resolv.conf manpage

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computing: software: operating systems: Linux: configuration files: resolv.conf: manpage



      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 var‐
      ious types of resolver information.
      On a normally configured system this file should not be necessary.  The only name server to
      be queried will be on the local machine; the domain name is determined from the  host  name
      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  key‐
             word.  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 host
             name  returned by gethostname(); the domain part is taken to be everything after the
             first ‘.’.  Finally, if the host name 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  allows  addresses  returned  by gethostbyname 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. E.g.,
             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.
                    sets a threshold for the number of dots which must appear in a name given  to
                    res_query()  (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.
                    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>).
                    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>).
             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.
                    sets RES_NOCHECKNAME in _res.options, which disables the modern BIND checking
                    of incoming host names and mail names for invalid characters such  as  under‐
                    score (_), 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() function, and  of  mapping
                    IPv4 responses in IPv6 ‘‘tunnelled form’’ if no AAAA records are found but an
                    A record set exists.
      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
      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.


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


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