3. Name Server Configuration

In this chapter we provide some suggested configurations, along with guidelines for their use. We suggest reasonable values for certain option settings.

3.1. Sample Configurations

3.1.1. A Caching-only Name Server

The following sample configuration is appropriate for a caching-only name server for use by clients internal to a corporation. All queries from outside clients are refused using the allow-query option. The same effect can be achieved using suitable firewall rules.

// Two corporate subnets we wish to allow queries from.
acl corpnets { 192.168.4.0/24; 192.168.7.0/24; };
options {
     // Working directory
     directory "/etc/namedb";

     allow-query { corpnets; };
};
// Provide a reverse mapping for the loopback
// address 127.0.0.1
zone "0.0.127.in-addr.arpa" {
     type primary;
     file "localhost.rev";
     notify no;
};

3.1.2. An Authoritative-only Name Server

This sample configuration is for an authoritative-only server that is the primary server for example.com and a secondary server for the subdomain eng.example.com.

options {
     // Working directory
     directory "/etc/namedb";
     // Do not allow access to cache
     allow-query-cache { none; };
     // This is the default
     allow-query { any; };
     // Do not provide recursive service
     recursion no;
};

// Provide a reverse mapping for the loopback
// address 127.0.0.1
zone "0.0.127.in-addr.arpa" {
     type primary;
     file "localhost.rev";
     notify no;
};
// We are the primary server for example.com
zone "example.com" {
     type primary;
     file "example.com.db";
     // IP addresses of secondary servers allowed to
     // transfer example.com
     allow-transfer {
      192.168.4.14;
      192.168.5.53;
     };
};
// We are a secondary server for eng.example.com
zone "eng.example.com" {
     type secondary;
     file "eng.example.com.bk";
     // IP address of eng.example.com primary server
     masters { 192.168.4.12; };
};

3.2. Load Balancing

A primitive form of load balancing can be achieved in the DNS by using multiple records (such as multiple A records) for one name.

For example, assuming three HTTP servers with network addresses of 10.0.0.1, 10.0.0.2, and 10.0.0.3, a set of records such as the following means that clients will connect to each machine one-third of the time:

Name

TTL

CLASS

TYPE

Resource Record (RR) Data

www

600

IN

A

10.0.0.1

600

IN

A

10.0.0.2

600

IN

A

10.0.0.3

When a resolver queries for these records, BIND rotates them and responds to the query with the records in a different order. In the example above, clients randomly receive records in the order 1, 2, 3; 2, 3, 1; and 3, 1, 2. Most clients use the first record returned and discard the rest.

For more detail on ordering responses, check the rrset-order sub-statement in the options statement; see RRset Ordering.

3.3. Name Server Operations

3.3.1. Tools for Use With the Name Server Daemon

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

3.3.1.1. Diagnostic Tools

The dig, host, and nslookup programs are all command-line tools for manually querying name servers. They differ in style and output format.

dig

dig is the most versatile and complete of these lookup tools. It has two modes: simple interactive mode for a single query, and batch mode, which executes a query for each in a list of several query lines. All query options are accessible from the command line.

For more information and a list of available commands and options, see dig - DNS lookup utility.

host

The host utility emphasizes simplicity and ease of use. By default, it converts between host names and Internet addresses, but its functionality can be extended with the use of options.

For more information and a list of available commands and options, see host - DNS lookup utility.

nslookup

nslookup has two modes: interactive and non-interactive. Interactive mode allows the user to query name servers for information about various hosts and domains, or to print a list of hosts in a domain. Non-interactive mode is used to print just the name and requested information for a host or domain.

Due to its arcane user interface and frequently inconsistent behavior, we do not recommend the use of nslookup. Use dig instead.

3.3.1.2. Administrative Tools

Administrative tools play an integral part in the management of a server.

named-checkconf

The named-checkconf program checks the syntax of a named.conf file.

For more information and a list of available commands and options, see named-checkconf - named configuration file syntax checking tool.

named-checkzone

The named-checkzone program checks a zone file for syntax and consistency.

For more information and a list of available commands and options, see named-checkzone - zone file validation tool.

named-compilezone

This tool is similar to named-checkzone but it always dumps the zone content to a specified file (typically in a different format).

For more information and a list of available commands and options, see named-compilezone - zone file converting tool.

rndc

The remote name daemon control (rndc) program allows the system administrator to control the operation of a name server.

See rndc - name server control utility for details of the available rndc commands.

rndc requires a configuration file, since all communication with the server is authenticated with digital signatures that rely on a shared secret, and there is no way to provide that secret other than with a configuration file. The default location for the rndc configuration file is /etc/rndc.conf, but an alternate location can be specified with the -c option. If the configuration file is not found, rndc also looks in /etc/rndc.key (or whatever sysconfdir was defined when the BIND build was configured). The rndc.key file is generated by running rndc-confgen -a as described in controls Statement Definition and Usage.

The format of the configuration file is similar to that of named.conf, but is limited to only four statements: the options, key, server, and include statements. These statements are what associate the secret keys to the servers with which they are meant to be shared. The order of statements is not significant.

The options statement has three clauses: default-server, default-key, and default-port. default-server takes a host name or address argument and represents the server that is contacted if no -s option is provided on the command line. default-key takes the name of a key as its argument, as defined by a key statement. default-port specifies the port to which rndc should connect if no port is given on the command line or in a server statement.

The key statement defines a key to be used by rndc when authenticating with named. Its syntax is identical to the key statement in named.conf. The keyword key is followed by a key name, which must be a valid domain name, though it need not actually be hierarchical; thus, a string like rndc_key is a valid name. The key statement has two clauses: algorithm and secret. While the configuration parser accepts any string as the argument to algorithm, currently only the strings hmac-md5, hmac-sha1, hmac-sha224, hmac-sha256, hmac-sha384, and hmac-sha512 have any meaning. The secret is a Base64-encoded string as specified in RFC 3548.

The server statement associates a key defined using the key statement with a server. The keyword server is followed by a host name or address. The server statement has two clauses: key and port. The key clause specifies the name of the key to be used when communicating with this server, and the port clause can be used to specify the port rndc should connect to on the server.

A sample minimal configuration file is as follows:

key rndc_key {
     algorithm "hmac-sha256";
     secret
       "c3Ryb25nIGVub3VnaCBmb3IgYSBtYW4gYnV0IG1hZGUgZm9yIGEgd29tYW4K";
};
options {
     default-server 127.0.0.1;
     default-key    rndc_key;
};

This file, if installed as /etc/rndc.conf, allows the command:

$ rndc reload

to connect to 127.0.0.1 port 953 and causes the name server to reload, if a name server on the local machine is running with the following controls statements:

controls {
    inet 127.0.0.1
        allow { localhost; } keys { rndc_key; };
};

and it has an identical key statement for rndc_key.

Running the rndc-confgen program conveniently creates an rndc.conf file, and also displays the corresponding controls statement needed to add to named.conf. Alternatively, it is possible to run rndc-confgen -a to set up an rndc.key file and not modify named.conf at all.

3.3.2. Signals

Certain Unix signals cause the name server to take specific actions, as described in the following table. These signals can be sent using the kill command.

SIGHUP

Causes the server to read named.conf and reload the database.

SIGTERM

Causes the server to clean up and exit.

SIGINT

Causes the server to clean up and exit.

3.4. Plugins

Plugins are a mechanism to extend the functionality of named using dynamically loadable libraries. By using plugins, core server functionality can be kept simple for the majority of users; more complex code implementing optional features need only be installed by users that need those features.

The plugin interface is a work in progress, and is expected to evolve as more plugins are added. Currently, only “query plugins” are supported; these modify the name server query logic. Other plugin types may be added in the future.

The only plugin currently included in BIND is filter-aaaa.so, which replaces the filter-aaaa feature that previously existed natively as part of named. The code for this feature has been removed from named and can no longer be configured using standard named.conf syntax, but linking in the filter-aaaa.so plugin provides identical functionality.

3.4.1. Configuring Plugins

A plugin is configured with the plugin statement in named.conf:

plugin query "library.so" {
    parameters
};

In this example, file library.so is the plugin library. query indicates that this is a query plugin.

Multiple plugin statements can be specified, to load different plugins or multiple instances of the same plugin.

parameters are passed as an opaque string to the plugin’s initialization routine. Configuration syntax differs depending on the module.

3.4.2. Developing Plugins

Each plugin implements four functions:

  • plugin_register to allocate memory, configure a plugin instance, and attach to hook points within named ,

  • plugin_destroy to tear down the plugin instance and free memory,

  • plugin_version to check that the plugin is compatible with the current version of the plugin API,

  • plugin_check to test syntactic correctness of the plugin parameters.

At various locations within the named source code, there are “hook points” at which a plugin may register itself. When a hook point is reached while named is running, it is checked to see whether any plugins have registered themselves there; if so, the associated “hook action” - a function within the plugin library - is called. Hook actions may examine the runtime state and make changes: for example, modifying the answers to be sent back to a client or forcing a query to be aborted. More details can be found in the file lib/ns/include/ns/hooks.h.