Table of Contents
In this chapter we provide some suggested configurations along with guidelines for their use. We suggest reasonable values for certain option settings.
The following sample configuration is appropriate for a caching-only name server for use by clients internal to a corporation. All queries from outside clients are refused using the allow-query option. Alternatively, the same effect could be achieved using suitable firewall rules.
// Two corporate subnets we wish to allow queries from.
acl corpnets { 192.168.4.0/24; 192.168.7.0/24; };
options {
// 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 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 {
// 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 master;
file "localhost.rev";
notify no;
};
// We are the master server for example.com
zone "example.com" {
type master;
file "example.com.db";
// IP addresses of slave servers allowed to
// transfer example.com
allow-transfer {
192.168.4.14;
192.168.5.53;
};
};
// We are a slave server for eng.example.com
zone "eng.example.com" {
type slave;
file "eng.example.com.bk";
// IP address of eng.example.com master server
masters { 192.168.4.12; };
};
A primitive form of load balancing can be achieved in the DNS by using multiple records (such as multiple A records) for one name.
For example, if you have 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 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
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 sub-statement in the options statement, see RRset Ordering.
This section describes several indispensable diagnostic, administrative and monitoring tools available to the system administrator for controlling and debugging the name server daemon.
The dig, host, and nslookup programs are all command line tools for manually querying name servers. They differ in style and output format.
dig is the most versatile and complete of these lookup tools. It has two modes: simple interactive mode for a single query, and batch mode which executes a query for each in a list of several query lines. All query options are accessible from the command line.
dig
[@server]
domain
[query-type]
[query-class]
[+query-option]
[-dig-option]
[%comment]
The usual simple use of dig will take the form
dig @server domain query-type query-class
For more information and a list of available commands and options, see the dig man page.
The host utility emphasizes simplicity and ease of use. By default, it converts between host names and Internet addresses, but its functionality can be extended with the use of options.
host
[-aCdlnrsTwv]
[-c class]
[-N ndots]
[-t type]
[-W timeout]
[-R retries]
[-m flag]
[-4]
[-6]
hostname
[server]
For more information and a list of available commands and options, see the host man page.
nslookup has two modes: interactive and non-interactive. Interactive mode allows the user to query name servers for information about various hosts and domains or to print a list of hosts in a domain. Non-interactive mode is used to print just the name and requested information for a host or domain.
nslookup
[-option...]
[
[host-to-find]
| [- [server]]
]
Interactive mode is entered when no arguments are given (the default name server will be used) or when the first argument is a hyphen (`-') and the second argument is the host name or Internet address of a name server.
Non-interactive mode is used when the name or Internet address of the host to be looked up is given as the first argument. The optional second argument specifies the host name or address of a name server.
Due to its arcane user interface and frequently inconsistent behavior, we do not recommend the use of nslookup. Use dig instead.
Administrative tools play an integral part in the management of a server.
The named-checkconf program
checks the syntax of a named.conf file.
named-checkconf
[-jvz]
[-t directory]
[filename]
The named-checkzone program checks a master file for syntax and consistency.
named-checkzone
[-djqvD]
[-c class]
[-o output]
[-t directory]
[-w directory]
[-k (ignore|warn|fail)]
[-n (ignore|warn|fail)]
[-W (ignore|warn)]
zone
[filename]
Similar to named-checkzone, but it always dumps the zone content to a specified file (typically in a different format).
The remote name daemon control (rndc) program allows the system administrator to control the operation of a name server. If you run rndc without any options, it will display a usage message as follows:
rndc
[-c config]
[-s server]
[-p port]
[-y key]
command
[command...]
See rndc(8) 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 will also look in
/etc/rndc.key (or whatever
sysconfdir was defined when
the BIND build was
configured).
The rndc.key file is
generated by
running rndc-confgen -a as
described in
the section called “controls Statement Definition and
Usage”.
The format of the configuration file is similar to
that of named.conf, but
limited to
only four statements, the options,
key, server and
include
statements. These statements are what associate the
secret keys to the servers with which they are meant to
be shared. The order of statements is not
significant.
The options statement has
three clauses:
default-server, default-key,
and default-port.
default-server takes a
host name or address argument and represents the server
that will
be contacted if no -s
option is provided on the command line.
default-key takes
the name of a key as its argument, as defined by a key statement.
default-port specifies the
port to which
rndc should connect if no
port is given on the command line or in a
server statement.
The key statement defines a
key to be used
by rndc when authenticating
with
named. Its syntax is
identical to the
key statement in named.conf.
The keyword key is
followed by a key name, which must be a valid
domain name, though it need not actually be hierarchical;
thus,
a string like "rndc_key" is a valid
name.
The key statement has two
clauses:
algorithm and secret.
While the configuration parser will accept any string as the
argument
to algorithm, currently only the 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,
would allow the command:
$ rndc reload
to connect to 127.0.0.1 port 953 and cause the name server to reload, if a name server on the local machine were running with following controls statements:
controls {
inet 127.0.0.1
allow { localhost; } keys { rndc_key; };
};
and it had an identical key statement for
rndc_key.
Running the rndc-confgen
program will
conveniently create a rndc.conf
file for you, and also display the
corresponding controls
statement that you need to
add to named.conf.
Alternatively,
you can run rndc-confgen -a
to set up
a rndc.key file and not
modify
named.conf at all.
Certain UNIX signals cause the name server to take specific actions, as described in the following table. These signals can be sent using the kill command.
|
SIGHUP |
Causes the server to read |
|
SIGTERM |
Causes the server to clean up and exit. |
|
SIGINT |
Causes the server to clean up and exit. |
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.
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 will differ depending on the module.
Each plugin implements four functions:
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" is called -
this is a function within the plugin library. 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.
BIND 9.17.0 (Development Release)