dns/bind9

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Configuration Reference

The operational functionality of BIND 9 is defined using the file named.conf, which is typically located in /etc or /usr/local/etc/namedb, depending on the operating system or distribution. A further file rndc.conf will be present if rndc is being run from a remote host, but is not required if rndc is being run from localhost (the same system as BIND 9 is running on).

Configuration File (named.conf)

The file named.conf may contain three types of entities:

Comment

Multiple comment formats are supported <comment_syntax>.

Block

Blocks <configuration_blocks> are containers for statements <Statement> which either have common functionality - for example, the definition of a cryptographic key in a key block - or which define the scope of the statement - for example, a statement which appears in a zone block has scope only for that zone.

Blocks are organized hierarchically within named.conf and may have a number of different properties:

Certain blocks cannot be nested inside other blocks and thus may be regarded as the topmost-level blocks: for example, the options block and the logging block.
Certain blocks can appear multiple times, in which case they have an associated name to disambiguate them: for example, the zone block (zone example.com { ... };) or the key block (key mykey { ... };).
Certain blocks may be "nested" within other blocks. For example, the zone block may appear within a view block.

The description of each block in this manual lists its permissible locations.

Statement

Statements define and control specific BIND behaviors.
Statements may have a single parameter (a Value) or multiple parameters (Argument/Value pairs). For example, the recursion statement takes a single value parameter - in this case, the string yes or no (recursion yes;) - while the port statement takes a numeric value defining the DNS port number (port 53;). More complex statements take one or more argument/value pairs. The also-notify statement may take a number of such argument/value pairs, such as also-notify port 5353;, where port is the argument and 5353 is the corresponding value.
Statements can appear in a single block <Block> - for example, an algorithm statement can appear only in a key block - or in multiple blocks - for example, an also-notify statement can appear in an options block where it has global (server-wide) scope, in a zone block where it has scope only for the specific zone (and overrides any global statement), or even in a view block where it has scope for only that view (and overrides any global statement).

The file named.conf may further contain one or more instances of the include <include_grammar> Directive. This directive is provided for administrative convenience in assembling a complete named.conf file and plays no subsequent role in BIND 9 operational characteristics or functionality.

Note

Over a period of many years the BIND ARM acquired a bewildering array of terminology. Many of the terms used described similar concepts and served only to add a layer of complexity, possibly confusion, and perhaps mystique to BIND 9 configuration. The ARM now uses only the terms Block, Statement, Argument, Value, and Directive to describe all entities used in BIND 9 configuration.

Comment Syntax

The BIND 9 comment syntax allows comments to appear anywhere that whitespace 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.

Syntax

/* 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

Definition and Usage

Comments can be inserted anywhere that whitespace may appear in a BIND configuration file.

C-style comments start with the two characters /* (slash, star) and end with */ (star, slash). Because they are completely delimited with these characters, they can be used to comment only a portion of a line or to span multiple lines.

C-style comments cannot be nested. For example, the following is not valid because the entire comment ends with the first */:

/* This is the start of a comment.
   This is still part of the comment.
/* This is an incorrect attempt at nesting a comment. */
   This is no longer in any comment. */

C++-style comments start with the two characters // (slash, slash) and continue to the end of the physical line. They cannot be continued across multiple physical lines; to have one logical comment span multiple lines, each line must use the // pair. For example:

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

Shell-style (or Perl-style) comments start with the character # (number/pound sign) and continue to the end of the physical line, as in C++ comments. For example:

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

Warning

The semicolon (;) character cannot start a comment, unlike in a zone file. The semicolon indicates the end of a configuration statement.

Configuration Layout Styles

BIND is very picky about opening and closing brackets/braces, semicolons, and all the other separators defined in the formal syntaxes in later sections. There are many layout styles that can assist in minimizing errors, as shown in the following examples:

// dense single-line style
zone "example.com" in{type secondary; file "secondary.example.com"; primaries {10.0.0.1;};};
//  single-statement-per-line style
zone "example.com" in{
    type secondary;
    file "secondary.example.com";
    primaries {10.0.0.1;};
};
// spot the difference
zone "example.com" in{
    type secondary;
file "sec.secondary.com";
primaries {10.0.0.1;}; };

`include` Directive

include filename;

`include` Directive Definition and Usage

The include directive inserts the specified file (or files if a valid glob expression is detected) at the point where the include directive is encountered. The include directive 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.

Address Match Lists

Syntax

An address match list is a list of semicolon-separated address_match_element s.

{ <address_match_element>; ... };

Each element is then defined as:

address_match_element

[ ! ] ( <ip_address> | <netprefix> | key <server_key> | <acl_name> | { address_match_list } )

Definition and Usage

Address match lists are primarily used to determine access control for various server operations. They are also used in the listen-on and sortlist statements. The elements which constitute an address match list can be any of the following:

ip_address: an IP address (IPv4 or IPv6)
netprefix: an IP prefix (in / notation)
server_key: a key ID, as defined by the key statement
acl_name: the name of an address match list defined with the acl statement
a nested address match list enclosed in braces

Elements can be negated with a leading exclamation mark (!), and the match list names "any", "none", "localhost", and "localnets" are predefined. More information on those names can be found in the description of the acl statement.

The addition of the key clause made the name of this syntactic element something of a misnomer, since security keys can be used to validate access without regard to a host or network address. Nonetheless, the term "address match list" is still used throughout the documentation.

When a given IP address or prefix is compared to an address match list, the comparison takes place in approximately O(1) time. However, key comparisons require that the list of keys be traversed until a matching key is found, and therefore may be somewhat slower.

The interpretation of a match depends on whether the list is being used for access control, defining listen-on ports, or in a sortlist, and whether the element was negated.

When used as an access control list, a non-negated match allows access and a negated match denies access. If there is no match, access is denied. The clauses allow-notify, allow-recursion, allow-recursion-on, allow-query, allow-query-on, allow-query-cache, allow-query-cache-on, allow-transfer, allow-update, allow-update-forwarding, and blackhole all use address match lists. Similarly, the listen-on option causes the server to refuse queries on any of the machine's addresses which do not match the list.

Order of insertion is significant. If more than one element in an ACL is found to match a given IP address or prefix, preference is given to the one that came first in the ACL definition. Because of this first-match behavior, an element that defines a subset of another element in the list should come before the broader element, regardless of whether either is negated. For example, in 1.2.3/24; ! 1.2.3.13; the 1.2.3.13 element is completely useless because the algorithm matches any lookup for 1.2.3.13 to the 1.2.3/24 element. Using ! 1.2.3.13; 1.2.3/24 fixes that problem by blocking 1.2.3.13 via the negation, but all other 1.2.3.* hosts pass through.

Glossary of Terms Used

Following is a list of terms used throughout the BIND configuration file documentation:

acl_name: The name of an address_match_list as defined by the acl statement.
address_match_list: See address_match_lists.
boolean: Either yes or no. The words true and false are also accepted, as are the numbers 1 and 0.
domain_name: A quoted string which is used as a DNS name; for example: my.test.domain.
fixedpoint: A non-negative real number that can be specified to the nearest one-hundredth. Up to five digits can be specified before a decimal point, and up to two digits after, so the maximum value is 99999.99. Acceptable values might be further limited by the contexts in which they are used.
integer: A non-negative 32-bit integer (i.e., a number between 0 and 4294967295, inclusive). Its acceptable value might be further limited by the context in which it is used.
ip_address: An ipv4_address or ipv6_address.
ipv4_address: An IPv4 address with exactly four integer elements valued 0 through 255 and separated by dots (.), such as 192.168.1.1 (a "dotted-decimal" notation with all four elements present).
ipv6_address: An IPv6 address, such as 2001:db8::1234. IPv6-scoped addresses that have ambiguity on their scope zones must be disambiguated by an appropriate zone ID with the percent character (%) as a delimiter. It is strongly recommended to use string zone names rather than numeric identifiers, to be robust against system configuration changes. However, since there is no standard mapping for such names and identifier values, only interface names as link identifiers are supported, assuming one-to-one mapping between interfaces and links. For example, a link-local address fe80::1 on the link attached to the interface ne0 can be specified as fe80::1%ne0. Note that on most systems link-local addresses always have ambiguity and need to be disambiguated.
netprefix: An IP network specified as an ip_address, followed by a slash (/) and then the number of bits in the netmask. Trailing zeros in an ip_address may be omitted. For example, 127/8 is the network 127.0.0.0 with netmask 255.0.0.0 and 1.2.3.0/28 is network 1.2.3.0 with netmask 255.255.255.240. When specifying a prefix involving an IPv6-scoped address, the scope may be omitted. In that case, the prefix matches packets from any scope.
percentage: An integer value followed by % to represent percent.
port: An IP port integer. It is limited to 0 through 65535, with values below 1024 typically restricted to use by processes running as root. In some cases, an asterisk (*) character can be used as a placeholder to select a random high-numbered port.
portrange: A list of a port or a port range. A port range is specified in the form of range followed by two port s, port_low and port_high, which represents port numbers from port_low through port_high, inclusive. port_low must not be larger than port_high. For example, range 1024 65535 represents ports from 1024 through 65535. The asterisk (*) character is not allowed as a valid port or as a port range boundary.
remote-servers: A named list of one or more ip_address es with optional tls_id, server_key, and/or port. A remote-servers list may include other remote-servers lists. See primaries block.
server_key: A domain_name representing the name of a shared key, to be used for transaction security <tsig>. Keys are defined using key blocks.

size sizeval A 64-bit unsigned integer. Integers may take values 0 <= value <= 18446744073709551615, though certain parameters (such as max-journal-size) may use a more limited range within these extremes. In most cases, setting a value to 0 does not literally mean zero; it means "undefined" or "as big as possible," depending on the context. See the explanations of particular parameters that use size for details on how they interpret its use. Numeric values can optionally be followed by a scaling factor: K or k for kilobytes, M or m for megabytes, and G or g for gigabytes, which scale by 1024, 1024*1024, and 1024*1024*1024 respectively.

Some statements also accept the keywords unlimited or default: unlimited generally means "as big as possible," and is usually the best way to safely set a very large number. default uses the limit that was in force when the server was started.

tls_id: A named TLS configuration object which defines a TLS key and certificate. See tls block.

Blocks

A BIND 9 configuration consists of blocks, statements, and comments.

The following blocks are supported:

acl

Defines a named IP address matching list, for access control and other uses.

controls

Declares control channels to be used by the rndc utility.

dnssec-policy

Describes a DNSSEC key and signing policy for zones. See dnssec-policy for details.

key

Specifies key information for use in authentication and authorization using TSIG.

logging

Specifies what information the server logs and where the log messages are sent.

masters

Synonym for primaries.

options

Controls global server configuration options and sets defaults for other statements.

parental-agents

Defines a named list of servers for inclusion in primary and secondary zones' parental-agents lists.

primaries

Defines a named list of servers for inclusion in stub and secondary zones' primaries or also-notify lists. (Note: this is a synonym for the original keyword masters, which can still be used, but is no longer the preferred terminology.)

server

Sets certain configuration options on a per-server basis.

statistics-channels

Declares communication channels to get access to named statistics.

tls

Specifies configuration information for a TLS connection, including a key-file, cert-file, ca-file, dhparam-file, remote-hostname, ciphers, protocols, prefer-server-ciphers, and session-tickets.

http

Specifies configuration information for an HTTP connection, including endpoints, listener-clients, and streams-per-connection.

trust-anchors

Defines DNSSEC trust anchors: if used with the initial-key or initial-ds keyword, trust anchors are kept up-to-date using 5011 trust anchor maintenance; if used with static-key or static-ds, keys are permanent.

managed-keys

Is identical to trust-anchors; this option is deprecated in favor of trust-anchors with the initial-key keyword, and may be removed in a future release.

trusted-keys

Defines permanent trusted DNSSEC keys; this option is deprecated in favor of trust-anchors with the static-key keyword, and may be removed in a future release.

view

Defines a view.

zone

Defines a zone.

The logging and options statements may only occur once per configuration.

`acl` Block Grammar

`acl` Block Definition and Usage

The acl statement assigns a symbolic name to an address match list. It gets its name from one of the primary uses of address match lists: Access Control Lists (ACLs).

The following ACLs are built-in:

any

Matches all hosts.

none

Matches no hosts.

localhost

Matches the IPv4 and IPv6 addresses of all network interfaces on the system. When addresses are added or removed, the localhost ACL element is updated to reflect the changes.

localnets

Matches any host on an IPv4 or IPv6 network for which the system has an interface. When addresses are added or removed, the localnets ACL element is updated to reflect the changes. Some systems do not provide a way to determine the prefix lengths of local IPv6 addresses; in such cases, localnets only matches the local IPv6 addresses, just like localhost.

`controls` Block Grammar

`controls` Block Definition and Usage

The controls statement declares control channels to be used by system administrators to manage 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.

read-only: If the read-only argument is on, the control channel is limited to the following set of read-only commands: nta -dump, null, status, showzone, testgen, and zonestatus. By default, read-only is not enabled and the control channel allows read-write access.

If no controls statement is present, named sets 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 attempts to load the command channel key from the file . To create an rndc.key file, run rndc-confgen -a.

To disable the command channel, use an empty controls statement: controls { };.

`key` Block Grammar

`key` Block Definition and Usage

The key statement defines a shared secret key for use with TSIG (see tsig) or the command channel (see controls).

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 must be defined at the top level.

The server_key, also known as the key name, is a domain name that uniquely identifies 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.

`logging` Block Grammar

`logging` Block Definition and Usage

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 desired. If there is no logging statement, the logging configuration is:

logging {
     category default { default_syslog; default_debug; };
     category unmatched { null; };
};

If named is started with the -L <named -L> option, it logs to the specified file at startup, instead of using syslog. In this case the logging configuration is:

logging {
     category default { default_logfile; default_debug; };
     category unmatched { null; };
};

The logging configuration is only established when the entire configuration file has been parsed. When the server starts up, all logging messages regarding syntax errors in the configuration file go to the default channels, or to standard error if the -g <named -g> option was specified.

The `channel` Phrase

All log output goes to one or more channels; there is no limit to the number of channels that can be created.

Every channel definition must include a destination clause that says whether messages selected for the channel go to a file, go to a particular syslog facility, go to the standard error stream, or are discarded. The definition can optionally also limit the message severity level that is accepted by the channel (the default is info), and whether to include a named-generated time stamp, the category name, and/or the severity level (the default is not to include any).

file

The file destination clause directs the channel to a disk file. It can include additional arguments to specify how large the file is allowed to become before it is rolled to a backup file (size), how many backup versions of the file are saved each time this happens (versions), and the format to use for naming backup versions (suffix).

The size option is used to limit log file growth. If the file ever exceeds the specified size, then named stops writing to the file unless it has a versions option associated with it. If backup versions are kept, the files are rolled as described below. If there is no versions option, no more data is written to the log until some out-of-band mechanism removes or truncates the log to less than the maximum size. The default behavior is not to limit the size of the file.

File rolling only occurs when the file exceeds the size specified with the size option. No backup versions are kept by default; any existing log file is simply appended. The versions option specifies how many backup versions of the file should be kept. If set to unlimited, there is no limit.

The suffix option can be set to either increment or timestamp. If set to timestamp, then when a log file is rolled, it is saved with the current timestamp as a file suffix. If set to increment, then backup files are saved with incrementing numbers as suffixes; older files are renamed when rolling. For example, if versions is set to 3 and suffix to increment, then when filename.log reaches the size specified by size, filename.log.1 is renamed to filename.log.2, filename.log.0 is renamed to filename.log.1, and filename.log is renamed to filename.log.0, whereupon a new filename.log is opened.

Here is an example using the size, versions, and suffix options:

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

The server can supply extensive debugging information when it is in debugging mode. If the server's global debug level is greater than zero, debugging mode is active. The global debug level is set either by starting the named server with the -d <named -d> flag followed by a positive integer, or by running rndc trace. The global debug level can be set to zero, and debugging mode turned off, by running rndc notrace. All debugging messages in the server have a debug level; higher debug levels give more detailed output. Channels that specify a specific debug severity, for example:

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

get debugging output of level 3 or less any time the server is in debugging mode, regardless of the global debugging level. Channels with dynamic severity use the server's global debug level to determine what messages to print.

Here is an example where all three print- options are on:

28-Feb-2000 15:05:32.863 general: notice: running

There are four predefined channels that are used for named's default logging, as follows. If named is started with the -L <named -L> option, then a fifth channel, default_logfile, is added. How they are used is described in category.

channel default_syslog {
    // send to syslog's daemon facility
    syslog daemon;
    // only send priority info and higher
    severity info;
};

channel default_debug {
    // write to named.run in the working directory
    // Note: stderr is used instead of "named.run" if
    // the server is started with the '-g' option.
    file "named.run";
    // log at the server's current debug level
    severity dynamic;
};

channel default_stderr {
    // writes to stderr
    stderr;
    // only send priority info and higher
    severity info;
};

channel null {
   // toss anything sent to this channel
   null;
};

channel default_logfile {
    // this channel is only present if named is
    // started with the -L option, whose argument
    // provides the file name
    file "...";
    // log at the server's current debug level
    severity dynamic;
};

The default_debug channel has the special property that it only produces output when the server's debug level is non-zero. It normally writes to a file called named.run in the server's working directory.

For security reasons, when the -u <named -u> command-line option is used, the named.run file is created only after named has changed to the new UID, and any debug output generated while named is starting -and still running as root - is discarded. To capture this output, run the server with the -L <named -L> option to specify a default logfile, or the -g <named -g> option to log to standard error which can be redirected to a file.

Once a channel is defined, it cannot be redefined. The built-in channels cannot be altered directly, but the default logging can be modified by pointing categories at defined channels.

The `category` Phrase

There are many categories, so desired logs can be sent anywhere while unwanted logs are ignored. If a list of channels is not specified for a category, log messages in that category are sent to the default category instead. If no default category is specified, the following "default default" is used:

category default { default_syslog; default_debug; };

If named is started with the -L <named -L> option, the default category is:

category default { default_logfile; default_debug; };

As an example, let's say a user wants to log security events to a file, but also wants to keep the default logging behavior. They would specify the following:

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

To discard all messages in a category, specify the null channel:

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

The following are the available categories and brief descriptions of the types of log information they contain. More categories may be added in future BIND releases.

The `query-errors` Category

The query-errors category is used to indicate why and how specific queries resulted in responses which indicate an error. Normally, these messages are logged at debug logging levels; note, however, that if query logging is active, some are logged at info. The logging levels are described below:

At debug level 1 or higher - or at info when query logging is active - each response with the rcode of SERVFAIL is logged as follows:

client 127.0.0.1#61502: query failed (SERVFAIL) for www.example.com/IN/AAAA at query.c:3880

This means an error resulting in SERVFAIL was detected at line 3880 of source file query.c. Log messages of this level are particularly helpful in identifying the cause of SERVFAIL for an authoritative server.

At debug level 2 or higher, detailed context information about recursive resolutions that resulted in SERVFAIL is logged. The log message looks like this:

fetch completed at resolver.c:2970 for www.example.com/A
in 10.000183: timed out/success [domain:example.com,
referral:2,restart:7,qrysent:8,timeout:5,lame:0,quota:0,neterr:0,
badresp:1,adberr:0,findfail:0,valfail:0]

The first part before the colon shows that a recursive resolution for AAAA records of www.example.com completed in 10.000183 seconds, and the final result that led to the SERVFAIL was determined at line 2970 of source file resolver.c.

The next part shows the detected final result and the latest result of DNSSEC validation. The latter is always "success" when no validation attempt was made. In this example, this query probably resulted in SERVFAIL because all name servers are down or unreachable, leading to a timeout in 10 seconds. DNSSEC validation was probably not attempted.

The last part, enclosed in square brackets, shows statistics collected for this particular resolution attempt. The domain field shows the deepest zone that the resolver reached; it is the zone where the error was finally detected. The meaning of the other fields is summarized in the following list.

referral: The number of referrals the resolver received throughout the resolution process. In the above example.com there are two.
restart: The number of cycles that the resolver tried remote servers at the domain zone. In each cycle, the resolver sends one query (possibly resending it, depending on the response) to each known name server of the domain zone.
qrysent: The number of queries the resolver sent at the domain zone.
timeout: The number of timeouts the resolver received since the last response.
lame: The number of lame servers the resolver detected at the domain zone. A server is detected to be lame either by an invalid response or as a result of lookup in BIND 9's address database (ADB), where lame servers are cached.
quota: The number of times the resolver was unable to send a query because it had exceeded the permissible fetch quota for a server.
neterr: The number of erroneous results that the resolver encountered in sending queries at the domain zone. One common case is when the remote server is unreachable and the resolver receives an "ICMP unreachable" error message.
badresp: The number of unexpected responses (other than lame) to queries sent by the resolver at the domain zone.
adberr: Failures in finding remote server addresses of thedomain zone in the ADB. One common case of this is that the remote server's name does not have any address records.
findfail: Failures to resolve remote server addresses. This is a total number of failures throughout the resolution process.
valfail: Failures of DNSSEC validation. Validation failures are counted throughout the resolution process (not limited to the domain zone), but should only happen in domain.

At debug level 3 or higher, the same messages as those at debug level 1 are logged for errors other than SERVFAIL. Note that negative responses such as NXDOMAIN are not errors, and are not logged at this debug level.

At debug level 4 or higher, the detailed context information logged at debug level 2 is logged for errors other than SERVFAIL and for negative responses such as NXDOMAIN.

`parental-agents` Block Grammar

`parental-agents` Block Definition and Usage

parental-agents lists allow for a common set of parental agents to be easily used by multiple primary and secondary zones. A parental agent is the entity that is allowed to change a zone's delegation information (defined in 7344).

`primaries` Block Grammar

`primaries` Block Definition and Usage

primaries lists allow for a common set of primary servers to be easily used by multiple stub and secondary zones in their primaries or also-notify lists. (Note: primaries is a synonym for the original keyword masters, which can still be used, but is no longer the preferred terminology.)

To force the zone transfer requests to be sent over TLS, use tls keyword, e.g. primaries { 192.0.2.1 tls tls-configuration-name; };, where tls-configuration-name refers to a previously defined tls statement <tls>.

Warning

Please note that TLS connections to primaries are not authenticated unless remote-hostname or ca-file are specified within the tls statement <tls> in use (see information on Strict TLS <strict-tls> and Mutual TLS <mutual-tls> for more details). Not authenticated mode (Opportunistic TLS <opportunistic-tls>) provides protection from passive observers but does not protect from man-in-the-middle attacks on zone transfers.

`options` Block Grammar

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

`options` Block Definition and Usage

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 is used.

dnssec-policy

This specifies which key and signing policy (KASP) should be used for this zone. This is a string referring to a dnssec-policy block. The default is none.

max-zone-ttl

tags: deprecated
short: Specifies a maximum permissible time-to-live (TTL) value, in seconds.

This should now be configured as part of dnssec-policy. Use of this option in options, view and zone blocks is a fatal error if dnssec-policy has also been configured for the same zone. In zones without dnssec-policy, this option is deprecated, and will be rendered non-operational in a future release.

max-zone-ttl specifies a maximum permissible TTL value in seconds. For convenience, TTL-style time-unit suffixes may be used to specify the maximum value. When a zone file is loaded, any record encountered with a TTL higher than max-zone-ttl causes the zone to be rejected.

This is needed in DNSSEC-maintained zones because when rolling to a new DNSKEY, the old key needs to remain available until RRSIG records have expired from caches. The max-zone-ttl option guarantees that the largest TTL in the zone is no higher than the set value.

When used in options, view and zone blocks, setting max-zone-ttl to zero is equivalent to "unlimited".

Boolean Options

Forwarding

The forwarding facility can be used to create a large site-wide cache on a few servers, reducing traffic over links to external name servers. It can also be used to allow queries by servers that do not have direct access to the Internet, but wish to look up exterior names anyway. Forwarding occurs only on those queries for which the server is not authoritative and does not have the answer in its cache.

Forwarding can also be configured on a per-domain basis, allowing for the global forwarding options to be overridden in a variety of ways. Particular domains can be set to use different forwarders, or have a different forward only/first behavior, or not forward at all; see zone.

Dual-stack Servers

Dual-stack servers are used as servers of last resort, to work around problems in reachability due to the lack of support for either IPv4 or IPv6 on the host machine.

Access Control

Access to the server can be restricted based on the IP address of the requesting system. See address_match_lists for details on how to specify IP address lists.

Warning

Please note that incoming TLS connections are not authenticated at the TLS level by default. Please use tsig to authenticate requestors or consider implementing Mutual TLS <mutual-tls> authentication.

Interfaces

The interfaces, ports, and protocols that the server can use to answer queries may be specified using the listen-on and listen-on-v6 options.

Query Address

Zone Transfers

BIND has mechanisms in place to facilitate zone transfers and set limits on the amount of load that transfers place on the system. The following options apply to zone transfers.

Server Resource Limits

The following options set limits on the server's resource consumption that are enforced internally by the server rather than by the operating system.

Periodic Task Intervals

The `sortlist` Statement

The response to a DNS query may consist of multiple resource records (RRs) forming a resource record set (RRset). The name server normally returns the RRs within the RRset in an indeterminate order (but see the rrset-order statement in rrset_ordering). The client resolver code should rearrange the RRs as appropriate: that is, using any addresses on the local net in preference to other addresses. However, not all resolvers can do this or are correctly configured. When a client is using a local server, the sorting can be performed in the server, based on the client's address. This only requires configuring the name servers, not all the clients.

sortlist {
    // IF the local host
    // THEN first fit on the following nets
    { localhost;
    { localnets;
        192.168.1/24;
        { 192.168.2/24; 192.168.3/24; }; }; };
    // IF on class C 192.168.1 THEN use .1, or .2 or .3
    { 192.168.1/24;
    { 192.168.1/24;
        { 192.168.2/24; 192.168.3/24; }; }; };
    // IF on class C 192.168.2 THEN use .2, or .1 or .3
    { 192.168.2/24;
    { 192.168.2/24;
        { 192.168.1/24; 192.168.3/24; }; }; };
    // IF on class C 192.168.3 THEN use .3, or .1 or .2
    { 192.168.3/24;
    { 192.168.3/24;
        { 192.168.1/24; 192.168.2/24; }; }; };
    // IF .4 or .5 THEN prefer that net
    { { 192.168.4/24; 192.168.5/24; };
    };
};

The following example illlustrates reasonable behavior for the local host and hosts on directly connected networks. Responses sent to queries from the local host favor any of the directly connected networks. Responses sent to queries from any other hosts on a directly connected network prefer addresses on that same network. Responses to other queries are not sorted.

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

RRset Ordering

Note

While alternating the order of records in a DNS response between subsequent queries is a known load distribution technique, certain caveats apply (mostly stemming from caching) which usually make it a suboptimal choice for load balancing purposes when used on its own.

Tuning

Built-in Server Information Zones

The server provides some helpful diagnostic information through a number of built-in zones under the pseudo-top-level-domain bind in the CHAOS class. These zones are part of a built-in view (see view) of class CHAOS, which is separate from the default view of class IN. Most global configuration options (allow-query, etc.) apply to this view, but some are locally overridden: notify, recursion, and allow-new-zones are always set to no, and rate-limit is set to allow three responses per second.

To disable these zones, use the options below or hide the built-in CHAOS view by defining an explicit view of class CHAOS that matches all clients.

Built-in Empty Zones

The named server has some built-in empty zones, for SOA and NS records only. These are for zones that should normally be answered locally and for which queries should not be sent to the Internet's root servers. The official servers that cover these namespaces return NXDOMAIN responses to these queries. In particular, these cover the reverse namespaces for addresses from 1918, 4193, 5737, and 6598. They also include the reverse namespace for the IPv6 local address (locally assigned), IPv6 link local addresses, the IPv6 loopback address, and the IPv6 unknown address.

The server attempts to determine if a built-in zone already exists or is active (covered by a forward-only forwarding declaration) and does not create an empty zone if either is true.

The current list of empty zones is:

10.IN-ADDR.ARPA
16.172.IN-ADDR.ARPA
17.172.IN-ADDR.ARPA
18.172.IN-ADDR.ARPA
19.172.IN-ADDR.ARPA
20.172.IN-ADDR.ARPA
21.172.IN-ADDR.ARPA
22.172.IN-ADDR.ARPA
23.172.IN-ADDR.ARPA
24.172.IN-ADDR.ARPA
25.172.IN-ADDR.ARPA
26.172.IN-ADDR.ARPA
27.172.IN-ADDR.ARPA
28.172.IN-ADDR.ARPA
29.172.IN-ADDR.ARPA
30.172.IN-ADDR.ARPA
31.172.IN-ADDR.ARPA
168.192.IN-ADDR.ARPA
64.100.IN-ADDR.ARPA
65.100.IN-ADDR.ARPA
66.100.IN-ADDR.ARPA
67.100.IN-ADDR.ARPA
68.100.IN-ADDR.ARPA
69.100.IN-ADDR.ARPA
70.100.IN-ADDR.ARPA
71.100.IN-ADDR.ARPA
72.100.IN-ADDR.ARPA
73.100.IN-ADDR.ARPA
74.100.IN-ADDR.ARPA
75.100.IN-ADDR.ARPA
76.100.IN-ADDR.ARPA
77.100.IN-ADDR.ARPA
78.100.IN-ADDR.ARPA
79.100.IN-ADDR.ARPA
80.100.IN-ADDR.ARPA
81.100.IN-ADDR.ARPA
82.100.IN-ADDR.ARPA
83.100.IN-ADDR.ARPA
84.100.IN-ADDR.ARPA
85.100.IN-ADDR.ARPA
86.100.IN-ADDR.ARPA
87.100.IN-ADDR.ARPA
88.100.IN-ADDR.ARPA
89.100.IN-ADDR.ARPA
90.100.IN-ADDR.ARPA
91.100.IN-ADDR.ARPA
92.100.IN-ADDR.ARPA
93.100.IN-ADDR.ARPA
94.100.IN-ADDR.ARPA
95.100.IN-ADDR.ARPA
96.100.IN-ADDR.ARPA
97.100.IN-ADDR.ARPA
98.100.IN-ADDR.ARPA
99.100.IN-ADDR.ARPA
100.100.IN-ADDR.ARPA
101.100.IN-ADDR.ARPA
102.100.IN-ADDR.ARPA
103.100.IN-ADDR.ARPA
104.100.IN-ADDR.ARPA
105.100.IN-ADDR.ARPA
106.100.IN-ADDR.ARPA
107.100.IN-ADDR.ARPA
108.100.IN-ADDR.ARPA
109.100.IN-ADDR.ARPA
110.100.IN-ADDR.ARPA
111.100.IN-ADDR.ARPA
112.100.IN-ADDR.ARPA
113.100.IN-ADDR.ARPA
114.100.IN-ADDR.ARPA
115.100.IN-ADDR.ARPA
116.100.IN-ADDR.ARPA
117.100.IN-ADDR.ARPA
118.100.IN-ADDR.ARPA
119.100.IN-ADDR.ARPA
120.100.IN-ADDR.ARPA
121.100.IN-ADDR.ARPA
122.100.IN-ADDR.ARPA
123.100.IN-ADDR.ARPA
124.100.IN-ADDR.ARPA
125.100.IN-ADDR.ARPA
126.100.IN-ADDR.ARPA
127.100.IN-ADDR.ARPA
0.IN-ADDR.ARPA
127.IN-ADDR.ARPA
254.169.IN-ADDR.ARPA
2.0.192.IN-ADDR.ARPA
100.51.198.IN-ADDR.ARPA
113.0.203.IN-ADDR.ARPA
255.255.255.255.IN-ADDR.ARPA
0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.IP6.ARPA
1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.IP6.ARPA
8.B.D.0.1.0.0.2.IP6.ARPA
D.F.IP6.ARPA
8.E.F.IP6.ARPA
9.E.F.IP6.ARPA
A.E.F.IP6.ARPA
B.E.F.IP6.ARPA
EMPTY.AS112.ARPA
HOME.ARPA

Empty zones can be set at the view level and only apply to views of class IN. Disabled empty zones are only inherited from options if there are no disabled empty zones specified at the view level. To override the options list of disabled zones, disable the root zone at the view level. For example:

disable-empty-zone ".";

If using the address ranges covered here, reverse zones covering the addresses should already be in place. In practice this appears to not be the case, with many queries being made to the infrastructure servers for names in these spaces. So many, in fact, that sacrificial servers had to be deployed to channel the query load away from the infrastructure servers.

Note

The real parent servers for these zones should disable all empty zones under the parent zone they serve. For the real root servers, this is all built-in empty zones. This enables them to return referrals to deeper in the tree.

Content Filtering

If a response message is rejected due to the filtering, the entire message is discarded without being cached, and a SERVFAIL error is returned to the client.

This filtering is intended to prevent "DNS rebinding attacks," in which an attacker, in response to a query for a domain name the attacker controls, returns an IP address within the user's own network or an alias name within the user's own domain. A naive web browser or script could then serve as an unintended proxy, allowing the attacker to get access to an internal node of the local network that could not be externally accessed otherwise. See the paper available at https://dl.acm.org/doi/10.1145/1315245.1315298 for more details about these attacks.

For example, with a domain named "example.net" and an internal network using an IPv4 prefix 192.0.2.0/24, an administrator might specify the following rules:

deny-answer-addresses { 192.0.2.0/24; } except-from { "example.net"; };
deny-answer-aliases { "example.net"; };

If an external attacker let a web browser in the local network look up an IPv4 address of "attacker.example.com", the attacker's DNS server would return a response like this:

attacker.example.com. A 192.0.2.1

in the answer section. Since the rdata of this record (the IPv4 address) matches the specified prefix 192.0.2.0/24, this response would be ignored.

On the other hand, if the browser looked up a legitimate internal web server "www.example.net" and the following response were returned to the BIND 9 server:

www.example.net. A 192.0.2.2

it would be accepted, since the owner name "www.example.net" matches the except-from element, "example.net".

Note that this is not really an attack on the DNS per se. In fact, there is nothing wrong with having an "external" name mapped to an "internal" IP address or domain name from the DNS point of view; it might actually be provided for a legitimate purpose, such as for debugging. As long as the mapping is provided by the correct owner, it either is not possible or does not make sense to detect whether the intent of the mapping is legitimate within the DNS. The "rebinding" attack must primarily be protected at the application that uses the DNS. For a large site, however, it may be difficult to protect all possible applications at once. This filtering feature is provided only to help such an operational environment; turning it on is generally discouraged unless there is no other choice and the attack is a real threat to applications.

Care should be particularly taken if using this option for addresses within 127.0.0.0/8. These addresses are obviously "internal," but many applications conventionally rely on a DNS mapping from some name to such an address. Filtering out DNS records containing this address spuriously can break such applications.

Response Policy Zone (RPZ) Rewriting

BIND 9 includes a limited mechanism to modify DNS responses for requests analogous to email anti-spam DNS rejection lists. Responses can be changed to deny the existence of domains (NXDOMAIN), deny the existence of IP addresses for domains (NODATA), or contain other IP addresses or data.

Rules encoded in response policy zones are processed after those defined in access_control. All queries from clients which are not permitted access to the resolver are answered with a status code of REFUSED, regardless of configured RPZ rules.

Five policy triggers can be encoded in RPZ records.

RPZ-CLIENT-IP

IP records are triggered by the IP address of the DNS client. Client IP address triggers are encoded in records that have owner names that are subdomains of rpz-client-ip, relativized to the policy zone origin name, and that encode an address or address block. IPv4 addresses are represented as prefixlength.B4.B3.B2.B1.rpz-client-ip. The IPv4 prefix length must be between 1 and 32. All four bytes - B4, B3, B2, and B1 - must be present. B4 is the decimal value of the least significant byte of the IPv4 address as in IN-ADDR.ARPA.

IPv6 addresses are encoded in a format similar to the standard IPv6 text representation, prefixlength.W8.W7.W6.W5.W4.W3.W2.W1.rpz-client-ip. Each of W8,...,W1 is a one- to four-digit hexadecimal number representing 16 bits of the IPv6 address as in the standard text representation of IPv6 addresses, but reversed as in IP6.ARPA. (Note that this representation of IPv6 addresses is different from IP6.ARPA, where each hex digit occupies a label.) All 8 words must be present except when one set of consecutive zero words is replaced with .zz., analogous to double colons (::) in standard IPv6 text encodings. The IPv6 prefix length must be between 1 and 128.

QNAME

QNAME policy records are triggered by query names of requests and targets of CNAME records resolved to generate the response. The owner name of a QNAME policy record is the query name relativized to the policy zone.

RPZ-IP

IP triggers are IP addresses in an A or AAAA record in the ANSWER section of a response. They are encoded like client-IP triggers, except as subdomains of rpz-ip.

RPZ-NSDNAME

NSDNAME triggers match names of authoritative servers for the query name, a parent of the query name, a CNAME for the query name, or a parent of a CNAME. They are encoded as subdomains of rpz-nsdname, relativized to the RPZ origin name. NSIP triggers match IP addresses in A and AAAA RRsets for domains that can be checked against NSDNAME policy records. The nsdname-enable phrase turns NSDNAME triggers off or on for a single policy zone or for all zones.

If authoritative name servers for the query name are not yet known, named recursively looks up the authoritative servers for the query name before applying an RPZ-NSDNAME rule, which can cause a processing delay. To speed up processing at the cost of precision, the nsdname-wait-recurse option can be used; when set to no, RPZ-NSDNAME rules are only applied when authoritative servers for the query name have already been looked up and cached. If authoritative servers for the query name are not in the cache, the RPZ-NSDNAME rule is ignored, but the authoritative servers for the query name are looked up in the background and the rule is applied to subsequent queries. The default is yes, meaning RPZ-NSDNAME rules are always applied, even if authoritative servers for the query name need to be looked up first.

RPZ-NSIP

NSIP triggers match the IP addresses of authoritative servers. They are encoded like IP triggers, except as subdomains of rpz-nsip. NSDNAME and NSIP triggers are checked only for names with at least min-ns-dots dots. The default value of min-ns-dots is 1, to exclude top-level domains. The nsip-enable phrase turns NSIP triggers off or on for a single policy zone or for all zones.

If a name server's IP address is not yet known, named recursively looks up the IP address before applying an RPZ-NSIP rule, which can cause a processing delay. To speed up processing at the cost of precision, the nsip-wait-recurse option can be used; when set to no, RPZ-NSIP rules are only applied when a name server's IP address has already been looked up and cached. If a server's IP address is not in the cache, the RPZ-NSIP rule is ignored, but the address is looked up in the background and the rule is applied to subsequent queries. The default is yes, meaning RPZ-NSIP rules are always applied, even if an address needs to be looked up first.

The query response is checked against all response policy zones, so two or more policy records can be triggered by a response. Because DNS responses are rewritten according to at most one policy record, a single record encoding an action (other than DISABLED actions) must be chosen. Triggers, or the records that encode them, are chosen for rewriting in the following order:

Choose the triggered record in the zone that appears first in the response-policy option.
Prefer CLIENT-IP to QNAME to IP to NSDNAME to NSIP triggers in a single zone.
Among NSDNAME triggers, prefer the trigger that matches the smallest name under the DNSSEC ordering.
Among IP or NSIP triggers, prefer the trigger with the longest prefix.
Among triggers with the same prefix length, prefer the IP or NSIP trigger that matches the smallest IP address.

When the processing of a response is restarted to resolve DNAME or CNAME records and a policy record set has not been triggered, all response policy zones are again consulted for the DNAME or CNAME names and addresses.

RPZ record sets are any types of DNS record, except DNAME or DNSSEC, that encode actions or responses to individual queries. Any of the policies can be used with any of the triggers. For example, while the TCP-only policy is commonly used with client-IP triggers, it can be used with any type of trigger to force the use of TCP for responses with owner names in a zone.

PASSTHRU

The auto-acceptance policy is specified by a CNAME whose target is rpz-passthru. It causes the response to not be rewritten and is most often used to "poke holes" in policies for CIDR blocks.

DROP

The auto-rejection policy is specified by a CNAME whose target is rpz-drop. It causes the response to be discarded. Nothing is sent to the DNS client.

TCP-Only

The "slip" policy is specified by a CNAME whose target is rpz-tcp-only. It changes UDP responses to short, truncated DNS responses that require the DNS client to try again with TCP. It is used to mitigate distributed DNS reflection attacks.

NXDOMAIN

The "domain undefined" response is encoded by a CNAME whose target is the root domain (.).

NODATA

The empty set of resource records is specified by a CNAME whose target is the wildcard top-level domain (*.). It rewrites the response to NODATA or ANCOUNT=0.

Local Data

A set of ordinary DNS records can be used to answer queries. Queries for record types not in the set are answered with NODATA.

A special form of local data is a CNAME whose target is a wildcard such as *.example.com. It is used as if an ordinary CNAME after the asterisk (*) has been replaced with the query name. This special form is useful for query logging in the walled garden's authoritative DNS server.

All of the actions specified in all of the individual records in a policy zone can be overridden with a policy clause in the response-policy option. An organization using a policy zone provided by another organization might use this mechanism to redirect domains to its own walled garden.

GIVEN: The placeholder policy says "do not override but perform the action specified in the zone."
DISABLED: The testing override policy causes policy zone records to do nothing but log what they would have done if the policy zone were not disabled. The response to the DNS query is written (or not) according to any triggered policy records that are not disabled. Disabled policy zones should appear first, because they are often not logged if a higher-precedence trigger is found first.
PASSTHRU; DROP; TCP-Only; NXDOMAIN; NODATA: These settings each override the corresponding per-record policy.
CNAME domain: This causes all RPZ policy records to act as if they were "cname domain" records.

By default, the actions encoded in a response policy zone are applied only to queries that ask for recursion (RD=1). That default can be changed for a single policy zone, or for all response policy zones in a view, with a recursive-only no clause. This feature is useful for serving the same zone files both inside and outside an 1918 cloud and using RPZ to delete answers that would otherwise contain 1918 values on the externally visible name server or view.

Also by default, RPZ actions are applied only to DNS requests that either do not request DNSSEC metadata (DO=0) or when no DNSSEC records are available for the requested name in the original zone (not the response policy zone). This default can be changed for all response policy zones in a view with a break-dnssec yes clause. In that case, RPZ actions are applied regardless of DNSSEC. The name of the clause option reflects the fact that results rewritten by RPZ actions cannot verify.

No DNS records are needed for a QNAME or Client-IP trigger; the name or IP address itself is sufficient, so in principle the query name need not be recursively resolved. However, not resolving the requested name can leak the fact that response policy rewriting is in use, and that the name is listed in a policy zone, to operators of servers for listed names. To prevent that information leak, by default any recursion needed for a request is done before any policy triggers are considered. Because listed domains often have slow authoritative servers, this behavior can cost significant time. The qname-wait-recurse no option overrides the default and enables that behavior when recursion cannot change a non-error response. The option does not affect QNAME or client-IP triggers in policy zones listed after other zones containing IP, NSIP, and NSDNAME triggers, because those may depend on the A, AAAA, and NS records that would be found during recursive resolution. It also does not affect DNSSEC requests (DO=1) unless break-dnssec yes is in use, because the response would depend on whether RRSIG records were found during resolution. Using this option can cause error responses such as SERVFAIL to appear to be rewritten, since no recursion is being done to discover problems at the authoritative server.

The TTL of a record modified by RPZ policies is set from the TTL of the relevant record in the policy zone. It is then limited to a maximum value. The max-policy-ttl clause changes the maximum number of seconds from its default of 5. For convenience, TTL-style time-unit suffixes may be used to specify the value. It also accepts ISO 8601 duration formats.

For example, an administrator might use this option statement:

response-policy { zone "badlist"; };

and this zone statement:

zone "badlist" {type primary; file "primary/badlist"; allow-query {none;}; };

with this zone file:

$TTL 1H
@                       SOA LOCALHOST. named-mgr.example.com (1 1h 15m 30d 2h)
            NS  LOCALHOST.

; QNAME policy records.  There are no periods (.) after the owner names.
nxdomain.domain.com     CNAME   .               ; NXDOMAIN policy
*.nxdomain.domain.com   CNAME   .               ; NXDOMAIN policy
nodata.domain.com       CNAME   *.              ; NODATA policy
*.nodata.domain.com     CNAME   *.              ; NODATA policy
bad.domain.com          A       10.0.0.1        ; redirect to a walled garden
            AAAA    2001:2::1
bzone.domain.com        CNAME   garden.example.com.

; do not rewrite (PASSTHRU) OK.DOMAIN.COM
ok.domain.com           CNAME   rpz-passthru.

; redirect x.bzone.domain.com to x.bzone.domain.com.garden.example.com
*.bzone.domain.com      CNAME   *.garden.example.com.

; IP policy records that rewrite all responses containing A records in 127/8
;       except 127.0.0.1
8.0.0.0.127.rpz-ip      CNAME   .
32.1.0.0.127.rpz-ip     CNAME   rpz-passthru.

; NSDNAME and NSIP policy records
ns.domain.com.rpz-nsdname   CNAME   .
48.zz.2.2001.rpz-nsip       CNAME   .

; auto-reject and auto-accept some DNS clients
112.zz.2001.rpz-client-ip    CNAME   rpz-drop.
8.0.0.0.127.rpz-client-ip    CNAME   rpz-drop.

; force some DNS clients and responses in the example.com zone to TCP
16.0.0.1.10.rpz-client-ip   CNAME   rpz-tcp-only.
example.com                 CNAME   rpz-tcp-only.
*.example.com               CNAME   rpz-tcp-only.

Response policy zones can be configured to set an Extended DNS Error (EDE) code on the responses which have been modified by the response policy:

response-policy { zone "badlist" ede filtered; };

The following settings are supported for the ede option:

none: No Extended DNS Error code is set (default).
forged: Extended DNS Error code 4 - Forged Answer.
blocked: Extended DNS Error code 15 - Blocked.
censored: Extended DNS Error code 16 - Censored.
filtered: Extended DNS Error code 17 - Filtered.
prohibited: Extended DNS Error code 18 - Prohibited.

See 8914 for more information about the Extended DNS Error codes.

RPZ can affect server performance. Each configured response policy zone requires the server to perform one to four additional database lookups before a query can be answered. For example, a DNS server with four policy zones, each with all four kinds of response triggers (QNAME, IP, NSIP, and NSDNAME), requires a total of 17 times as many database lookups as a similar DNS server with no response policy zones. A BIND 9 server with adequate memory and one response policy zone with QNAME and IP triggers might achieve a maximum queries-per-second (QPS) rate about 20% lower. A server with four response policy zones with QNAME and IP triggers might have a maximum QPS rate about 50% lower.

Responses rewritten by RPZ are counted in the RPZRewrites statistics.

The log clause can be used to optionally turn off rewrite logging for a particular response policy zone. By default, all rewrites are logged.

The add-soa option controls whether the RPZ's SOA record is added to the section for traceback of changes from this zone. This can be set at the individual policy zone level or at the response-policy level. The default is yes.

Updates to RPZ zones are processed asynchronously; if there is more than one update pending they are bundled together. If an update to a RPZ zone (for example, via IXFR) happens less than min-update-interval seconds after the most recent update, the changes are not carried out until this interval has elapsed. The default is 60 seconds. For convenience, TTL-style time-unit suffixes may be used to specify the value. It also accepts ISO 8601 duration formats.

Response Rate Limiting

NXDOMAIN Redirection

named supports NXDOMAIN redirection via two methods:

Redirect zone <type redirect>
Redirect namespace

With either method, when named gets an NXDOMAIN response it examines a separate namespace to see if the NXDOMAIN response should be replaced with an alternative response.

With a redirect zone (zone "." { type redirect; };), the data used to replace the NXDOMAIN is held in a single zone which is not part of the normal namespace. All the redirect information is contained in the zone; there are no delegations.

If both a redirect zone and a redirect namespace are configured, the redirect zone is tried first.

`server` Block Grammar

`server` Block Definition and Usage

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.

It is possible to override the following values defined in view and options blocks:

edns-udp-size

max-udp-size

notify-source-v6

notify-source

provide-ixfr

query-source-v6

query-source

request-expire

request-ixfr

request-nsid

require-cookie

send-cookie

transfer-format

transfer-source-v6

transfer-source

`statistics-channels` Block Grammar

`statistics-channels` Block Definition and Usage

The statistics-channels statement declares communication channels to be used by system administrators to get access to statistics information on the name server.

This statement is intended to be flexible to support multiple communication protocols in the future, but currently only HTTP access is supported. It requires that BIND 9 be compiled with libxml2 and/or json-c (also known as libjson0); the statistics-channels statement is still accepted even if it is built without the library, but any HTTP access fails with an error.

An inet control channel is a TCP socket listening at the specified port on the specified ip_address, which can be an IPv4 or IPv6 address. An ip_address of * (asterisk) is interpreted as the IPv4 wildcard address; connections are accepted on any of the system's IPv4 addresses. To listen on the IPv6 wildcard address, use an ip_address of ::.

If no port is specified, port 80 is used for HTTP channels. The asterisk (*) cannot be used for port.

Attempts to open a statistics channel are restricted by the optional allow clause. Connections to the statistics channel are permitted based on the address_match_list. If no allow clause is present, named accepts connection attempts from any address. Since the statistics may contain sensitive internal information, the source of connection requests must be restricted appropriately so that only trusted parties can access the statistics channel.

Gathering data exposed by the statistics channel locks various subsystems in named, which could slow down query processing if statistics data is requested too often.

An issue in the statistics channel would be considered a security issue only if it could be exploited by unprivileged users circumventing the access control list. In other words, any issue in the statistics channel that could be used to access information unavailable otherwise, or to crash named, is not considered a security issue if it can be avoided through the use of a secure configuration.

If no statistics-channels statement is present, named does not open any communication channels.

The statistics are available in various formats and views, depending on the URI used to access them. For example, if the statistics channel is configured to listen on 127.0.0.1 port 8888, then the statistics are accessible in XML format at http://127.0.0.1:8888/ or http://127.0.0.1:8888/xml. A CSS file is included, which can format the XML statistics into tables when viewed with a stylesheet-capable browser, and into charts and graphs using the Google Charts API when using a JavaScript-capable browser.

Broken-out subsets of the statistics can be viewed at http://127.0.0.1:8888/xml/v3/status (server uptime and last reconfiguration time), http://127.0.0.1:8888/xml/v3/server (server and resolver statistics), http://127.0.0.1:8888/xml/v3/zones (zone statistics), http://127.0.0.1:8888/xml/v3/net (network status and socket statistics), http://127.0.0.1:8888/xml/v3/mem (memory manager statistics), and http://127.0.0.1:8888/xml/v3/traffic (traffic sizes).

The full set of statistics can also be read in JSON format at http://127.0.0.1:8888/json, with the broken-out subsets at http://127.0.0.1:8888/json/v1/status (server uptime and last reconfiguration time), http://127.0.0.1:8888/json/v1/server (server and resolver statistics), http://127.0.0.1:8888/json/v1/zones (zone statistics), http://127.0.0.1:8888/json/v1/net (network status and socket statistics), http://127.0.0.1:8888/json/v1/mem (memory manager statistics), and http://127.0.0.1:8888/json/v1/traffic (traffic sizes).

`tls` Block Grammar

`tls` Block Definition and Usage

The tls statement is used to configure a TLS connection; this configuration can then be referenced by a listen-on or listen-on-v6 statement to cause named to listen for incoming requests via TLS, or in the primaries statement for a zone of type secondary to cause zone transfer requests to be sent via TLS.

tls can only be set at the top level of named.conf.

The following options can be specified in a tls statement:

Warning

TLS configuration is subject to change and incompatible changes might be introduced in the future. Users of TLS are encouraged to carefully read release notes when upgrading.

The options described above are used to control different aspects of TLS functioning. Thus, most of them have no well-defined default values, as these depend on the cryptographic library version in use and system-wide cryptographic policy. On the other hand, by specifying the needed options one could have a uniform configuration deployable across a range of platforms.

An example of privacy-oriented, perfect forward secrecy enabled configuration can be found below. It can be used as a starting point.

tls local-tls {
    key-file "/path/to/key.pem";
    cert-file "/path/to/fullchain_cert.pem";
    dhparam-file "/path/to/dhparam.pem";
    ciphers "HIGH:!kRSA:!aNULL:!eNULL:!RC4:!3DES:!MD5:!EXP:!PSK:!SRP:!DSS:!SHA1:!SHA256:!SHA384";
    prefer-server-ciphers yes;
    session-tickets no;
};

A Diffie-Hellman parameters file can be generated using e.g. OpenSSL, like follows:

openssl dhparam -out /path/to/dhparam.pem <3072_or_4096>

Ensure that it gets generated on a machine with enough entropy from external sources (e.g. the computer you work on should be fine, the remote virtual machine or server might be not). These files do not contain any sensitive data and can be shared if required.

There are two built-in TLS connection configurations: ephemeral, uses a temporary key and certificate created for the current named session only, and none, which can be used when setting up an HTTP listener with no encryption.

BIND supports the following TLS authentication mechanisms described in the RFC 9103, Section 9.3: Opportunistic TLS, Strict TLS, and Mutual TLS.

Opportunistic TLS provides encryption for data but does not provide any authentication for the channel. This mode is the default one and it is used whenever remote-hostname and ca-file options are not set in tls statements in use. RFC 9103 allows optional fallback to clear-text DNS in the cases when TLS is not available. Still, BIND intentionally does not support that in order to protect from unexpected data leaks due to misconfiguration. Both BIND and its complementary tools either successfully establish a secure channel via TLS when instructed to do so or fail to establish a connection otherwise.

Strict TLS provides server authentication via a pre-configured hostname for outgoing connections. This mechanism offers both channel confidentiality and channel authentication (of the server). In order to achieve Strict TLS, one needs to use remote-hostname and, optionally, ca-file options in the tls statements used for establishing outgoing connections (e.g. the ones used to download zone from primaries via TLS). Providing any of the mentioned options will enable server authentication. If remote-hostname is provided but ca-file is missing, then the platform-specific certificate authority certificates are used for authentication. The set roughly corresponds to the one used by WEB-browsers to authenticate HTTPS hosts. On the other hand, if ca-file is provided but remote-hostname is missing, then the remote side's IP address is used instead.

Mutual TLS is an extension to Strict TLS that provides channel confidentiality and mutual channel authentication. It builds up upon the clients offering client certificates when establishing connections and them doing the server authentication as in the case of Strict TLS. The server verifies the provided client certificates and accepts the TLS connection in case of successful verification or rejects it otherwise. In order to instruct the server to require and verify client TLS certificates, one needs to specify the ca-file option in tls configurations used to configure server listeners. The provided file must contain certificate authority certificates used to issue client certificates. In most cases, one should build one's own TLS certificate authority specifically to issue client certificates and include the certificate authority certificate into the file.

For authenticating zone transfers over TLS, Mutual TLS might be considered a standalone solution, while Strict TLS paired with TSIG-based authentication and, optionally, IP-based access lists, might be considered acceptable for most practical purposes. Mutual TLS has the advantage of not requiring TSIG and thus, not having security issues related to shared cryptographic secrets.

`http` Block Grammar

`http` Block Definition and Usage

The http statement is used to configure HTTP endpoints on which to listen for DNS-over-HTTPS (DoH) queries. This configuration can then be referenced by a listen-on or listen-on-v6 statement to cause named to listen for incoming requests over HTTPS.

http can only be set at the top level of named.conf.

The following options can be specified in an http statement:

Any of the options above could be omitted. In such a case, a global value specified in the options statement is used (see http-listener-clients, http-streams-per-connection.

For example, the following configuration enables DNS-over-HTTPS queries on all local addresses:

http local {
    endpoints { "/dns-query"; };
};

options {
    ....
    listen-on tls ephemeral http local { any; };
    listen-on-v6 tls ephemeral http local { any; };
};

`trust-anchors` Block Grammar

`trust-anchors` Block Definition and Usage

The trust-anchors statement defines DNSSEC trust anchors. DNSSEC is described in DNSSEC.

A trust anchor is defined when the public key or public key digest 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 or digest has been configured as a trust anchor, it is treated as if it has been validated and proven secure.

The resolver attempts DNSSEC validation on all DNS data in subdomains of configured trust anchors. Validation below specified names can be temporarily disabled by using rndc nta, or permanently disabled with the validate-except option.

All keys listed in trust-anchors, and their corresponding zones, are deemed to exist regardless of what parent zones say. Only keys configured as trust anchors are used to validate the DNSKEY RRset for the corresponding name. The parent's DS RRset is not used.

trust-anchors may be set at the top level of named.conf or within a view. If it is set in both places, the configurations are additive; keys defined at the top level are inherited by all views, but keys defined in a view are only used within that view.

The trust-anchors statement can contain multiple trust-anchor entries, each consisting of a domain name, followed by an "anchor type" keyword indicating the trust anchor's format, followed by the key or digest data.

If the anchor type is static-key or initial-key, then it is followed with the key's flags, protocol, and algorithm, plus the Base64 representation of the public key data. This is identical to the text representation of a DNSKEY record. Spaces, tabs, newlines, and carriage returns are ignored in the key data, so the configuration may be split into multiple lines.

If the anchor type is static-ds or initial-ds, it is followed with the key tag, algorithm, digest type, and the hexadecimal representation of the key digest. This is identical to the text representation of a DS record. Spaces, tabs, newlines, and carriage returns are ignored.

Trust anchors configured with the static-key or static-ds anchor types are immutable, while keys configured with initial-key or initial-ds can be kept up-to-date automatically, without intervention from the resolver operator. (static-key keys are identical to keys configured using the deprecated trusted-keys statement.)

Suppose, for example, that a zone's key-signing key was compromised, and the zone owner had to revoke and replace the key. A resolver which had the original key configured using static-key or static-ds would be unable to validate this zone any longer; it would reply with a SERVFAIL response code. This would continue until the resolver operator had updated the trust-anchors statement with the new key.

If, however, the trust anchor had been configured using initial-key or initial-ds instead, the zone owner could add a "stand-by" key to the zone in advance. named would store the stand-by key, and when the original key was revoked, named would be able to transition smoothly to the new key. It would also recognize that the old key had been revoked and cease using that key to validate answers, minimizing the damage that the compromised key could do. This is the process used to keep the ICANN root DNSSEC key up-to-date.

Whereas static-key and static-ds trust anchors continue to be trusted until they are removed from named.conf, an initial-key or initial-ds is only trusted once: for as long as it takes to load the managed key database and start the 5011 key maintenance process.

It is not possible to mix static with initial trust anchors for the same domain name.

The first time named runs with an initial-key or initial-ds configured in named.conf, it fetches the DNSKEY RRset directly from the zone apex, and validates it using the trust anchor specified in trust-anchors. If the DNSKEY RRset is validly signed by a key matching the trust anchor, then it is used as the basis for a new managed-keys database.

From that point on, whenever named runs, it sees the initial-key or initial-ds listed in trust-anchors, checks to make sure 5011 key maintenance has already been initialized for the specified domain, and if so, simply moves on. The key specified in the trust-anchors statement is not used to validate answers; it is superseded by the key or keys stored in the managed-keys database.

The next time named runs after an initial-key or initial-ds has been removed from the trust-anchors statement (or changed to a static-key or static-ds), the corresponding zone is removed from the managed-keys database, and 5011 key maintenance is no longer used for that domain.

In the current implementation, the managed-keys database is stored as a master-format zone file.

On servers which do not use views, this file is named managed-keys.bind. When views are in use, there is a separate managed-keys database for each view; the filename is the view name (or, if a view name contains characters which would make it illegal as a filename, a hash of the view name), followed by the suffix .mkeys.

When the key database is changed, the zone is updated. As with any other dynamic zone, changes are written into a journal file, e.g., managed-keys.bind.jnl or internal.mkeys.jnl. Changes are committed to the primary file as soon as possible afterward, usually within 30 seconds. Whenever named is using automatic key maintenance, the zone file and journal file can be expected to exist in the working directory. (For this reason, among others, the working directory should be always be writable by named.)

If the dnssec-validation option is set to auto, named automatically sets up an initial-key for the root zone. This initializing key is built in to named, and is current as of the release date. When the root zone key changes, a running server will detect the change and roll to the new key, but newly-installed servers being run for the first time will need to be from a recent enough version of BIND to have been built with the current key.

`dnssec-policy` Block Grammar

`dnssec-policy` Block Definition and Usage

The dnssec-policy statement defines a key and signing policy (KASP) for zones.

A KASP determines how one or more zones are signed with DNSSEC. For example, it specifies how often keys should roll, which cryptographic algorithms to use, and how often RRSIG records need to be refreshed. Multiple key and signing policies can be configured with unique policy names.

A policy for a zone is selected using a dnssec-policy statement in the zone block, specifying the name of the policy that should be used.

There are three built-in policies:

default, which uses the default policy <dnssec_policy_default>,
insecure, to be used when you want to gracefully unsign your zone,
none, which means no DNSSEC policy (the same as not selecting dnssec-policy at all; the zone is not signed.)

Keys are not shared among zones, which means that one set of keys per zone is generated even if they have the same policy. If multiple views are configured with different versions of the same zone, each separate version uses the same set of signing keys.

The dnssec-policy statement requires dynamic DNS to be set up, or inline-signing to be enabled.

If inline-signing is enabled, this means that a signed version of the zone is maintained separately and is written out to a different file on disk (the zone's filename plus a .signed extension).

If the zone is dynamic because it is configured with an update-policy or allow-update, the DNSSEC records are written to the filename set in the original zone's file, unless inline-signing is explicitly set.

Key rollover timing is computed for each key according to the key lifetime defined in the KASP. The lifetime may be modified by zone TTLs and propagation delays, to prevent validation failures. When a key reaches the end of its lifetime, named generates and publishes a new key automatically, then deactivates the old key and activates the new one; finally, the old key is retired according to a computed schedule.

Zone-signing key (ZSK) rollovers require no operator input. Key-signing key (KSK) and combined-signing key (CSK) rollovers require action to be taken to submit a DS record to the parent. Rollover timing for KSKs and CSKs is adjusted to take into account delays in processing and propagating DS updates.

Policy default causes the zone to be signed with a single combined-signing key (CSK) using algorithm ECDSAP256SHA256; this key has an unlimited lifetime. (A verbose copy of this policy may be found in the source tree, in the file doc/misc/dnssec-policy.default.conf.)

Note

The default signing policy may change in future releases. This could require changes to a signing policy when upgrading to a new version of BIND. Check the release notes carefully when upgrading to be informed of such changes. To prevent policy changes on upgrade, use an explicitly defined dnssec-policy, rather than default.

If a dnssec-policy statement is modified and the server restarted or reconfigured, named attempts to change the policy smoothly from the old one to the new. For example, if the key algorithm is changed, then a new key is generated with the new algorithm, and the old algorithm is retired when the existing key's lifetime ends.

Note

Rolling to a new policy while another key rollover is already in progress is not yet supported, and may result in unexpected behavior.

The following options can be specified in a dnssec-policy statement:

keys

This is a list specifying the algorithms and roles to use when generating keys and signing the zone. Entries in this list do not represent specific DNSSEC keys, which may be changed on a regular basis, but the roles that keys play in the signing policy. For example, configuring a KSK of algorithm RSASHA256 ensures that the DNSKEY RRset always includes a key-signing key for that algorithm.

Here is an example (for illustration purposes only) of some possible entries in a keys list:

keys {
    ksk key-directory lifetime unlimited algorithm rsasha256 2048;
    zsk lifetime P30D algorithm 8;
    csk lifetime P6MT12H3M15S algorithm ecdsa256;
};

This example specifies that three keys should be used in the zone. The first token determines which role the key plays in signing RRsets. If set to ksk, then this is a key-signing key; it has the KSK flag set and is only used to sign DNSKEY, CDS, and CDNSKEY RRsets. If set to zsk, this is a zone-signing key; the KSK flag is unset, and the key signs all RRsets except DNSKEY, CDS, and CDNSKEY. If set to csk, the key has the KSK flag set and is used to sign all RRsets.

An optional second token determines where the key is stored. Currently, keys can only be stored in the configured key-directory. This token may be used in the future to store keys in hardware security modules or separate directories.

The lifetime parameter specifies how long a key may be used before rolling over. In the example above, the first key has an unlimited lifetime, the second key may be used for 30 days, and the third key has a rather peculiar lifetime of 6 months, 12 hours, 3 minutes, and 15 seconds. A lifetime of 0 seconds is the same as unlimited.

Note that the lifetime of a key may be extended if retiring it too soon would cause validation failures. The key lifetime must be longer than the time it takes to do a rollover; that is, the lifetime must be more than the publication interval (which is the sum of dnskey-ttl, publish-safety, and zone-propagation-delay). It must also be more than the retire interval (which is the sum of max-zone-ttl, retire-safety and zone-propagation-delay for ZSKs, and the sum of parent-ds-ttl, retire-safety, and parent-propagation-delay for KSKs and CSKs). BIND 9 treats a key lifetime that is too short as an error.

The algorithm parameter specifies the key's algorithm, expressed either as a string ("rsasha256", "ecdsa384", etc.) or as a decimal number. An optional second parameter specifies the key's size in bits. If it is omitted, as shown in the example for the second and third keys, an appropriate default size for the algorithm is used. Each KSK/ZSK pair must have the same algorithm. A CSK combines the functionality of a ZSK and a KSK.

Automated KSK Rollovers

BIND has mechanisms in place to facilitate automated KSK rollovers. It publishes CDS and CDNSKEY records that can be used by the parent zone to publish or withdraw the zone's DS records. BIND will query the parental agents to see if the new DS is actually published before withdrawing the old DNSSEC key.

Note

The DS response is not validated so it is recommended to set up a trust relationship with the parental agent. For example, use TSIG to authenticate the parental agent, or point to a validating resolver.

The following options apply to DS queries sent to parental-agents:

`managed-keys` Block Grammar

`managed-keys` Block Definition and Usage

The managed-keys statement has been deprecated in favor of trust-anchors with the initial-key keyword.

`trusted-keys` Block Grammar

`trusted-keys` Block Definition and Usage

The trusted-keys statement has been deprecated in favor of trust-anchors with the static-key keyword.

`view` Block Grammar

view view_name [ class ] {
    match-clients { address_match_list } ;
    match-destinations { address_match_list } ;
    match-recursive-only <boolean> ;
  [ view_option ; ... ]
  [ zone_statement ; ... ]
} ;

`view` Block Definition and Usage

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.

Zones defined within a view statement are only 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 applies 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 primary;
        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 primary;
       file "example-external.db";
      };
};

`zone` Block Grammar

`zone` Block Definition and Usage

Zone Types

Class

The zone's name may optionally be followed by a class. If a class is not specified, class IN (for Internet) is assumed. This is correct for the vast majority of cases.

The hesiod class is named for an information service from MIT's Project Athena. It was used to share information about various systems databases, such as users, groups, printers, and so on. The keyword HS is a synonym for hesiod.

Another MIT development is Chaosnet, a LAN protocol created in the mid-1970s. Zone data for it can be specified with the CHAOS class.

Zone Options

allow-notify: See the description of allow-notify in access_control.
allow-query: See the description of allow-query in access_control.
allow-query-on: See the description of allow-query-on in access_control.
allow-transfer: See the description of allow-transfer in access_control.
allow-update: See the description of allow-update in access_control.
update-policy: This specifies a "Simple Secure Update" policy. See dynamic_update_policies.
allow-update-forwarding: See the description of allow-update-forwarding in access_control.
also-notify: This option is only meaningful if notify is active for this zone. The set of machines that receive a DNS NOTIFY message for this zone is made up of all the listed name servers (other than the primary) for the zone, plus any IP addresses specified with also-notify. A port may be specified with each also-notify address to send the notify messages to a port other than the default of 53. A TSIG key may also be specified to cause the NOTIFY to be signed by the given key. also-notify is not meaningful for stub zones. The default is the empty list.
check-names: This option is used to restrict the character set and syntax of certain domain names in primary files and/or DNS responses received from the network. The default varies according to zone type. For primary <type primary> zones the default is fail; for secondary <type secondary> zones the default is warn. It is not implemented for hint <type hint> zones.
check-mx: See the description of check-mx in boolean_options.
check-spf: See the description of check-spf in boolean_options.
check-wildcard: See the description of check-wildcard in boolean_options.
check-integrity: See the description of check-integrity in boolean_options.
check-sibling: See the description of check-sibling in boolean_options.
zero-no-soa-ttl: See the description of zero-no-soa-ttl in boolean_options.
update-check-ksk: See the description of update-check-ksk in boolean_options.
dnssec-loadkeys-interval: See the description of dnssec-loadkeys-interval in options.
dnssec-update-mode: See the description of dnssec-update-mode in options.
dnssec-dnskey-kskonly: See the description of dnssec-dnskey-kskonly in boolean_options.
try-tcp-refresh: See the description of try-tcp-refresh in boolean_options.

dialup: See the description of dialup in boolean_options.

forward: This option is only meaningful if the zone has a forwarders list. The only value causes the lookup to fail after trying the forwarders and getting no answer, while first allows a normal lookup to be tried.
forwarders: This is used to override the list of global forwarders. If it is not specified in a zone of type forward, no forwarding is done for the zone and the global options are not used.

max-ixfr-ratio: See the description of max-ixfr-ratio in options.
max-journal-size: See the description of max-journal-size in server_resource_limits.
max-records: See the description of max-records in server_resource_limits.
max-transfer-time-in: See the description of max-transfer-time-in in zone_transfers.
max-transfer-idle-in: See the description of max-transfer-idle-in in zone_transfers.
max-transfer-time-out: See the description of max-transfer-time-out in zone_transfers.
max-transfer-idle-out: See the description of max-transfer-idle-out in zone_transfers.
notify: See the description of notify in boolean_options.
notify-delay: See the description of notify-delay in tuning.
notify-to-soa: See the description of notify-to-soa in boolean_options.
zone-statistics: See the description of zone-statistics in options.

sig-validity-interval: See the description of sig-validity-interval in tuning.
sig-signing-nodes: See the description of sig-signing-nodes in tuning.
sig-signing-signatures: See the description of sig-signing-signatures in tuning.
sig-signing-type: See the description of sig-signing-type in tuning.
transfer-source: See the description of transfer-source in zone_transfers.
transfer-source-v6: See the description of transfer-source-v6 in zone_transfers.
notify-source: See the description of notify-source in zone_transfers.
notify-source-v6: See the description of notify-source-v6 in zone_transfers.
min-refresh-time; max-refresh-time; min-retry-time; max-retry-time: See the descriptions in tuning.
ixfr-from-differences: See the description of ixfr-from-differences in boolean_options. (Note that the ixfr-from-differences choices of primary <type primary> and secondary <type secondary> are not available at the zone level.)
key-directory: See the description of key-directory in options.
auto-dnssec: See the description of auto-dnssec in options.
serial-update-method: See the description of serial-update-method in options.

multi-master: See the description of multi-master in boolean_options.
masterfile-format: See the description of masterfile-format in tuning.
max-zone-ttl: See the description of max-zone-ttl in options. The use of this option in zone blocks is deprecated and will be rendered nonoperational in a future release.

Dynamic Update Policies

BIND 9 supports two methods of granting clients the right to perform dynamic updates to a zone:

allow-update - a simple access control list
update-policy - fine-grained access control

In both cases, BIND 9 writes the updates to the zone's filename set in file.

In the case of a DNSSEC zone, DNSSEC records are also written to the zone's filename, unless inline-signing is enabled.

Note

The zone file can no longer be manually updated while named is running; it is now necessary to perform rndc freeze, edit, and then perform rndc thaw. Comments and formatting in the zone file are lost when dynamic updates occur.

Multiple Views

When multiple views are in use, a zone may be referenced by more than one of them. Often, the views contain different zones with the same name, allowing different clients to receive different answers for the same queries. At times, however, it is desirable for multiple views to contain identical zones. The in-view zone option provides an efficient way to do this; it allows a view to reference a zone that was defined in a previously configured view. For example:

view internal {
    match-clients { 10/8; };

    zone example.com {
    type primary;
    file "example-external.db";
    };
};

view external {
    match-clients { any; };

    zone example.com {
    in-view internal;
    };
};

An in-view option cannot refer to a view that is configured later in the configuration file.

A zone statement which uses the in-view option may not use any other options, with the exception of forward and forwarders. (These options control the behavior of the containing view, rather than change the zone object itself.)

Zone-level ACLs (e.g., allow-query, allow-transfer), and other configuration details of the zone, are all set in the view the referenced zone is defined in. Be careful to ensure that ACLs are wide enough for all views referencing the zone.

An in-view zone cannot be used as a response policy zone.

An in-view zone is not intended to reference a forward zone.

Statements

BIND 9 supports many hundreds of statements; finding the right statement to control a specific behavior or solve a particular problem can be a daunting task. To simplify the task for users, all statements have been assigned one or more tags. Tags are designed to group together statements that have broadly similar functionality; thus, for example, all statements that control the handling of queries or of zone transfers are respectively tagged under query and transfer.

dnssec_tag_statements are those that relate to or control DNSSEC.

logging_tag_statements relate to or control logging, and typically only appear in a logging block.

query_tag_statements relate to or control queries.

security_tag_statements relate to or control security features.

server_tag_statements relate to or control server behavior, and typically only appear in a server block.

transfer_tag_statements relate to or control zone transfers.

view_tag_statements relate to or control view selection criteria, and typically only appear in a view block.

zone_tag_statements relate to or control zone behavior, and typically only appear in a zone block.

deprecated_tag_statements are those that are now deprecated, but are included here for historical reference.

The following table lists all statements permissible in named.conf, with their associated tags; the next section groups the statements by tag. Please note that these sections are a work in progress.

Statements by Tag

These tables group the various statements permissible in named.conf by their corresponding tag.

DNSSEC Tag Statements

Logging Tag Statements

Query Tag Statements

Security Tag Statements

Server Tag Statements

Transfer Tag Statements

View Tag Statements

Zone Tag Statements

Deprecated Tag Statements

BIND 9 Statistics

BIND 9 maintains lots of statistics information and provides several interfaces for users to access those statistics. The available statistics include all statistics counters that are meaningful in BIND 9, and other information that is considered useful.

The statistics information is categorized into the following sections:

Incoming Requests

The number of incoming DNS requests for each OPCODE.

Incoming Queries

The number of incoming queries for each RR type.

Outgoing Queries

The number of outgoing queries for each RR type sent from the internal resolver, maintained per view.

Name Server Statistics

Statistics counters for incoming request processing.

Zone Maintenance Statistics

Statistics counters regarding zone maintenance operations, such as zone transfers.

Resolver Statistics

Statistics counters for name resolutions performed in the internal resolver, maintained per view.

Cache DB RRsets

Statistics counters related to cache contents, maintained per view.

The "NXDOMAIN" counter is the number of names that have been cached as nonexistent. Counters named for RR types indicate the number of active RRsets for each type in the cache database.

If an RR type name is preceded by an exclamation point (!), it represents the number of records in the cache which indicate that the type does not exist for a particular name; this is also known as "NXRRSET". If an RR type name is preceded by a hash mark (#), it represents the number of RRsets for this type that are present in the cache but whose TTLs have expired; these RRsets may only be used if stale answers are enabled. If an RR type name is preceded by a tilde (~), it represents the number of RRsets for this type that are present in the cache database but are marked for garbage collection; these RRsets cannot be used.

Socket I/O Statistics

Statistics counters for network-related events.

A subset of Name Server Statistics is collected and shown per zone for which the server has the authority, when zone-statistics is set to full (or yes), for backward compatibility. See the description of zone-statistics in options for further details.

These statistics counters are shown with their zone and view names. The view name is omitted when the server is not configured with explicit views.

There are currently two user interfaces to get access to the statistics. One is in plain-text format, dumped to the file specified by the statistics-file configuration option; the other is remotely accessible via a statistics channel when the statistics-channels statement is specified in the configuration file.

The Statistics File

The text format statistics dump begins with a line, like:

+++ Statistics Dump +++ (973798949)

The number in parentheses is a standard Unix-style timestamp, measured in seconds since January 1, 1970. Following that line is a set of statistics information, which is categorized as described above. Each section begins with a line, like:

++ Name Server Statistics ++

Each section consists of lines, each containing the statistics counter value followed by its textual description; see below for available counters. For brevity, counters that have a value of 0 are not shown in the statistics file.

The statistics dump ends with the line where the number is identical to the number in the beginning line; for example:

--- Statistics Dump --- (973798949)

Statistics Counters

The following lists summarize the statistics counters that BIND 9 provides. For each counter, the abbreviated symbol name is given; these symbols are shown in the statistics information accessed via an HTTP statistics channel. The description of the counter is also shown in the statistics file but, in this document, may be slightly modified for better readability.

Name Server Statistics Counters

Requestv4: This indicates the number of IPv4 requests received. Note: this also counts non-query requests.
Requestv6: This indicates the number of IPv6 requests received. Note: this also counts non-query requests.
ReqEdns0: This indicates the number of requests received with EDNS(0).
ReqBadEDN SVer: This indicates the number of requests received with an unsupported EDNS version.
ReqTSIG: This indicates the number of requests received with TSIG.
ReqSIG0: This indicates the number of requests received with SIG(0).
ReqBadSIG: This indicates the number of requests received with an invalid (TSIG or SIG(0)) signature.
ReqTCP: This indicates the number of TCP requests received.
AuthQryRej: This indicates the number of rejected authoritative (non-recursive) queries.
RecQryRej: This indicates the number of rejected recursive queries.
XfrRej: This indicates the number of rejected zone transfer requests.
UpdateRej: This indicates the number of rejected dynamic update requests.
Response: This indicates the number of responses sent.
RespTruncated: This indicates the number of truncated responses sent.
RespEDNS0: This indicates the number of responses sent with EDNS(0).
RespTSIG: This indicates the number of responses sent with TSIG.
RespSIG0: This indicates the number of responses sent with SIG(0).
QrySuccess: This indicates the number of queries that resulted in a successful answer, meaning queries which return a NOERROR response with at least one answer RR. This corresponds to the success counter of previous versions of BIND 9.
QryAuthAns: This indicates the number of queries that resulted in an authoritative answer.
QryNoauthAns: This indicates the number of queries that resulted in a non-authoritative answer.
QryReferral: This indicates the number of queries that resulted in a referral answer. This corresponds to the referral counter of previous versions of BIND 9.
QryNxrrset: This indicates the number of queries that resulted in NOERROR responses with no data. This corresponds to the nxrrset counter of previous versions of BIND 9.
QrySERVFAIL: This indicates the number of queries that resulted in SERVFAIL.
QryFORMERR: This indicates the number of queries that resulted in FORMERR.
QryNXDOMAIN: This indicates the number of queries that resulted in NXDOMAIN. This corresponds to the nxdomain counter of previous versions of BIND 9.
QryRecursion: This indicates the number of queries that caused the server to perform recursion in order to find the final answer. This corresponds to the recursion counter of previous versions of BIND 9.
QryDuplicate: This indicates the number of queries which the server attempted to recurse but for which it discovered an existing query with the same IP address, port, query ID, name, type, and class already being processed. This corresponds to the duplicate counter of previous versions of BIND 9.
QryDropped: This indicates the number of recursive queries dropped by the server as a result of configured limits. These limits include the settings of the fetches-per-zone, fetches-per-server, clients-per-query, and max-clients-per-query options, as well as the rate-limit option. This corresponds to the dropped counter of previous versions of BIND 9.
QryFailure: This indicates the number of query failures. This corresponds to the failure counter of previous versions of BIND 9. Note: this counter is provided mainly for backward compatibility with previous versions; normally, more fine-grained counters such as AuthQryRej and RecQryRej that would also fall into this counter are provided, so this counter is not of much interest in practice.
QryNXRedir: This indicates the number of queries that resulted in NXDOMAIN that were redirected.
QryNXRedirRLookup: This indicates the number of queries that resulted in NXDOMAIN that were redirected and resulted in a successful remote lookup.
XfrReqDone: This indicates the number of requested and completed zone transfers.
UpdateReqFwd: This indicates the number of forwarded update requests.
UpdateRespFwd: This indicates the number of forwarded update responses.
UpdateFwdFail: This indicates the number of forwarded dynamic updates that failed.
UpdateDone: This indicates the number of completed dynamic updates.
UpdateFail: This indicates the number of failed dynamic updates.
UpdateBadPrereq: This indicates the number of dynamic updates rejected due to a prerequisite failure.
UpdateQuota: This indicates the number of times a dynamic update or update forwarding request was rejected because the number of pending requests exceeded update-quota.
RateDropped: This indicates the number of responses dropped due to rate limits.
RateSlipped: This indicates the number of responses truncated by rate limits.
RPZRewrites: This indicates the number of response policy zone rewrites.

Zone Maintenance Statistics Counters

NotifyOutv4: This indicates the number of IPv4 notifies sent.
NotifyOutv6: This indicates the number of IPv6 notifies sent.
NotifyInv4: This indicates the number of IPv4 notifies received.
NotifyInv6: This indicates the number of IPv6 notifies received.
NotifyRej: This indicates the number of incoming notifies rejected.
SOAOutv4: This indicates the number of IPv4 SOA queries sent.
SOAOutv6: This indicates the number of IPv6 SOA queries sent.
AXFRReqv4: This indicates the number of requested IPv4 AXFRs.
AXFRReqv6: This indicates the number of requested IPv6 AXFRs.
IXFRReqv4: This indicates the number of requested IPv4 IXFRs.
IXFRReqv6: This indicates the number of requested IPv6 IXFRs.
XfrSuccess: This indicates the number of successful zone transfer requests.
XfrFail: This indicates the number of failed zone transfer requests.

Resolver Statistics Counters

Queryv4: This indicates the number of IPv4 queries sent.
Queryv6: This indicates the number of IPv6 queries sent.
Responsev4: This indicates the number of IPv4 responses received.
Responsev6: This indicates the number of IPv6 responses received.
NXDOMAIN: This indicates the number of NXDOMAINs received.
SERVFAIL: This indicates the number of SERVFAILs received.
FORMERR: This indicates the number of FORMERRs received.
OtherError: This indicates the number of other errors received.
EDNS0Fail: This indicates the number of EDNS(0) query failures.
Mismatch: This indicates the number of mismatched responses received, meaning the DNS ID, response's source address, and/or the response's source port does not match what was expected. (The port must be 53 or as defined by the port option.) This may be an indication of a cache poisoning attempt.
Truncated: This indicates the number of truncated responses received.
Lame: This indicates the number of lame delegations received.
Retry: This indicates the number of query retries performed.
QueryAbort: This indicates the number of queries aborted due to quota control.
QuerySockFail: This indicates the number of failures in opening query sockets. One common reason for such failures is due to a limitation on file descriptors.
QueryCurUDP: This indicates the number of UDP queries in progress.
QueryCurTCP: This indicates the number of TCP queries in progress.
QueryTimeout: This indicates the number of query timeouts.
GlueFetchv4: This indicates the number of IPv4 NS address fetches invoked.
GlueFetchv6: This indicates the number of IPv6 NS address fetches invoked.
GlueFetchv4Fail: This indicates the number of failed IPv4 NS address fetches.
GlueFetchv6Fail: This indicates the number of failed IPv6 NS address fetches.
ValAttempt: This indicates the number of attempted DNSSEC validations.
ValOk: This indicates the number of successful DNSSEC validations.
ValNegOk: This indicates the number of successful DNSSEC validations on negative information.
ValFail: This indicates the number of failed DNSSEC validations.
QryRTTnn: This provides a frequency table on query round-trip times (RTTs). Each nn specifies the corresponding frequency. In the sequence of nn_1, nn_2, ..., nn_m, the value of nn_i is the number of queries whose RTTs are between nn_(i-1) (inclusive) and nn_i (exclusive) milliseconds. For the sake of convenience, we define nn_0 to be 0. The last entry should be represented as nn_m+, which means the number of queries whose RTTs are equal to or greater than nn_m milliseconds.
NumFetch: This indicates the number of active fetches.
BucketSize: This indicates the number the resolver's internal buckets (a static number).
REFUSED: This indicates the number of REFUSED responses received.
ClientCookieOut: This indicates the number of COOKIE sent with client cookie only.
ServerCookieOut: This indicates the number of COOKIE sent with client and server cookie.
CookieIn: This indicates the number of COOKIE replies received.
CookieClientOk: This indicates the number of COOKIE client ok.
BadEDNSVersion: This indicates the number of bad EDNS version replies received.
BadCookieRcode: This indicates the number of bad cookie rcode replies received.
ZoneQuota: This indicates the number of queries spilled due to zone quota.
ServerQuota: This indicates the number of queries spilled due to server quota.
ClientQuota: This indicates the number of queries spilled due to clients per query quota.
NextItem: This indicates the number of waits for next item, when an invalid response is received.
Priming: This indicates the number of priming fetches performed by the resolver.

Socket I/O Statistics Counters

Socket I/O statistics counters are defined per socket type, which are UDP4 (UDP/IPv4), UDP6 (UDP/IPv6), TCP4 (TCP/IPv4), TCP6 (TCP/IPv6), Unix (Unix Domain), and FDwatch (sockets opened outside the socket module). In the following list, <TYPE> represents a socket type. Not all counters are available for all socket types; exceptions are noted in the descriptions.

<TYPE>Open: This indicates the number of sockets opened successfully. This counter does not apply to the FDwatch type.
<TYPE>OpenFail: This indicates the number of failures to open sockets. This counter does not apply to the FDwatch type.
<TYPE>Close: This indicates the number of closed sockets.
<TYPE>BindFail: This indicates the number of failures to bind sockets.
<TYPE>ConnFail: This indicates the number of failures to connect sockets.
<TYPE>Conn: This indicates the number of connections established successfully.
<TYPE>AcceptFail: This indicates the number of failures to accept incoming connection requests. This counter does not apply to the UDP and FDwatch types.
<TYPE>Accept: This indicates the number of incoming connections successfully accepted. This counter does not apply to the UDP and FDwatch types.
<TYPE>SendErr: This indicates the number of errors in socket send operations.
<TYPE>RecvErr: This indicates the number of errors in socket receive operations, including errors of send operations on a connected UDP socket, notified by an ICMP error message.

356 KiB Raw Blame History

Configuration Reference

Configuration File (named.conf)

Comment Syntax

Syntax

Definition and Usage

Configuration Layout Styles

include Directive

include Directive Definition and Usage

Address Match Lists

Syntax

Definition and Usage

Glossary of Terms Used

Blocks

acl Block Grammar

acl Block Definition and Usage

controls Block Grammar

controls Block Definition and Usage

key Block Grammar

key Block Definition and Usage

logging Block Grammar

logging Block Definition and Usage

The channel Phrase

The category Phrase

The query-errors Category

parental-agents Block Grammar

parental-agents Block Definition and Usage

primaries Block Grammar

primaries Block Definition and Usage

options Block Grammar

options Block Definition and Usage

Boolean Options

Forwarding

Dual-stack Servers

Access Control

Interfaces

Query Address

Zone Transfers

Server Resource Limits

Periodic Task Intervals

The sortlist Statement

RRset Ordering

Tuning

Built-in Server Information Zones

Built-in Empty Zones

Content Filtering

Response Policy Zone (RPZ) Rewriting

Response Rate Limiting

NXDOMAIN Redirection

server Block Grammar

server Block Definition and Usage

statistics-channels Block Grammar

statistics-channels Block Definition and Usage

tls Block Grammar

tls Block Definition and Usage

http Block Grammar

http Block Definition and Usage

trust-anchors Block Grammar

trust-anchors Block Definition and Usage

dnssec-policy Block Grammar

dnssec-policy Block Definition and Usage

Automated KSK Rollovers

managed-keys Block Grammar

managed-keys Block Definition and Usage

trusted-keys Block Grammar

trusted-keys Block Definition and Usage

view Block Grammar

view Block Definition and Usage

zone Block Grammar

zone Block Definition and Usage

Zone Types

Class

Zone Options

Dynamic Update Policies

Multiple Views

Statements

Statements by Tag

DNSSEC Tag Statements

Logging Tag Statements

Query Tag Statements

356 KiB

Raw Blame History

`include` Directive

`include` Directive Definition and Usage

`acl` Block Grammar

`acl` Block Definition and Usage

`controls` Block Grammar

`controls` Block Definition and Usage

`key` Block Grammar

`key` Block Definition and Usage

`logging` Block Grammar

`logging` Block Definition and Usage

The `channel` Phrase

The `category` Phrase

The `query-errors` Category

`parental-agents` Block Grammar

`parental-agents` Block Definition and Usage

`primaries` Block Grammar

`primaries` Block Definition and Usage

`options` Block Grammar

`options` Block Definition and Usage

The `sortlist` Statement

`server` Block Grammar

`server` Block Definition and Usage

`statistics-channels` Block Grammar

`statistics-channels` Block Definition and Usage

`tls` Block Grammar

`tls` Block Definition and Usage

`http` Block Grammar

`http` Block Definition and Usage

`trust-anchors` Block Grammar

`trust-anchors` Block Definition and Usage

`dnssec-policy` Block Grammar

`dnssec-policy` Block Definition and Usage

`managed-keys` Block Grammar

`managed-keys` Block Definition and Usage

`trusted-keys` Block Grammar

`trusted-keys` Block Definition and Usage

`view` Block Grammar

`view` Block Definition and Usage

`zone` Block Grammar

`zone` Block Definition and Usage