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  con-
       sidered a trusted source of DNS information (e.g., DNSSEC AD-bit infor-
       mation will be returned unmodified from this source).

       If this file does not exist, only the name server on the local  machine
       will  be  queried;  the domain name is determined from the hostname and
       the domain search path is constructed from the domain name.

       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 name-
              server 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.)

       domain Local domain name.
              Most queries for names within this domain can  use  short  names
              relative to the local domain.  If set to '.', the root domain is
              considered.  If no domain entry is present, the domain is deter-
              mined  from  the  local hostname returned by gethostname(2); the
              domain part is taken to  be  everything  after  the  first  '.'.
              Finally,  if  the  hostname  does not contain a domain part, the
              root domain is assumed.

       search Search list for host-name lookup.
              The search list is normally determined  from  the  local  domain
              name;  by default, it contains only the local domain name.  This
              may be changed by listing the desired domain search path follow-
              ing 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 avail-
              able for one of the domains.

              The search list is currently limited to six domains with a total
              of 256 characters.

       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  sepa-
              rated  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  modi-
              fied.  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  sin-
                     gle   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).

                     Some programs behave strangely when this option is turned
                     on.

              ip6-bytestring (since glibc 2.3.4)
                     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 avail-
                     able 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 per-
                     forms  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 hard-
                     ware mistakenly sends back only  one  reply.   When  that
                     happens  the client system will sit and wait for the sec-
                     ond reply.  Turning this option on changes this  behavior
                     so  that  if two requests from the same port are not han-
                     dled 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.

       The domain and search keywords are mutually exclusive.   If  more  than
       one instance of these keywords is present, the last instance wins.

       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),    nss-
       witch.conf(5), hostname(7), named(8)

       Name Server Operations Guide for BIND

COLOPHON
       This page is part of release 4.15 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         2017-09-15                    RESOLV.CONF(5)
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