resolv.conf - resolver configuration file
/etc/resolv.confThe resolver 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. The
configuration 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:
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 possibly dot) notation as per RFC 2373. Up to
MAXNS (currently 3, see <resolv.h>) 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.)
By default, the search list contains one entry, the local domain name. It is determined from the local hostname returned by gethostname(2); the local 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 attempted 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 unlimited.
The domain directive is an obsolete name for the search directive that handles one search list entry only.
This option 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 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 allows certain internal resolver variables to be modified. The syntax is
options
option...
where option is one of the following:
Sets RES_DEBUG in _res.options (effective
only if glibc was built with debug support; see
resolver(3)).
nSets 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.
nSets 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_TIMEOUT (currently 5, see
<resolv.h>). The value for this option is silently capped
to 30.
nSets 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 <resolv.h>). The value for this option is silently
capped to 5.
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 having all clients try the first listed server
first every time.
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.
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
characters.
Sets RES_USE_INET6 in _res.options. This
has the effect of trying an AAAA query before an A query inside the
gethostbyname(3) function, 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 getaddrinfo(3), rather than
gethostbyname(3).
Some programs behave strangely when this option is turned on.
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.
Clear/set RES_NOIP6DOTINT in _res.options.
When this option is clear (ip6-dotint), reverse IPv6
lookups are made in the (deprecated) ip6.int zone; when this
option is set (no-ip6-dotint), reverse 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.
Sets RES_USE_EDNS0 in _res.options. This
enables support for the DNS extensions described in RFC 2671.
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 perform the IPv6 and IPv4 requests sequentially (at the cost of
some slowdown of the resolving process).
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
sending the second request.
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 effect if neither RES_DEFNAMES or RES_DNSRCH is set.
Sets RES_USEVC in _res.options. This option
forces the use of TCP for DNS resolutions.
Sets RES_NORELOAD in _res.options. This
option disables automatic reloading of a changed configuration file.
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-validating 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 resolver sets the AD bit in outgoing DNS queries (to
enable AD bit support), and preserves the AD bit in responses. Without
this option, 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 responses.
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-separated 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-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.
Lines that contain a semicolon (;) or hash character (#) in the first column are treated as comments.
/etc/resolv.conf, <resolv.h>
gethostbyname(3), resolver(3), host.conf(5), hosts(5), nsswitch.conf(5), hostname(7), named(8)
Name Server Operations Guide for BIND