resolv.conf(5) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | FILES | SEE ALSO | COLOPHON

RESOLV.CONF(5)          Linux Programmer's Manual         RESOLV.CONF(5)

NAME         top

       resolv.conf - resolver configuration file

SYNOPSIS         top

       /etc/resolv.conf

DESCRIPTION         top

       The 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:

       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 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.)

       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 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.

       sortlist
              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
              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_TIMEOUT
                     (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 application.  The
                     default is RES_DFLRETRY (currently 2, see
                     <resolv.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
                     having all clients try the first listed server
                     first every time.

              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
                     characters.

              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) 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).

              ip6-bytestring (since glibc 2.3.4 to 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 2.24)
                     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 in glibc
                     versions up to 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 version 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).

              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 sending
                     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 effect 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-
                     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 version 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.

FILES         top

       /etc/resolv.conf, <resolv.h>

SEE ALSO         top

       gethostbyname(3), resolver(3), host.conf(5), hosts(5),
       nsswitch.conf(5), hostname(7), named(8)

       Name Server Operations Guide for BIND

COLOPHON         top

       This page is part of release 5.13 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

4th Berkeley Distribution      2021-03-22                 RESOLV.CONF(5)

Pages that refer to this page: pmdanetcheck(1)pmhostname(1)resolvectl(1)resolver(3)host.conf(5)hosts(5)resolved.conf(5)systemd.network(5)hostname(7)systemd-resolved.service(8)