In this section we provide some suggested configurations along with guidelines for their use. We also address the topic of reasonable option setting.
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.
// Two corporate subnets we wish to allow queries from.
acl "corpnets" { 192.168.4.0/24; 192.168.7.0/24; };
options {
directory "/etc/namedb"; // Working directory
pid-file "named.pid"; // Put pid file in working dir
allow-query { "corpnets"; };
};
// Root server hints
zone "." { type hint; file "root.hint"; };
// Provide a reverse mapping for the loopback address 127.0.0.1
zone "0.0.127.in-addr.arpa" {
type master;
file "localhost.rev";
notify no;
};
This sample configuration is for an authoritative-only server that is the master server for "example.com" and a slave for the subdomain "eng.example.com".
options {
directory "/etc/namedb"; // Working directory
pid-file "named.pid"; // Put pid file in working dir
allow-query { any; }; // This is the default
recursion no; // Do not provide recursive service
};
// Root server hints
zone "." { type hint; file "root.hint"; };
// Provide a reverse mapping for the loopback address 127.0.0.1
zone "0.0.127.in-addr.arpa" {
type master;
file "localhost.rev";
notify no;
};
// We are the master server for example.com
zone "example.com" {
type master;
file "example.com.db";
// IP addresses of slave servers allowed to transfer example.com
allow-transfer {
192.168.4.14;
192.168.5.53;
};
};
// We are a slave server for eng.example.com
zone "eng.example.com" {
type slave;
file "eng.example.com.bk";
// IP address of eng.example.com master server
masters { 192.168.4.12; };
};
Primitive load balancing can be achieved in DNS using multiple A records for one name.
For example, if you have three WWW servers with network addresses of 10.0.0.1, 10.0.0.2 and 10.0.0.3, a set of records such as the following means that clients will connect to each machine one third of the time:
When a resolver queries for these records, BIND will rotate them and respond to the query with the records in a different order. In the example above, clients will randomly receive records in the order 1, 2, 3; 2, 3, 1; and 3, 1, 2. Most clients will use the first record returned and discard the rest.
For more detail on ordering responses, check the rrset-order substatement in the options statement, see RRset Ordering. This substatement is not supported in BIND 9, and only the ordering scheme described above is available.
DNS Notify is a mechanism that allows master nameservers to notify their slave servers of changes to a zone's data. In response to a NOTIFY from a master server, the slave will check to see that its version of the zone is the current version and, if not, initiate a transfer.
DNS Notify is fully documented in RFC 1996. See also the description of the zone option also-notify, see Section 6.2.14.6. For more information about notify, see Section 6.2.14.1.
There are several indispensable diagnostic, administrative and monitoring tools available to the system administrator for controlling and debugging the nameserver daemon. We describe several in this section
The domain information groper (dig) is a command line tool that can be used to gather information from the Domain Name System servers. Dig has two modes: simple interactive mode for a single query, and batch mode which executes a query for each in a list of several query lines. All query options are accessible from the command line.
dig [@server] domain [query-type] [query-class] [+query-option] [-dig-option] [%comment]
The usual simple use of dig will take the form
dig @server domain query-type query-class
For more information and a list of available commands and options, see the dig man page.
The host utility provides a simple DNS lookup using a command-line interface for looking up Internet hostnames. By default, the utility converts between host names and Internet addresses, but its functionality can be extended with the use of options.
host [-aCdlrTwv] [-c class] [-N ndots] [-t type] [-W timeout] [-R retries] hostname [server]
For more information and a list of available commands and options, see the host man page.
nslookup is a program used to query Internet domain nameservers. nslookup has two modes: interactive and non-interactive. Interactive mode allows the user to query nameservers for information about various hosts and domains or to print a list of hosts in a domain. Non-interactive mode is used to print just the name and requested information for a host or domain.
nslookup [-option...] [host-to-find | - [server]]
Interactive mode is entered when no arguments are given (the default nameserver will be used) or when the first argument is a hyphen (`-') and the second argument is the host name or Internet address of a nameserver.
Non-interactive mode is used when the name or Internet address of the host to be looked up is given as the first argument. The optional second argument specifies the host name or address of a nameserver.
Due to its arcane user interface and frequently inconsistent behavior, we do not recommend the use of nslookup. Use dig instead.
Administrative tools play an integral part in the management of a server.
The named-checkconf program checks the syntax of a named.conf file.
named-checkconf [-t directory] [filename]
The named-checkzone program checks a master file for syntax and consistency.
named-checkzone [-dq] [-c class] zone [filename]
The remote name daemon control (rndc) program allows the system administrator to control the operation of a nameserver. If you run rndc without any options it will display a usage message as follows:
rndc [-c config] [-s server] [-p port] [-y key] command [command...]
command is one of the following:
Reload configuration file and zones.
Reload the given zone.
Schedule zone maintenance for the given zone.
Reload the configuration file and load new zones, but do not reload existing zone files even if they have changed. This is faster than a full reload when there is a large number of zones because it avoids the need to examine the modification times of the zones files.
Write server statistics to the statistics file.
Toggle query logging. Query logging can also be enabled by explictly directing the queries category to a channel in the logging section of named.conf.
Dump the server's caches to the dump file.
Stop the server, making sure any recent changes made through dynamic update or IXFR are first saved to the master files of the updated zones.
Stop the server immediately. Recent changes made through dynamic update or IXFR are not saved to the master files, but will be rolled forward from the journal files when the server is restarted.
Increment the servers debugging level by one.
Sets the server's debugging level to an explicit value.
Sets the server's debugging level to 0.
Flushes the server's cache.
Display status of the server. Note the number of zones includes the internal bind/CH zone and the default ./IN hint zone if there is not a explicit root zone configured.
In BIND 9.2, rndc supports all the commands of the BIND 8 ndc utility except ndc start, which was also not supported in ndc's channel mode.
A configuration file is required, since all communication with the server is authenticated with digital signatures that rely on a shared secret, and there is no way to provide that secret other than with a configuration file. The default location for the rndc configuration file is /etc/rndc.conf, but an alternate location can be specified with the -c option. If the configuration file is not found, rndc will also look in /etc/rndc.key (or whatever sysconfdir was defined when the BIND build was configured). The rndc.key file is generated by running rndc-confgen -a as described in Section 6.2.4.
The format of the configuration file is similar to that of named.conf, but limited to only four statements, the options, key, server and include statements. These statements are what associate the secret keys to the servers with which they are meant to be shared. The order of statements is not significant.
The options statement has three clauses: default-server, default-key, and default-port. default-server takes a host name or address argument and represents the server that will be contacted if no -s option is provided on the command line. default-key takes the name of 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 names a key with its string argument. The string is required by the server to be a valid domain name, though it need not actually be hierarchical; thus, a string like "rndc_key" is a valid name. The key statement has two clauses: algorithm and secret. While the configuration parser will accept any string as the argument to algorithm, currently only the string "hmac-md5" has any meaning. The secret is a base-64 encoded string.
The server statement uses the key clause to associate a key-defined key with a server. The argument to the server statement is a host name or address (addresses must be double quoted). The argument to the key clause is the name of the key as defined by the key statement. The port clause can be used to specify the port to which rndc should connect on the given server.
A sample minimal configuration file is as follows:
key rndc_key {
algorithm "hmac-md5";
secret "c3Ryb25nIGVub3VnaCBmb3IgYSBtYW4gYnV0IG1hZGUgZm9yIGEgd29tYW4K";
};
options {
default-server localhost;
default-key rndc_key;
};
This file, if installed as /etc/rndc.conf, would allow the command:
$ rndc reload
to connect to 127.0.0.1 port 953 and cause the nameserver to reload, if a nameserver on the local machine were running with following controls statements:
controls {
inet 127.0.0.1 allow { localhost; } keys { rndc_key; };
};
and it had an identical key statement for rndc_key.
Running the rndc-confgen program will conveniently create a rndc.conf file for you, and also display the corresponding controls statement that you need to add to named.conf. Alternatively, you can run rndc-confgen -a to set up a rndc.key file and not modify named.conf at all.
Certain UNIX signals cause the name server to take specific actions, as described in the following table. These signals can be sent using the kill command.