DNS. However, sorting of multiple RRs is permitted
for optimization purposes, for example, to specify that a particular
nearby server be tried first. See the
section called “The sortlist Statement” and the
section called “RRset Ordering”.
The components of a Resource Record are:
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owner name |
The domain name where the RR is found. |
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type |
An encoded 16-bit value that specifies the type of the |
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TTL |
The time-to-live of the RR. This field is a 32-bit integer |
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class |
An encoded 16-bit value that identifies a protocol family |
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RDATA |
The resource data. The format of the data is type (and |
The following are types of valid
RRs:
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A |
A host address. In the IN class, this is a 32-bit IP address. |
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AAAA |
IPv6 address. Described in RFC 1886. |
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A6 |
IPv6 address. This can be a partial address (a suffix) |
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AFSDB |
Location of AFS database servers. Experimental. Described |
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APL |
Address prefix list. Experimental. Described in RFC 3123. |
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CERT |
Holds a digital certificate. Described in RFC 2538. |
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CNAME |
Identifies the canonical name of an alias. Described in |
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DNAME |
Replaces the domain name specified with another name to |
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DNSKEY |
Stores a public key associated with a signed DNS zone. |
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DS |
Stores the hash of a public key associated with a signed |
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GPOS |
Specifies the global position. Superseded by LOC. |
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HINFO |
Identifies the CPU and OS used by a host. Described in |
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ISDN |
Representation of ISDN addresses. Experimental. Described |
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KEY |
Stores a public key associated with a DNS name. Used in |
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KX |
Identifies a key exchanger for this DNS name. Described |
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LOC |
For storing GPS info. Described in RFC 1876. Experimental. |
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MX |
Identifies a mail exchange for the domain with a 16-bit |
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NAPTR |
Name authority pointer. Described in RFC 2915. |
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NSAP |
A network service access point. Described in RFC 1706. |
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NS |
The authoritative name server for the domain. Described |
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NSEC |
Used in DNSSECbis to securely indicate that RRs with an |
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NXT |
Used in DNSSEC to securely indicate that RRs with an owner |
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PTR |
A pointer to another part of the domain name space. Described |
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PX |
Provides mappings between RFC 822 and X.400 addresses. |
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RP |
Information on persons responsible for the domain. Experimental. |
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RRSIG |
Contains DNSSECbis signature data. Described in RFC 4034. |
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RT |
Route-through binding for hosts that do not have their |
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SIG |
Contains DNSSEC signature data. Used in original DNSSEC; |
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SOA |
Identifies the start of a zone of authority. Described |
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SRV |
Information about well known network services (replaces |
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TXT |
Text records. Described in RFC 1035. |
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WKS |
Information about which well known network services, such |
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X25 |
Representation of X.25 network addresses. Experimental. |
The following classes of resource
records are currently valid in the DNS:
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IN |
The Internet. |
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CH |
CHAOSnet, a LAN protocol created at MIT in the mid-1970s. |
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HS |
Hesiod, an information service developed by MIT's Project |
The owner name is often implicit, rather than forming an integral part
of the RR. For example, many name servers internally form tree or hash
structures for the name space, and chain RRs off nodes. The remaining
RR parts are the fixed header (type, class, TTL) which is consistent
for all RRs, and a variable part (RDATA) that fits the needs of the resource
being described.
The meaning of the TTL field is a time limit on how long an RR can
be kept in a cache. This limit does not apply to authoritative data in
zones; it is also timed out, but by the refreshing policies for the zone.
The TTL is assigned by the administrator for the zone where the data
originates. While short TTLs can be used to minimize caching, and a zero
TTL prohibits caching, the realities of Internet performance suggest
that these times should be on the order of days for the typical host.
If a change can be anticipated, the TTL can be reduced prior to the change
to minimize inconsistency during the change, and then increased back
to its former value following the change.
The data in the RDATA section of RRs is carried as a combination of
binary strings and domain names. The domain names are frequently used
as "pointers" to other data in the DNS.
RRs are represented in binary form in the packets of the DNS protocol,
and are usually represented in highly encoded form when stored in a name
server or resolver. In the examples provided in RFC 1034, a style similar
to that used in master files was employed in order to show the contents
of RRs. In this format, most RRs are shown on a single line, although
continuation lines are possible using parentheses.
The start of the line gives the owner of the RR. If a line begins with
a blank, then the owner is assumed to be the same as that of the previous
RR. Blank lines are often included for readability.
Following the owner, we list the TTL, type, and class of the RR. Class
and type use the mnemonics defined above, and TTL is an integer before
the type field. In order to avoid ambiguity in parsing, type and class
mnemonics are disjoint, TTLs are integers, and the type mnemonic is always
last. The IN class and TTL values are often omitted from examples in
the interests of clarity.
The resource data or RDATA section of the RR are given using knowledge
of the typical representation for the data.
For example, we might show the RRs carried in a message as:
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The MX RRs have an RDATA section which consists of a 16-bit number
followed by a domain name. The address RRs use a standard IP address
format to contain a 32-bit internet address.
The above example shows six RRs, with two RRs at each of three domain
names.
Similarly we might see:
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This example shows two addresses for XX.LCS.MIT.EDU,
each of a different class.
As described above, domain servers store information as a series of resource
records, each of which contains a particular piece of information about
a given domain name (which is usually, but not always, a host). The simplest
way to think of a RR is as a typed pair of data, a domain name matched
with a relevant datum, and stored with some additional type information
to help systems determine when the RR is relevant.
MX records are used to control delivery of email. The data specified
in the record is a priority and a domain name. The priority controls the
order in which email delivery is attempted, with the lowest number first.
If two priorities are the same, a server is chosen randomly. If no servers
at a given priority are responding, the mail transport agent will fall
back to the next largest priority. Priority numbers do not have any absolute
meaning — they are relevant only respective to other MX records for
that domain name. The domain name given is the machine to which the mail
will be delivered. It must have
an associated address record (A or AAAA) — CNAME is not sufficient.
For a given domain, if there is both a CNAME record and an MX record,
the MX record is in error, and will be ignored. Instead, the mail will
be delivered to the server specified in the MX record pointed to by the
CNAME.
For example:
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Mail delivery will be attempted to mail.example.com and mail2.example.com (in
any order), and if neither of those succeed, delivery to mail.backup.org will
be attempted.
The time-to-live of the RR field is a 32-bit integer represented in units
of seconds, and is primarily used by resolvers when they cache RRs. The
TTL describes how long a RR can be cached before it should be discarded.
The following three types of TTL are currently used in a zone file.
|
SOA |
The last field in the SOA is the negative caching TTL. This The maximum time for negative caching is 3 hours (3h). |
|
$TTL |
The $TTL directive at the top of the zone file (before the |
|
RR TTLs |
Each RR can have a TTL as the second field in the RR, which |
All of these TTLs default to units of seconds, though units can be explicitly
specified, for example, 1h30m.
Reverse name resolution (that is, translation from IP address to name)
is achieved by means of the in-addr.arpa domain
and PTR records. Entries in the in-addr.arpa domain are made in least-to-most
significant order, read left to right. This is the opposite order to the
way IP addresses are usually written. Thus, a machine with an IP address
of 10.1.2.3 would have a corresponding in-addr.arpa name of 3.2.1.10.in-addr.arpa.
This name should have a PTR resource record whose data field is the name
of the machine or, optionally, multiple PTR records if the machine has
more than one name. For example, in the [example.com]
domain:
|
|
|
|
|
|
Note
The $ORIGIN lines in
the examples are for providing context to the examples only-they do not
necessarily appear in the actual usage. They are only used here to indicate
that the example is relative to the listed origin.
The Master File Format was initially defined in RFC 1035 and has subsequently
been extended. While the Master File Format itself is class independent
all records in a Master File must be of the same class.
Master File Directives include $ORIGIN, $INCLUDE,
and $TTL.
Syntax: $ORIGIN domain-name [comment]
$ORIGIN sets the domain
name that will be appended to any unqualified records. When a zone is
first read in there is an implicit $ORIGIN <zone-name>. The
current $ORIGIN is appended
to the domain specified in the $ORIGIN argument
if it is not absolute.
$ORIGIN example.com. WWW CNAME MAIN-SERVER
is equivalent to
WWW.EXAMPLE.COM. CNAME MAIN-SERVER.EXAMPLE.COM.
Syntax: $INCLUDE filename [ origin ]
[ comment ]
Read and process the file filename as
if it were included into the file at this point. If origin is
specified the file is processed with $ORIGIN set
to that value, otherwise the current $ORIGIN is
used.
The origin and the current domain name revert to the values they had
prior to the $INCLUDE once
the file has been read.
Note
RFC 1035 specifies that the current origin should be restored after
an $INCLUDE, but it is
silent on whether the current domain name should also be restored.
BIND 9 restores both of them. This could be construed as a deviation
from RFC 1035, a feature, or both.
Syntax: $GENERATE range lhs [ttl]
[class] type rhs [comment]
$GENERATE is used to create
a series of resource records that only differ from each other by an iterator. $GENERATE can
be used to easily generate the sets of records required to support sub
/24 reverse delegations described in RFC 2317: Classless IN-ADDR.ARPA delegation.
$ORIGIN 0.0.192.IN-ADDR.ARPA. $GENERATE 1-2 0 NS SERVER$.EXAMPLE. $GENERATE 1-127 $ CNAME $.0
is equivalent to
0.0.0.192.IN-ADDR.ARPA NS SERVER1.EXAMPLE.
0.0.0.192.IN-ADDR.ARPA. NS SERVER2.EXAMPLE.
1.0.0.192.IN-ADDR.ARPA. CNAME 1.0.0.0.192.IN-ADDR.ARPA.
2.0.0.192.IN-ADDR.ARPA. CNAME 2.0.0.0.192.IN-ADDR.ARPA.
...
127.0.0.192.IN-ADDR.ARPA. CNAME 127.0.0.0.192.IN-ADDR.ARPA.
|
range |
This can be one of two forms: start-stop or start-stop/step. |
|
lhs |
lhs describes For compatibility with earlier versions, $$ is |
|
ttl |
Specifies the time-to-live of the generated records. If not class and ttl can |
|
class |
Specifies the class of the generated records. This must match class and ttl can |
|
type |
At present the only supported types are PTR, CNAME, DNAME, |
|
rhs |
A domain name. It is processed similarly to lhs. |
The $GENERATE directive
is a BIND extension and not part of
the standard zone file format.
BIND 8 does not support the optional TTL and CLASS fields.
In addition to the standard textual format, BIND 9 supports the ability
to read or dump to zone files in other formats. The raw format
is currently available as an additional format. It is a binary format representing
BIND 9's internal data structure directly, thereby remarkably improving
the loading time.
For a primary server, a zone file in the raw format
is expected to be generated from a textual zone file by the named-compilezone command.
For a secondary server or for a dynamic zone, it is automatically generated
(if this format is specified by the masterfile-format option)
when named dumps the zone
contents after zone transfer or when applying prior updates.
If a zone file in a binary format needs manual modification, it first
must be converted to a textual form by the named-compilezone command.
All necessary modification should go to the text file, which should then
be converted to the binary form by the named-compilezone command
again.
Although the raw format uses the network
byte order and avoids architecture-dependent data alignment so that it
is as much portable as possible, it is primarily expected to be used inside
the same single system. In order to export a zone file in the raw format
or make a portable backup of the file, it is recommended to convert the
file to the standard textual representation.
Table of Contents
Access Control Lists (ACLs), are address match lists that you can set up
and nickname for future use in allow-notify, allow-query, allow-recursion, blackhole, allow-transfer,
etc.
Using ACLs allows you to have finer control over who can access your name
server, without cluttering up your config files with huge lists of IP addresses.
It is a good idea to use ACLs, and
to control access to your server. Limiting access to your server by outside
parties can help prevent spoofing and denial of service (DoS) attacks against
your server.
Here is an example of how to properly apply ACLs:
// Set up an ACL named "bogusnets" that will block RFC1918 space
// and some reserved space, which is commonly used in spoofing attacks.
acl bogusnets {
0.0.0.0/8; 1.0.0.0/8; 2.0.0.0/8; 192.0.2.0/24; 224.0.0.0/3;
10.0.0.0/8; 172.16.0.0/12; 192.168.0.0/16;
};
// Set up an ACL called our-nets. Replace this with the real IP numbers.
acl our-nets { x.x.x.x/24; x.x.x.x/21; };
options {
...
...
allow-query { our-nets; };
allow-recursion { our-nets; };
...
blackhole { bogusnets; };
...
};
zone "example.com" {
type master;
file "m/example.com";
allow-query { any; };
};
This allows recursive queries of the server from the outside unless recursion
has been previously disabled.
For more information on how to use ACLs to protect your server, see the AUSCERT advisory
at:
ftp://ftp.auscert.org.au/pub/auscert/advisory/AL-1999.004.dns_dos
On UNIX servers, it is possible to run BIND in
a chrooted environment (using the chroot() function)
by specifying the "-t"
option. This can help improve system security by placing BIND in
a "sandbox", which will limit the damage done if a server is compromised.
Another useful feature in the UNIX version of BIND is
the ability to run the daemon as an unprivileged user ( -u user ).
We suggest running as an unprivileged user when using the chroot feature.
Here is an example command line to load BIND in
a chroot sandbox, /var/named,
and to run named setuid to
user 202:
/usr/local/bin/named -u 202 -t /var/named
In order for a chroot environment
to work properly in a particular directory (for example, /var/named),
you will need to set up an environment that includes everything BIND needs
to run. From BIND's point of view, /var/named is
the root of the filesystem. You will need to adjust the values of options
like like directory and pid-file to
account for this.
Unlike with earlier versions of BIND, you will typically not need
to compile named statically
nor install shared libraries under the new root. However, depending on
your operating system, you may need to set up things like /dev/zero, /dev/random, /dev/log,
and /etc/localtime.
Prior to running the named daemon,
use the touch utility (to
change file access and modification times) or the chown utility
(to set the user id and/or group id) on files to which you want BIND to
write.
Note
Note that if the named daemon
is running as an unprivileged user, it will not be able to bind to new
restricted ports if the server is reloaded.
Access to the dynamic update facility should be strictly limited. In earlier
versions of BIND, the only way to do this
was based on the IP address of the host requesting the update, by listing
an IP address or network prefix in the allow-update zone
option. This method is insecure since the source address of the update UDP
packet is easily forged. Also note that if the IP addresses allowed by the allow-update option
include the address of a slave server which performs forwarding of dynamic
updates, the master can be trivially attacked by sending the update to the
slave, which will forward it to the master with its own source IP address
causing the master to approve it without question.
For these reasons, we strongly recommend that updates be cryptographically
authenticated by means of transaction signatures (TSIG). That is, the allow-update option
should list only TSIG key names, not IP addresses or network prefixes. Alternatively,
the new update-policy option
can be used.
Some sites choose to keep all dynamically-updated DNS data in a subdomain
and delegate that subdomain to a separate zone. This way, the top-level zone
containing critical data such as the IP addresses of public web and mail
servers need not allow dynamic update at all.
Table of Contents
The best solution to solving installation and configuration issues is
to take preventative measures by setting up logging files beforehand. The
log files provide a source of hints and information that can be used to
figure out what went wrong and how to fix the problem.
Zone serial numbers are just numbers-they aren't date related. A lot of
people set them to a number that represents a date, usually of the form YYYYMMDDRR.
A number of people have been testing these numbers for Y2K compliance and
have set the number to the year 2000 to see if it will work. They then try
to restore the old serial number. This will cause problems because serial
numbers are used to indicate that a zone has been updated. If the serial
number on the slave server is lower than the serial number on the master,
the slave server will attempt to update its copy of the zone.
Setting the serial number to a lower number on the master server than the
slave server means that the slave will not perform updates to its copy of
the zone.
The solution to this is to add 2147483647 (2^31-1) to the number, reload
the zone and make sure all slaves have updated to the new zone serial number,
then reset the number to what you want it to be, and reload the zone again.
The Internet Systems Consortium (ISC)
offers a wide range of support and service agreements for BIND and DHCP servers.
Four levels of premium support are available and each level includes support
for all ISC programs, significant discounts
on products and training, and a recognized priority on bug fixes and non-funded
feature requests. In addition, ISC offers
a standard support agreement package which includes services ranging from
bug fix announcements to remote support. It also includes training in BIND and DHCP.
To discuss arrangements for support, contact info@isc.org or
visit the ISC web page at http://www.isc.org/services/support/ to
read more.
Table of Contents
Although the "official" beginning of the Domain Name System occurred
in 1984 with the publication of RFC 920, the core of the new system was
described in 1983 in RFCs 882 and 883. From 1984 to 1987, the ARPAnet (the
precursor to today's Internet) became a testbed of experimentation for
developing the new naming/addressing scheme in a rapidly expanding, operational
network environment. New RFCs were written and published in 1987 that modified
the original documents to incorporate improvements based on the working
model. RFC 1034,
"Domain Names-Concepts and Facilities", and RFC 1035, "Domain Names-Implementation
and Specification" were published and became the standards upon which all DNS implementations
are built.
The first working domain name server, called "Jeeves", was written in
1983-84 by Paul Mockapetris for operation on DEC Tops-20 machines located
at the University of Southern California's Information Sciences Institute
(USC-ISI) and SRI International's Network Information Center (SRI-NIC).
A DNS server for Unix machines, the
Berkeley Internet Name Domain (BIND)
package, was written soon after by a group of graduate students at the
University of California at Berkeley under a grant from the US Defense
Advanced Research Projects Administration (DARPA).
Versions of BIND through 4.8.3 were
maintained by the Computer Systems Research Group (CSRG) at UC Berkeley.
Douglas Terry, Mark Painter, David Riggle and Songnian Zhou made up the
initial BIND project team. After that,
additional work on the software package was done by Ralph Campbell. Kevin
Dunlap, a Digital Equipment Corporation employee on loan to the CSRG, worked
on BIND for 2 years, from 1985 to 1987.
Many other people also contributed to BIND development
during that time: Doug Kingston, Craig Partridge, Smoot Carl-Mitchell,
Mike Muuss, Jim Bloom and Mike Schwartz. BIND maintenance
was subsequently handled by Mike Karels and O. Kure.
BIND versions 4.9 and 4.9.1 were released
by Digital Equipment Corporation (now Compaq Computer Corporation). Paul
Vixie, then a DEC employee, became BIND's
primary caretaker. He was assisted by Phil Almquist, Robert Elz, Alan Barrett,
Paul Albitz, Bryan Beecher, Andrew Partan, Andy Cherenson, Tom Limoncelli,
Berthold Paffrath, Fuat Baran, Anant Kumar, Art Harkin, Win Treese, Don
Lewis, Christophe Wolfhugel, and others.
BIND version 4.9.2 was sponsored by
Vixie Enterprises. Paul Vixie became BIND's
principal architect/programmer.
BIND versions from 4.9.3 onward have
been developed and maintained by the Internet Systems Consortium and its
predecessor, the Internet Software Consortium, with support being provided
by ISC's sponsors. As co-architects/programmers, Bob Halley and Paul Vixie
released the first production-ready version of BIND version
8 in May 1997.
BIND development work is made possible
today by the sponsorship of several corporations, and by the tireless work
efforts of numerous individuals.
IPv6 addresses are 128-bit identifiers for interfaces and sets of interfaces
which were introduced in the DNS to
facilitate scalable Internet routing. There are three types of addresses: Unicast,
an identifier for a single interface; Anycast,
an identifier for a set of interfaces; and Multicast,
an identifier for a set of interfaces. Here we describe the global Unicast
address scheme. For more information, see RFC 3587.
IPv6 unicast addresses consist of a global
routing prefix, a subnet identifier,
and an interface identifier.
The global routing prefix is provided by the upstream provider or ISP,
and (roughly) corresponds to the IPv4 network section
of the address range. The subnet identifier is for local subnetting, much
the same as subnetting an IPv4 /16 network into /24 subnets. The interface
identifier is the address of an individual interface on a given network;
in IPv6, addresses belong to interfaces rather than to machines.
The subnetting capability of IPv6 is much more flexible than that of
IPv4: subnetting can be carried out on bit boundaries, in much the same
way as Classless InterDomain Routing (CIDR), and the DNS PTR representation
("nibble" format) makes setting up reverse zones easier.
The Interface Identifier must be unique on the local link, and is usually
generated automatically by the IPv6 implementation, although it is usually
possible to override the default setting if necessary. A typical IPv6 address
might look like: 2001:db8:201:9:a00:20ff:fe81:2b32
IPv6 address specifications often contain long strings of zeros, so the
architects have included a shorthand for specifying them. The double colon
(`::') indicates the longest possible string of zeros that can fit, and
can be used only once in an address.
Specification documents for the Internet protocol suite, including the DNS,
are published as part of the Request for Comments (RFCs) series of technical
notes. The standards themselves are defined by the Internet Engineering
Task Force (IETF) and the Internet Engineering Steering Group (IESG). RFCs
can be obtained online via FTP at:
ftp://www.isi.edu/in-notes/RFCxxxx.txt
(where xxxx is the number of
the RFC). RFCs are also available via the Web at:
Standards
Proposed Standards
[RFC2136] P. Vixie, S. Thomson, Y. Rekhter,
and J. Bound. Dynamic
Updates in the Domain Name System. April
1997.
DNS Security Proposed
Standards
[RFC4033] R. Arends, R. Austein, M. Larson, D. Massey,
and S. Rose. DNS
Security Introduction and Requirements. March
2005.
Other Important RFCs About DNS Implementation
[RFC1535] E. Gavron. A
Security Problem and Proposed Correction With Widely Deployed DNS Software.. October
1993.
Resource Record Types
[RFC1183] C.F. Everhart, L.
A. Mamakos, R. Ullmann,
and P. Mockapetris. New DNS RR
Definitions. October 1990.
[RFC2168] R. Daniel and M. Mealling. Resolution
of Uniform Resource Identifiers using the Domain Name System. June
1997.
[RFC1876] C. Davis, P. Vixie, T.,
and I. Dickinson. A
Means for Expressing Location Information in the Domain Name System. January
1996.
[RFC2052] A. Gulbrandsen and P. Vixie. A DNS RR
for Specifying the Location of Services.. October
1996.
[RFC2163] A. Allocchio. Using
the Internet DNS to Distribute
MIXER Conformant Global Address Mapping. January
1998.
[RFC2538] D. Eastlake, 3rd and O. Gudmundsson. Storing
Certificates in the Domain Name System (DNS). March
1999.
[RFC2539] D. Eastlake, 3rd. Storage
of Diffie-Hellman Keys in the Domain Name System (DNS). March
1999.
[RFC2782] A. Gulbrandsen. P. Vixie. L. Esibov. A
DNS RR for specifying the location of services (DNS SRV). February
2000.
[RFC2915] M. Mealling. R. Daniel. The
Naming Authority Pointer (NAPTR) DNS Resource Record. September
2000.
DNS and the Internet
DNS Operations
Internationalized Domain Names
[RFC2825] IAB and R. Daigle. A
Tangled Web: Issues of I18N, Domain Names, and the Other Internet
protocols. May 2000.
[RFC3490] P. Faltstrom, P. Hoffman,
and A. Costello. Internationalizing
Domain Names in Applications (IDNA). March
2003.
Other DNS-related
RFCs
Note
Note: the following list of RFCs, although DNS-related,
are not concerned with implementing software.
[RFC1464] R. Rosenbaum. Using
the Domain Name System To Store Arbitrary String Attributes. May
1993.
Obsolete and Unimplemented Experimental RFC
Obsoleted DNS Security RFCs
Note
Most of these have been consolidated into RFC4033, RFC4034 and
RFC4035 which collectively describe DNSSECbis.
[RFC3655] B. Wellington and O. Gudmundsson. Redefinition
of DNS Authenticated Data (AD) bit. November
2003.
Internet Drafts (IDs) are rough-draft working documents of the Internet
Engineering Task Force. They are, in essence, RFCs in the preliminary stages
of development. Implementors are cautioned not to regard IDs as archival,
and they should not be quoted or cited in any formal documents unless accompanied
by the disclaimer that they are "works in progress." IDs have a lifespan
of six months after which they are deleted unless updated by their authors.
Table of Contents
- dig — DNS
lookup utility - host — DNS
lookup utility - dnssec-keygen — DNSSEC
key generation tool - dnssec-signzone — DNSSEC
zone signing tool - named-checkconf — named
configuration file syntax checking tool - named-checkzone — zone
file validity checking or converting tool - named — Internet
domain name server - rndc — name
server control utility -
rndc.conf— rndc
configuration file - rndc-confgen — rndc
key generation tool
Name
dig — DNS lookup utility
Synopsis
dig [@server] [-b ]address
[-c ]class
[-f ]filename
[-k ]filename
[-p ]port#
[-q ]name
[-t ]type
[-x ]addr
[-y ][hmac:]name:key
[-4] [-6] [name]
[type] [class] [queryopt...]
dig [-h]
dig [global-queryopt...] [query...]
DESCRIPTION
dig (domain information
groper) is a flexible tool for interrogating DNS name servers. It performs
DNS lookups and displays the answers that are returned from the name server(s)
that were queried. Most DNS administrators use dig to
troubleshoot DNS problems because of its flexibility, ease of use and clarity
of output. Other lookup tools tend to have less functionality than dig.
Although dig is normally
used with command-line arguments, it also has a batch mode of operation
for reading lookup requests from a file. A brief summary of its command-line
arguments and options is printed when the -h option
is given. Unlike earlier versions, the BIND9 implementation of dig allows
multiple lookups to be issued from the command line.
Unless it is told to query a specific name server, dig will
try each of the servers listed in /etc/resolv.conf.
When no command line arguments or options are given, will perform an
NS query for "." (the root).
It is possible to set per-user defaults for dig via ${HOME}/.digrc.
This file is read and any options in it are applied before the command
line arguments.
The IN and CH class names overlap with the IN and CH top level domains
names. Either use the -t and -c options
to specify the type and class or use the -q the
specify the domain name or use "IN." and "CH." when looking up these top
level domains.
SIMPLE USAGE
A typical invocation of dig looks
like:
dig @server name type
where:
server-
is the name or IP address of the name server to query. This can
be an IPv4 address in dotted-decimal notation or an IPv6 address
in colon-delimited notation. When the suppliedserverargument
is a hostname, dig resolves
that name before querying that name server. If noserverargument
is provided, dig consults/etc/resolv.confand
queries the name servers listed there. The reply from the name server
that responds is displayed. name-
is the name of the resource record that is to be looked up.
type-
indicates what type of query is required —
ANY, A, MX, SIG, etc.typecan
be any valid query type. If notypeargument
is supplied, dig will
perform a lookup for an A record.
OPTIONS
The -b option sets the source IP address
of the query to address. This must
be a valid address on one of the host's network interfaces or "0.0.0.0" or "::".
An optional port may be specified by appending "#<port>"
The default query class (IN for internet) is overridden by the -c option. class is
any valid class, such as HS for Hesiod records or CH for CHAOSNET records.
The -f option makes dig operate
in batch mode by reading a list of lookup requests to process from the
file filename. The file contains
a number of queries, one per line. Each entry in the file should be organised
in the same way they would be presented as queries to dig using
the command-line interface.
If a non-standard port number is to be queried, the -p option
is used. port# is the port number
that dig will send its queries
instead of the standard DNS port number 53. This option would be used to
test a name server that has been configured to listen for queries on a
non-standard port number.
The -4 option forces dig to
only use IPv4 query transport. The -6 option
forces dig to only use IPv6
query transport.
The -t option sets the query type to type.
It can be any valid query type which is supported in BIND9. The default
query type "A", unless the -x option is supplied
to indicate a reverse lookup. A zone transfer can be requested by specifying
a type of AXFR. When an incremental zone transfer (IXFR) is required, type is
set to ixfr=N. The incremental zone transfer
will contain the changes made to the zone since the serial number in the
zone's SOA record was N.
The -q option sets the query name to name.
This useful do distingish the name from
other arguments.
Reverse lookups - mapping addresses to names - are simplified by the -x option. addr is
an IPv4 address in dotted-decimal notation, or a colon-delimited IPv6 address.
When this option is used, there is no need to provide the name, class and type arguments. dig automatically
performs a lookup for a name like 11.12.13.10.in-addr.arpa and
sets the query type and class to PTR and IN respectively. By default, IPv6
addresses are looked up using nibble format under the IP6.ARPA domain.
To use the older RFC1886 method using the IP6.INT domain specify the -i option.
Bit string labels (RFC2874) are now experimental and are not attempted.
To sign the DNS queries sent by dig and
their responses using transaction signatures (TSIG), specify a TSIG key
file using the -k option. You can also specify
the TSIG key itself on the command line using the -y option; hmac is
the type of the TSIG, default HMAC-MD5, name is
the name of the TSIG key and key is
the actual key. The key is a base-64 encoded string, typically generated
by dnssec-keygen(8).
Caution should be taken when using the -y option
on multi-user systems as the key can be visible in the output from ps(1) or
in the shell's history file. When using TSIG authentication with dig,
the name server that is queried needs to know the key and algorithm that
is being used. In BIND, this is done by providing appropriate key and server statements
in named.conf.
QUERY OPTIONS
dig provides a number of
query options which affect the way in which lookups are made and the results
displayed. Some of these set or reset flag bits in the query header, some
determine which sections of the answer get printed, and others determine
the timeout and retry strategies.
Each query option is identified by a keyword preceded by a plus sign
(+). Some keywords set or reset an option.
These may be preceded by the string no to
negate the meaning of that keyword. Other keywords assign values to options
like the timeout interval. They have the form +keyword=value.
The query options are:
+[no]tcp-
Use [do not use] TCP when querying name servers. The default behaviour
is to use UDP unless an AXFR or IXFR query is requested, in which
case a TCP connection is used. +[no]vc-
Use [do not use] TCP when querying name servers. This alternate
syntax to+[no]tcpis provided
for backwards compatibility. The "vc" stands for "virtual circuit". +[no]ignore-
Ignore truncation in UDP responses instead of retrying with TCP.
By default, TCP retries are performed. +domain=somename-
Set the search list to contain the single domain
somename,
as if specified in a domain directive
in/etc/resolv.conf, and enable search
list processing as if the+searchoption
were given. +[no]search-
Use [do not use] the search list defined by the searchlist or domain
directive inresolv.conf(if any).
The search list is not used by default. +[no]showsearch-
Perform [do not perform] a search showing intermediate results.
+[no]defname-
Deprecated, treated as a synonym for
+[no]search +[no]aaonly-
Sets the "aa" flag in the query.
+[no]aaflag-
A synonym for
+[no]aaonly. +[no]adflag-
Set [do not set] the AD (authentic data) bit in the query. The
AD bit currently has a standard meaning only in responses, not in
queries, but the ability to set the bit in the query is provided
for completeness. +[no]cdflag-
Set [do not set] the CD (checking disabled) bit in the query. This
requests the server to not perform DNSSEC validation of responses. +[no]cl-
Display [do not display] the CLASS when printing the record.
+[no]ttlid-
Display [do not display] the TTL when printing the record.
+[no]recurse-
Toggle the setting of the RD (recursion desired) bit in the query.
This bit is set by default, which means dig normally
sends recursive queries. Recursion is automatically disabled when
the+nssearchor+tracequery
options are used. +[no]nssearch-
When this option is set, dig attempts
to find the authoritative name servers for the zone containing the
name being looked up and display the SOA record that each name server
has for the zone. +[no]trace-
Toggle tracing of the delegation path from the root name servers
for the name being looked up. Tracing is disabled by default. When
tracing is enabled, dig makes
iterative queries to resolve the name being looked up. It will follow
referrals from the root servers, showing the answer from each server
that was used to resolve the lookup. +[no]cmd-
toggles the printing of the initial comment in the output identifying
the version of dig and
the query options that have been applied. This comment is printed
by default. +[no]short-
Provide a terse answer. The default is to print the answer in a
verbose form. +[no]identify-
Show [or do not show] the IP address and port number that supplied
the answer when the+shortoption
is enabled. If short form answers are requested, the default is not
to show the source address and port number of the server that provided
the answer. +[no]comments-
Toggle the display of comment lines in the output. The default
is to print comments. +[no]stats-
This query option toggles the printing of statistics: when the
query was made, the size of the reply and so on. The default behaviour
is to print the query statistics. +[no]qr-
Print [do not print] the query as it is sent. By default, the query
is not printed. +[no]question-
Print [do not print] the question section of a query when an answer
is returned. The default is to print the question section as a comment. +[no]answer-
Display [do not display] the answer section of a reply. The default
is to display it. +[no]authority-
Display [do not display] the authority section of a reply. The
default is to display it. +[no]additional-
Display [do not display] the additional section of a reply. The
default is to display it. +[no]all-
Set or clear all display flags.
+time=T-
Sets the timeout for a query to
Tseconds.
The default time out is 5 seconds. An attempt to setTto
less than 1 will result in a query timeout of 1 second being applied. +tries=T-
Sets the number of times to try UDP queries to server to
Tinstead
of the default, 3. IfTis
less than or equal to zero, the number of tries is silently rounded
up to 1. +retry=T-
Sets the number of times to retry UDP queries to server to
Tinstead
of the default, 2. Unlike+tries,
this does not include the initial query. +ndots=D-
Set the number of dots that have to appear in
nametoDfor
it to be considered absolute. The default value is that defined using
the ndots statement in/etc/resolv.conf