Copyright © 2004-2007 Internet Systems Consortium, Inc. ("ISC")
Copyright © 2000-2003 Internet Software Consortium.
Table of Contents
rndc.conf — rndc configuration file
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:
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To describe: |
We use the style: |
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a pathname, filename, URL, hostname, mailing list name, or new term or concept |
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literal user input |
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program output |
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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 |
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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:
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Name |
TTL |
CLASS |
TYPE |
Resource Record (RR) Data |
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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.
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.
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] [-c class] [-N ndots] [-t type] [-W timeout] [-R retries] hostname [server]
For more information and a list of available commands and options, see the host man page.
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.
The named-checkconf program
checks the syntax of a named.conf file.
named-checkconf [-jvz] [-t directory] [filename]
The named-checkzone program checks a master file for syntax and consistency.
named-checkzone [-djqvD] [-c class] [-o output] [-t directory] [-w directory] [-k (ignore|warn|fail)] [-n (ignore|warn|fail)] [-W (ignore|warn)] zone [filename]
Similar to named-checkzone, but it always dumps the zone content to a specified file (typically in a different format).
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 [-c config] [-s server] [-p port] [-y key] command [command...]
The command is one of the following:
reloadReload 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.
reconfigReload 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.
statsWrite 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 of
named.conf or by specifying
querylog yes; in the
options section of
named.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.
traceIncrement the servers debugging level by one.
trace levelSets the server's debugging level to an explicit value.
notraceSets the server's debugging level to 0.
flushFlushes the server's cache.
flushname nameFlushes the given name from the server's cache.
statusDisplay 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.
recursingDump 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 -c
option. If the configuration file is not found,
rndc will also look in
/etc/rndc.key (or whatever
sysconfdir was defined when
the BIND build was
configured).
The rndc.key file 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 -s
option 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 keyword key is
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 keyword server is 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 reload
to 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 a rndc.conf
file for you, and also display the
corresponding controls
statement that you need to
add to named.conf.
Alternatively,
you can run rndc-confgen -a
to set up
a rndc.key file and not
modify
named.conf at 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.
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SIGHUP |
Causes the server to read |
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SIGTERM |
Causes the server to clean up and exit. |
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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.
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:
site1
and
site2.example.com zones.
site1.internal and
site2.internal domains.
Hosts on the Internet will be able to:
site1
and
site2.example.com zones.
site1 and
site2.example.com zones.
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;
};
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