RESOLV.CONF(5) Linux Programmer's Manual RESOLV.CONF(5)
NAME
resolv.conf - resolver configuration file
SYNOPSIS
/etc/resolv.conf
DESCRIPTION
The ________ is a set of routines in the C library that provide access
to the Internet Domain Name System (DNS). The resolver configuration
file contains information that is read by the resolver routines the
first time they are invoked by a process. The file is designed to be
human readable and contains a list of keywords with values that provide
various types of resolver information.
If this file doesn't exist the only name server to be queried will be
on the local machine; the domain name is determined from the host name
and the domain search path is constructed from the domain name.
The different configuration options are:
nameserver Name server IP address
Internet address (in dot notation) of a name server that the
resolver should query. Up to MAXNS (currently 3, see
__________) name servers may be listed, one per keyword. If
there are multiple servers, the resolver library queries them in
the order listed. If no nameserver entries are present, the
default is to use the name server on the local machine. (The
algorithm used is to try a name server, and if the query times
out, try the next, until out of name servers, then repeat trying
all the name servers until a maximum number of retries are
made.)
domain Local domain name.
Most queries for names within this domain can use short names
relative to the local domain. If no domain entry is present,
the domain is determined from the local host name returned by
gethostname(3); the domain part is taken to be everything after
the first '.'. Finally, if the host name does not contain a
domain part, the root domain is assumed.
search Search list for host-name lookup.
The search list is normally determined from the local domain
name; by default, it contains only the local domain name. This
may be changed by listing the desired domain search path follow-
ing the ______ keyword with spaces or tabs separating the names.
Resolver queries having fewer than _____ dots (default is 1) in
them will be attempted using each component of the search path
in turn until a match is found. For environments with multiple
subdomains please read options ndots:_ below to avoid man-in-
the-middle attacks and unnecessary traffic for the root-dns-
servers. Note that this process may be slow and will generate a
lot of network traffic if the servers for the listed domains are
not local, and that queries will time out if no server is avail-
able for one of the domains.
The search list is currently limited to six domains with a total
of 256 characters.
sortlist
Sortlist allows addresses returned by gethostbyname(3) to be
sorted. A sortlist is specified by IP address netmask pairs.
The netmask is optional and defaults to the natural netmask of
the net. The IP address and optional network pairs are sepa-
rated by slashes. Up to 10 pairs may be specified. E.g.,
sortlist 130.155.160.0/255.255.240.0 130.155.0.0
options
Options allows certain internal resolver variables to be modi-
fied. The syntax is
options ______ ___
where ______ is one of the following:
debug sets RES_DEBUG in ____________.
ndots:_
sets a threshold for the number of dots which must appear
in a name given to res_query(3) (see resolver(3)) before
an _______ ________ _____ will be made. The default for
_ is 1, meaning that if there are any dots in a name, the
name will be tried first as an absolute name before any
______ ____ elements are appended to it.
timeout:_
sets the amount of time the resolver will wait for a
response from a remote name server before retrying the
query via a different name server. Measured in seconds,
the default is RES_TIMEOUT (currently 5, see __________).
attempts:_
sets the number of times the resolver will send a query
to its name servers before giving up and returning an
error to the calling application. The default is
RES_DFLRETRY (currently 2, see __________).
rotate sets RES_ROTATE in ____________, which causes round robin
selection of nameservers from among those listed. This
has the effect of spreading the query load among all
listed servers, rather than having all clients try the
first listed server first every time.
no-check-names
sets RES_NOCHECKNAME in ____________, which disables the
modern BIND checking of incoming host names and mail
names for invalid characters such as underscore (_), non-
ASCII, or control characters.
inet6 sets RES_USE_INET6 in ____________. This has the effect
of trying a AAAA query before an A query inside the geth-
ostbyname(3) function, and of mapping IPv4 responses in
IPv6 "tunneled form" if no AAAA records are found but an
A record set exists.
Some programs behave strangely when this option is turned
on.
The ______ and ______ keywords are mutually exclusive. If more than
one instance of these keywords is present, the last instance wins.
The ______ keyword of a system's ___________ file can be overridden on
a per-process basis by setting the environment variable LOCALDOMAIN to
a space-separated list of search domains.
The _______ keyword of a system's ___________ file can be amended on a
per-process basis by setting the environment variable RES_OPTIONS to a
space-separated list of resolver options as explained above under
options.
The keyword and value must appear on a single line, and the keyword
(e.g., nameserver) must start the line. The value follows the keyword,
separated by white space.
FILES
________________, __________
SEE ALSO
gethostbyname(3), resolver(3), hostname(7), named(8)
Name Server Operations Guide for BIND
COLOPHON
This page is part of release 2.77 of the Linux _________ project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
4th Berkeley Distribution 2004-10-31 RESOLV.CONF(5)