man > resolver(5)

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 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 re‐
       solver information.  The configuration file is considered a trusted source  of  DNS  informa‐
       tion; 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  ad‐
              dress  (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  deter‐
              mined  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 ap‐
                     pended 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 be‐
                     fore 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-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 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).

              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  re‐
                     moved  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 (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.

              edns0 (since glibc 2.6)
                     Sets  RES_USE_EDNS0  in  _res.options.  This enables support for the DNS exten‐
                     sions 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 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 res‐
                     olutions.

              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  ex‐
                     plicit 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 pre‐
                     serves 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  re‐
                     sponses 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.

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

4th Berkeley Distribution                    2023-10-31                               resolv.conf(5)