Provided by: manpages_6.7-2_all bug


       resolv.conf - resolver configuration file




       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.

              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.

              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.

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

              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  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

              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 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

       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),

       Name Server Operations Guide for BIND