8.2. Configuring the Resolver
configured in the
/etc/resolv.conf file. The resolver is not a separate and
distinct process; it is a library of routines called by network
processes. The resolv.conf file is read when a
process using the resolver starts, and is cached for the life of that
process. If the configuration file is not found, the resolver
attempts to connect to the named server running on
the local host. While this may work, I don't recommend it. By
allowing the resolver configuration to default, you give up control
over your system and become vulnerable to variations in the
techniques used by different systems to determine the default
configuration. For these reasons, the resolver configuration file
should be created on every system running BIND.
8.2.1. The Resolver Configuration File
The configuration file clearly documents the resolver configuration.
It allows you to identify up to three name servers, two of which
provide backup if the first server doesn't respond. It defines
the default domain and various other processing options. The
resolv.conf file is a
critical part of configuring name service.
resolv.conf is a simple, human-readable file.
There are system-specific variations in the commands used in the
file, but the entries supported by most systems are:
- nameserver address
The nameserver entries identify, by IP
address, the servers that the resolver is to query for domain
information. The name servers are queried in the order that they
appear in the file. If no response is received from a server, the
next server in the list is tried until the maximum number of servers
are tried. If no
nameserver entries are contained in the
resolv.conf file or if no
resolv.conf file exists, all queries are sent to
the local host. However, if there is a
resolv.conf file and it contains
nameserver entries, the local host is
not queried unless an entry points to it.
Specify the local host with its official IP address or with 0.0.0.0,
not with the loopback address. The official address avoids problems
seen on some versions of Unix when the loopback address is used. A
resolver-only configuration never contains a
nameserver entry that points to the local host.
- domain name
The domain entry defines the default
domain name. The resolver appends the default domain name to any
hostname that does not contain a dot. It then uses the expanded hostname
in the query it sends to the name server. For example, if the
hostname crab (which does not contain a dot) is
received by the resolver, the default domain name is appended to
crab to construct the query. If the value for
name in the domain
entry is wrotethebook.com, the resolver queries
for crab.wrotethebook.com. If the environment
LOCALDOMAIN is set, it overrides
the domain entry, and the value of LOCALDOMAIN is
used to expand the hostname.
- search domain ...
The search entry defines a series of
domains that is searched when a hostname does not contain a dot.
Assume the entry search
essex.wrotethebook.com butler.wrotethebook.com. A
query for the hostname cookbook is first tried
as cookbook.essex.wrotethebook.com. If that
fails to provide a successful match, the resolver queries for
cookbook.butler.wrotethebook.com. If that query
fails, no other attempts are made to resolve the hostname. Use either
a search statement or a domain
statement. (The search command is preferred.)
Never use both in the same configuration. If the environment variable
LOCALDOMAIN is set, it overrides the search entry.
- sortlist network[/netmask] ...
Addresses from the networks listed on the
sortlist command are preferred over
other addresses. If the resolver receives multiple addresses in
response to a query about a multi-homed host or a router, it reorders
the addresses so that an address from a network listed in the
sortlist statement is placed in front of the other
addresses. Normally addresses are returned to the application by the
resolver in the order in which they are received.
The sortlist command is rarely used because it
interferes with the servers' ability to reorder addresses for
load balancing and other purposes. The primary exception to this is
that sometimes sortlist is configured to prefer
addresses on a shared network over other addresses. Using this
configuration, if the computer running the resolver is connected to
network 172.16.0.0/16 and one of the addresses returned in a multiple
address response is from that network, the address from 172.16.0.0 is
placed in front of the other addresses.
- options option ...
The options entry is used to select
optional settings for the resolver. There are several possible
Turns on debugging, which prints debugging messages to standard
output. debug works only if the resolver was
compiled with the -DDEBUG option, and most weren't.
Sets the number of dots in a hostname used to determine whether or
not the search list is applied before sending the query to the name
server. The default is 1. Therefore a hostname with one dot does not
have a domain appended before it is sent to the name server. If
options ndots:2 is specified, a
hostname with one dot does have the search list domain added before
the query is sent out, but a hostname with two or more dots does not
have a domain added.
ndots may be useful for you if some component of
your domain could be confused with a top-level domain and your users
consistently truncate hostnames at that domain. In that case, the
queries would first be sent to the root servers for resolution in the
top-level domain before eventually getting back to your local server.
It is very bad form to bother the root servers over nothing. Use
ndots to force the resolver to extend the
troublesome hostnames with your local domain name so that they will
be resolved before reaching the root servers.
Sets the initial query timeout for the resolver. By default, the
timeout is 5 seconds for the first query to every server. Under the
Solaris 8 version of BIND, the syntax of this option is
Defines the number of times the resolver will retry a query. The
default value is 2, which means the resolver will retry a query two
times with every server in its server list before returning an error
to the application. Under the Solaris 8 version of BIND, the syntax
of this option is
retry:n, and the
default is 4.
Turns on round-robin selection of name servers. Normally, the
resolver sends the query to the first server in the name server list,
sending it to another server only if the first server does not
respond. The rotate option tells the resolver to
share the name server workload evenly among all of the servers.
Disables checking of domain names for compliance with RFC 952,
DOD Internet Host Table Specification. By
default, domain names that contain an underscore (_), non-ASCII
characters, or ASCII control characters are considered to be in
error. Use this option if you must work with hostnames that contain
Causes the resolver to query for IPv6 addresses. The version of the
Internet Protocol (IP) used in today's Internet is IPv4. IPv4
uses 32-bit addresses. IPv6 expands those to 128-bit addresses.
The most common
configuration defines the local domain name as the search list, the
local host as the first name server, and one or two backup name
servers. An example of this configuration is:
# Domain name resolver configuration file
# try yourself first
# try crab next
# finally try ora
The example is based on our imaginary network, so the default domain
name is wrotethebook.com. The configuration is
for rodent, and it specifies itself as the first
name server. The backup servers are crab and
ora. The configuration does not contain a sort
list or any options, as these are infrequently used. This is an
example of an average resolver configuration.
184.108.40.206. A resolver-only configuration
configuration is very simple. It is identical to the average
configuration except that it does not contain a
nameserver entry for the local system. A sample
resolv.conf file for a resolver-only system is
# Domain name resolver configuration file
# try crab
# next try ora
The configuration tells the resolver to pass all queries to
crab; if that fails, try
ora. Queries are never resolved locally. This
simple resolv.conf file is all that is required
for a resolver-only configuration.
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