RESOLV.CONF(4)RESOLV.CONF(4)NAMEresolv.conf - resolver configuration file
SYNOPSIS
/etc/resolv.conf
DESCRIPTION
The resolver is a set of routines that provide access to the Internet
Domain Name System. See resolver(3RESOLV). resolv.conf is a configura‐
tion file that contains the 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 val‐
ues that provide various types of resolver information.
The resolv.conf file contains the following configuration directives:
nameserver
Specifies the IPv4 or IPv6 Internet address of a
name server that the resolver is to query. Up to
MAXNS name servers may be listed, one per key‐
word. See <resolv.h>. If there are multiple
servers, the resolver library queries them in
the order listed. If no name server entries are
present, the resolver library queries the name
server on the local machine. The resolver
library follows the algorithm to try a name
server until the query times out. It then tries
the name servers that follow, until each query
times out. It repeats all the name servers until
a maximum number of retries are made.
domain
Specifies the 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 sys‐
info(2) or from gethostname(3C). (Everything
after the first `.' is presumed to be the domain
name.) If the host name does not contain a
domain part, the root domain is assumed. You can
use the LOCALDOMAIN environment variable to
override the domain name.
search
The 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. You can change the default
behavior by listing the desired domain search
path following the search keyword, with spaces
or tabs separating the names. Most resolver
queries will be attempted using each component
of the search path in turn until a match is
found. This process may be slow and will gener‐
ate a lot of network traffic if the servers for
the listed domains are not local. Queries will
time out if no server is available for one of
the domains.
The search list is currently limited to six
domains and a total of 256 characters.
sortlistaddresslist
Allows addresses returned by the libresolv-
internal gethostbyname() 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 separated by
slashes. Up to 10 pairs may be specified. For
example:
sortlist 130.155.160.0/255.255.240.0 130.155.0.0
options
Allows certain internal resolver variables to be
modified. The syntax is
options option ...
where option is one of the following:
debug
Sets RES_DEBUG in the
_res.options field.
ndots:n
Sets a threshold floor for the
number of dots which must
appear in a name given to
res_query() before an initial
absolute (as-is) query is per‐
formed. See resolver(3RESOLV).
The default value for n is 1,
which means that if there are
any dots in a name, the name
is tried first as an absolute
name before any search list
elements are appended to it.
timeout:n
retrans:n
Sets the amount of time the
resolver will wait for a
response from a remote name
server before retrying the
query by means of a different
name server. Measured in sec‐
onds, the default is RES_TIME‐
OUT. See <resolv.h>. The
timeout and retrans values are
the starting point for an
exponential back off procedure
where the timeout is doubled
for every retransmit attempt.
attempts:n
retry:n
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. See
<resolv.h>.
rotate
Sets RES_ROTATE in
_res.options. The name servers
are queried round-robin from
among those listed. The query
load is spread among all
listed servers, rather than
having all clients try the
first listed server first
every time.
no-check-names
Sets RES_NOCHECKNAME in
_res.options. This 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
_res.options. In the Solaris
BIND port, this has no effect
on gethostbyname(3NSL). To
retrieve IPv6 addresses or
IPv4 addresses, use getad‐
drinfo(3SOCKET) instead of
setting inet6.
The domain and search keywords are mutually exclusive. If more than one
instance of these keywords is present, the last instance takes prece‐
dence.
You can override the search keyword of the system resolv.conf file on a
per-process basis by setting the environment variable LOCALDOMAIN to a
space-separated list of search domains.
You can amend the options keyword of the system resolv.conf file on a
per-process basis by setting the environment variable RES_OPTIONS to a
space-separated list of resolver options.
The keyword and value must appear on a single line. Start the line with
the keyword, for example, nameserver, followed by the value, separated
by white space.
FILES
/etc/resolv.conf
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
┌────────────────────┬─────────────────────┐
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├────────────────────┼─────────────────────┤
│Interface Stability │ Standard BIND 8.3.3 │
└────────────────────┴─────────────────────┘
SEE ALSOdomainname(1M), sysinfo(2), gethostbyname(3NSL), getnameinfo(3SOCKET),
getipnodebyname(3SOCKET), gethostname(3C), resolver(3RESOLV),
attributes(5)
Vixie, Paul, Dunlap, Keven J., Karels, Michael J. Name Server Opera‐
tions Guide for BIND. Internet Software Consortium, 1996.
Dec 15, 2004 RESOLV.CONF(4)
