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 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.

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.

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; };
};

This section describes several indispensable diagnostic, administrative and monitoring tools available to the system administrator for controlling and debugging the name server daemon.

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.

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.

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.

Key: La/E5CjG9O+os1jq0a2jdA==

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).

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;
};

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.

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

/* 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. */

// This is the start of a comment.  The next line
// is a new comment, even though it is logically
// part of the previous comment.
          

# This is the start of a comment.  The next line
# is a new comment, even though it is logically
# part of the previous comment.

acl acl-name {
    address_match_list
};

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:

controls {
   [ inet ( ip_addr | * ) [ port ip_port ] allow {  address_match_list  }
                keys { key_list }; ]
   [ inet ...; ]
   [ unix path perm number owner number group number keys { 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 -a after BIND 9 is installed.

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 { };.

include filename;

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.

key key_id {
    algorithm string;
    secret string;
};

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 {
   [ channel channel_name {
     ( file path name

         [ versions ( number | unlimited ) ]
         [ size size spec ]
       | syslog syslog_facility

       | stderr
       | null );
     [ severity (critical | error | warning | notice |
                 info | debug [ level ] | dynamic ); ]
     [ print-category yes or no; ]
     [ print-severity yes or no; ]
     [ print-time yes or no; ]
   }; ]
   [ category category_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.

channel an_example_channel {
    file "example.log" versions 3 size 20m;
    print-time yes;
    print-category yes;
};

channel specific_debug_level {
    file "foo";
    severity debug 3;
};

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
};

category default { default_syslog; default_debug; };

channel my_security_channel {
    file "my_security_file";
    severity info;
};
category security {
    my_security_channel;
    default_syslog;
    default_debug;
};

category xfer-out { null; };
category notify { null; };

This is the grammar of the lwres statement in the named.conf file:

lwres {
    [ listen-on { ip_addr [port ip_port] ; [ ip_addr [port ip_port] ; ... ] }; ]
    [ view view_name; ]
    [ search { domain_name ; [ domain_name ; ... ] }; ]
    [ ndots number; ]
};

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.

masters name [port ip_port] { ( masters_list | ip_addr [port ip_port] [key key] ) ; [...] };

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.

listen-on { 5.6.7.8; };
listen-on port 1234 { !1.2.3.4; 1.2/16; };

{ any; }

listen-on-v6 { any; };
listen-on-v6 port 1234 { !2001:db8::/32; any; };

listen-on-v6 { none; };

query-source address * port *;
query-source-v6 address * port *;

topology {
    10/8;
    !1.2.3/24;
    { 1.2/16; 3/8; };
};

    topology { localhost; localnets; };

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
    };
};

sortlist {
           { localhost; localnets; };
           { localnets; };
};

rrset-order {
   class IN type A name "host.example.com" order random;
   order cyclic;
};

            disable-empty-zone ".";

server ip_addr[/prefixlen] {
    [ bogus yes_or_no ; ]
    [ provide-ixfr yes_or_no ; ]
    [ request-ixfr yes_or_no ; ]
    [ edns yes_or_no ; ]
    [ edns-udp-size number ; ]
    [ max-udp-size number ; ]
    [ transfers number ; ]
    [ transfer-format ( one-answer | many-answers ) ; ]]
    [ keys { string ; [ string ; [...]] } ; ]
    [ transfer-source (ip4_addr | *) [port ip_port] ; ]
    [ transfer-source-v6 (ip6_addr | *) [port ip_port] ; ]
    [ notify-source (ip4_addr | *) [port ip_port] ; ]
    [ notify-source-v6 (ip6_addr | *) [port ip_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.

view view_name
      [class] {
      match-clients { address_match_list };
      match-destinations { address_match_list };
      match-recursive-only yes_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";
      };
};

zone zone_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 [port ip_port] ; [ ip_addr [port ip_port] ; ... ] }; ]
    [ check-names (warn|fail|ignore) ; ]
    [ check-mx (warn|fail|ignore) ; ]
    [ check-wildcard yes_or_no; ]
    [ check-integrity yes_or_no ; ]
    [ dialup dialup_option ; ]
    [ file string ; ]
    [ masterfile-format (text|raw) ; ]
    [ journal string ; ]
    [ forward (only|first) ; ]
    [ forwarders { [ ip_addr [port ip_port] ; ... ] }; ]
    [ ixfr-base string ; ]
    [ ixfr-tmp-file string ; ]
    [ maintain-ixfr-base yes_or_no ; ]
    [ max-ixfr-log-size number ; ]
    [ max-transfer-idle-out number ; ]
    [ max-transfer-time-out number ; ]
    [ notify yes_or_no | explicit | master-only ; ]
    [ pubkey number number number string ; ]
    [ notify-source (ip4_addr | *) [port ip_port] ; ]
    [ notify-source-v6 (ip6_addr | *) [port ip_port] ; ]
    [ zone-statistics yes_or_no ; ]
    [ sig-validity-interval number ; ]
    [ database string ; ]
    [ min-refresh-time number ; ]
    [ max-refresh-time number ; ]
    [ min-retry-time number ; ]
    [ max-retry-time number ; ]
    [ key-directory path_name; ]
    [ zero-no-soa-ttl yes_or_no ; ]
};

zone zone_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-ksk yes_or_no; ]
    [ also-notify { ip_addr [port ip_port] ; [ ip_addr [port ip_port] ; ... ] }; ]
    [ check-names (warn|fail|ignore) ; ]
    [ dialup dialup_option ; ]
    [ file string ; ]
    [ masterfile-format (text|raw) ; ]
    [ journal string ; ]
    [ forward (only|first) ; ]
    [ forwarders { [ ip_addr [port ip_port] ; ... ] }; ]
    [ ixfr-base string ; ]
    [ ixfr-tmp-file string ; ]
    [ maintain-ixfr-base yes_or_no ; ]
    [ masters [port ip_port] { ( masters_list | ip_addr [port ip_port] [key key] ) ; [...] }; ]
    [ max-ixfr-log-size number ; ]
    [ max-transfer-idle-in number ; ]
    [ max-transfer-idle-out number ; ]
    [ max-transfer-time-in number ; ]
    [ max-transfer-time-out number ; ]
    [ notify yes_or_no | explicit | master-only ; ]
    [ pubkey number number number string ; ]
    [ 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] ; ]
    [ zone-statistics yes_or_no ; ]
    [ database string ; ]
    [ min-refresh-time number ; ]
    [ max-refresh-time number ; ]
    [ min-retry-time number ; ]
    [ max-retry-time number ; ]
    [ multi-master yes_or_no ; ]
    [ zero-no-soa-ttl yes_or_no ; ]
};

zone zone_name [class] {
    type hint;
    file string ;
    [ delegation-only yes_or_no ; ]
    [ check-names (warn|fail|ignore) ; // Not Implemented. ]
};

zone zone_name [class] {
    type stub;
    [ allow-query { address_match_list }; ]
    [ check-names (warn|fail|ignore) ; ]
    [ dialup dialup_option ; ]
    [ delegation-only yes_or_no ; ]
    [ file string ; ]
    [ masterfile-format (text|raw) ; ]
    [ forward (only|first) ; ]
    [ forwarders { [ ip_addr [port ip_port] ; ... ] }; ]
    [ masters [port ip_port] { ( masters_list | ip_addr [port ip_port] [key key] ) ; [...] }; ]
    [ max-transfer-idle-in number ; ]
    [ max-transfer-time-in number ; ]
    [ pubkey number number number string ; ]
    [ 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; ]
    [ zone-statistics yes_or_no ; ]
    [ database string ; ]
    [ min-refresh-time number ; ]
    [ max-refresh-time number ; ]
    [ min-retry-time number ; ]
    [ max-retry-time number ; ]
    [ multi-master yes_or_no ; ]
};

zone zone_name [class] {
    type forward;
    [ forward (only|first) ; ]
    [ forwarders { [ ip_addr [port ip_port] ; ... ] }; ]
    [ delegation-only yes_or_no ; ]
};

zone zone_name [class] {
    type delegation-only;
};

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.