resolver

RESOLV.CONF(5)              Linux Programmer's 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 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
       /etc/resolv.conf, <resolv.h>

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

       Name Server Operations Guide for BIND

COLOPHON
       This page is part of release 5.11 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)