plucky (5) resolver.5.gz

Provided by: manpages_6.9.1-1_all bug

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
       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.

       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 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 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 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.

              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 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)  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 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 resolutions.

              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 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