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.

       If  this  file  doesn't  exist  the only name server to be queried will be on the local machine; 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 (in dot notation) of a name server that the resolver should query.  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.)

       domain Local domain name.
              Most queries for names within this domain can use short names relative to the local domain.  If no domain entry is present,  the
              domain  is  determined  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 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.

              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 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.

              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.  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 nameservers 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  a  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.

                     Some programs behave strangely when this option is turned on.

              ip6-bytestring (since glibc 2.3.4)
                     sets RES_USE_BSTRING 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, then nibble format is used.

              ip6-dotint/no-ip6-dotint (since glibc 2.3.4)
                     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.  This option is set by default.

              edns0 (since glibc 2.6)
                     sets RES_USE_EDNSO in _res.options.  This enables support for the DNS extensions described in RFC 2671.

       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.

FILES

       /etc/resolv.conf, <resolv.h>

SEE ALSO

       gethostbyname(3), resolver(3), hostname(7), named(8)
       Name Server Operations Guide for BIND

COLOPHON

       This page is part of release 3.35 of the Linux man-pages project.  A description of the project, and information about reporting  bugs,
       can be found at http://man7.org/linux/man-pages/.
 

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