resolver(5) — Linux manual page


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

NAME         top

       resolv.conf - resolver configuration file

SYNOPSIS         top


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
       (e.g., DNSSEC AD-bit information will be returned unmodified from
       this source).

       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

              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:


              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)).

                     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.

                     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.

                     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.

                     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) zone; when this option
                     is set (no-ip6-dotint), reverse IPv6 lookups are made
                     in the 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_EDNSO 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

              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

       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 5.08 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

4th Berkeley Distribution        2020-08-13                   RESOLV.CONF(5)

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