RESOLV.CONF(5)
Configure the resolver library. Static snapshot from Ubuntu 24.04.
resolv.conf(5) File Formats Manual resolv.conf(5)
NAME
resolv.conf - resolver configuration file
SYNOPSIS
/etc/resolv.conf
DESCRIPTION
The resolver is a set of routines in the C library that provide access to the In‐
ternet Domain Name System (DNS). The resolver configuration file contains infor‐
mation 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 key‐
words with values that provide various types of resolver information. The con‐
figuration file is considered a trusted source of DNS information; see the trust-
ad option below for details.
If this file does not exist, only the name server on the local machine will be
queried, and the search list contains the local domain name determined from the
hostname.
The different configuration options are:
nameserver Name server IP address
Internet address of a name server that the resolver should query, either
an IPv4 address (in dot notation), or an IPv6 address in colon (and possi‐
bly dot) notation as per RFC 2373. Up to MAXNS (currently 3, see <re‐
solv.h>) name servers may be listed, one per keyword. If there are multi‐
ple 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.)
search Search list for host-name lookup.
By default, the search list contains one entry, the local domain name. It
is determined from the local hostname returned by gethostname(2); the lo‐
cal domain name is taken to be everything after the first '.'. Finally,
if the hostname does not contain a '.', the root domain is assumed as the
local domain name.
This may be changed by listing the desired domain search path following
the search keyword with spaces or tabs separating the names. Resolver
queries having fewer than ndots dots (default is 1) in them will be at‐
tempted using each component of the search path in turn until a match is
found. For environments with multiple subdomains please read options
ndots:n 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 available
for one of the domains.
If there are multiple search directives, only the search list from the
last instance is used.
In glibc 2.25 and earlier, the search list is limited to six domains with
a total of 256 characters. Since glibc 2.26, the search list is unlim‐
ited.
The domain directive is an obsolete name for the search directive that
handles one search list entry only.
sortlist
This option allows addresses returned by gethostbyname(3) to be sorted. A
sortlist is specified by IP-address-netmask pairs. The netmask is op‐
tional 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. Here is an example:
sortlist 130.155.160.0/255.255.240.0 130.155.0.0
options
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 _res.options (effective only if glibc was built
with debug support; see resolver(3)).
ndots:n
Sets a threshold for the number of dots which must appear in a name
given to res_query(3) (see resolver(3)) before an initial absolute
query will be made. The default for n is 1, meaning that if there
are any dots in a name, the name will be tried first as an absolute
name before any search list elements are appended to it. The value
for this option is silently capped to 15.
timeout:n
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. This may not be the total time taken by any resolver API
call and there is no guarantee that a single resolver API call maps
to a single timeout. Measured in seconds, the default is RES_TIME‐
OUT (currently 5, see <resolv.h>). The value for this option is
silently capped to 30.
attempts: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 ap‐
plication. The default is RES_DFLRETRY (currently 2, see <re‐
solv.h>). The value for this option is silently capped to 5.
rotate Sets RES_ROTATE in _res.options, which causes round-robin selection
of name servers from among those listed. This has the effect of
spreading the query load among all listed servers, rather than hav‐
ing all clients try the first listed server first every time.
no-aaaa (since glibc 2.36)
Sets RES_NOAAAA in _res.options, which suppresses AAAA queries made
by the stub resolver, including AAAA lookups triggered by NSS-based
interfaces such as getaddrinfo(3). Only DNS lookups are affected:
IPv6 data in hosts(5) is still used, getaddrinfo(3) with AI_PASSIVE
will still produce IPv6 addresses, and configured IPv6 name servers
are still used. To produce correct Name Error (NXDOMAIN) results,
AAAA queries are translated to A queries. This option is intended
preliminary for diagnostic purposes, to rule out that AAAA DNS
queries have adverse impact. It is incompatible with EDNS0 usage
and DNSSEC validation by applications.
no-check-names
Sets RES_NOCHECKNAME in _res.options, which disables the modern
BIND checking of incoming hostnames and mail names for invalid
characters such as underscore (_), non-ASCII, or control charac‐
ters.
inet6 Sets RES_USE_INET6 in _res.options. This has the effect of trying
an AAAA query before an A query inside the gethostbyname(3) func‐
tion, and of mapping IPv4 responses in IPv6 "tunneled form" if no
AAAA records are found but an A record set exists. Since glibc
2.25, this option is deprecated; applications should use getad‐
drinfo(3), rather than gethostbyname(3).
Some programs behave strangely when this option is turned on.
ip6-bytestring (since glibc 2.3.4 to glibc 2.24)
Sets RES_USEBSTRING in _res.options. This causes reverse IPv6
lookups to be made using the bit-label format described in
RFC 2673; if this option is not set (which is the default), then
nibble format is used. This option was removed in glibc 2.25,
since it relied on a backward-incompatible DNS extension that was
never deployed on the Internet.
ip6-dotint/no-ip6-dotint (glibc 2.3.4 to glibc 2.24)
Clear/set RES_NOIP6DOTINT in _res.options. When this option is
clear (ip6-dotint), reverse IPv6 lookups are made in the (depre‐
cated) ip6.int zone; when this option is set (no-ip6-dotint), re‐
verse IPv6 lookups are made in the ip6.arpa zone by default. These
options are available up to glibc 2.24, where no-ip6-dotint is the
default. Since ip6-dotint support long ago ceased to be available
on the Internet, these options were removed in glibc 2.25.
edns0 (since glibc 2.6)
Sets RES_USE_EDNS0 in _res.options. This enables support for the
DNS extensions described in RFC 2671.
single-request (since glibc 2.10)
Sets RES_SNGLKUP in _res.options. By default, glibc performs IPv4
and IPv6 lookups in parallel since glibc 2.9. Some appliance DNS
servers cannot handle these queries properly and make the requests
time out. This option disables the behavior and makes glibc per‐
form the IPv6 and IPv4 requests sequentially (at the cost of some
slowdown of the resolving process).
single-request-reopen (since glibc 2.9)
Sets RES_SNGLKUPREOP in _res.options. The resolver uses the same
socket for the A and AAAA requests. Some hardware mistakenly sends
back only one reply. When that happens the client system will sit
and wait for the second reply. Turning this option on changes this
behavior so that if two requests from the same port are not handled
correctly it will close the socket and open a new one before send‐
ing the second request.
no-tld-query (since glibc 2.14)
Sets RES_NOTLDQUERY in _res.options. This option causes
res_nsearch() to not attempt to resolve an unqualified name as if
it were a top level domain (TLD). This option can cause problems
if the site has ``localhost'' as a TLD rather than having localhost
on one or more elements of the search list. This option has no ef‐
fect if neither RES_DEFNAMES or RES_DNSRCH is set.
use-vc (since glibc 2.14)
Sets RES_USEVC in _res.options. This option forces the use of TCP
for DNS resolutions.
no-reload (since glibc 2.26)
Sets RES_NORELOAD in _res.options. This option disables automatic
reloading of a changed configuration file.
trust-ad (since glibc 2.31)
Sets RES_TRUSTAD in _res.options. This option controls the AD bit
behavior of the stub resolver. If a validating resolver sets the
AD bit in a response, it indicates that the data in the response
was verified according to the DNSSEC protocol. In order to rely on
the AD bit, the local system has to trust both the DNSSEC-validat‐
ing resolver and the network path to it, which is why an explicit
opt-in is required. If the trust-ad option is active, the stub re‐
solver sets the AD bit in outgoing DNS queries (to enable AD bit
support), and preserves the AD bit in responses. Without this op‐
tion, the AD bit is not set in queries, and it is always removed
from responses before they are returned to the application. This
means that applications can trust the AD bit in responses if the
trust-ad option has been set correctly.
In glibc 2.30 and earlier, the AD is not set automatically in
queries, and is passed through unchanged to applications in re‐
sponses.
The search keyword of a system's resolv.conf file can be overridden on a per-
process basis by setting the environment variable LOCALDOMAIN to a space-sepa‐
rated list of search domains.
The options keyword of a system's resolv.conf file can be amended on a per-
process basis by setting the environment variable RES_OPTIONS to a space-sepa‐
rated list of resolver options as explained above under options.
The keyword and value must appear on a single line, and the keyword (e.g., name‐
server) must start the line. The value follows the keyword, separated by white
space.
Lines that contain a semicolon (;) or hash character (#) in the first column are
treated as comments.
FILES
/etc/resolv.conf, <resolv.h>
SEE ALSO
gethostbyname(3), resolver(3), host.conf(5), hosts(5), nsswitch.conf(5), host‐
name(7), named(8)
Name Server Operations Guide for BIND
4th Berkeley Distribution 2023-10-31 resolv.conf(5)
Generated 2026-11-03 from resolv.conf.5 in the official Ubuntu 24.04 package manpages 6.7-2. Extracted without installing the package. Original copyright and license terms apply.