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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 Do-
       main 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 de-
       signed to be human readable and contains a list of keywords with values that provide vari-
       ous  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 de-
              fault 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 de-
              termined 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  in-
              stance 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 sep-
              arated 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 ele-
                     ments  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 guaran-
                     tee  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 de-
                     fault is RES_DFLRETRY (currently 2, see <resolv.h>).  The value for this op-
                     tion 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 check-
                     ing of incoming hostnames and mail names for invalid characters such as  un-
                     derscore (_), 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;  applica-
                     tions 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 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 re-
                     moved in glibc 2.25, since it relied on a backward-incompatible  DNS  exten-
                     sion 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 available in glibc  ver-
                     sions up to 2.24, where no-ip6-dotint is the default.  Since ip6-dotint sup-
                     port long ago ceased to be available on the Internet, these options were re-
                     moved in glibc 2.25.

              edns0 (since glibc 2.6)
                     Sets RES_USE_EDNSO in _res.options.  This enables support for the DNS exten-
                     sions 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 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 se-
                     quentially (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 re-
                     ply.  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  re-
                     sponse, 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  ac-
                     tive,  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  op-
                     tion,  the  AD  bit is not set in queries, and it is always removed from re-
                     sponses before they are returned to the application.  This means that appli-
                     cations  can  trust  the AD bit in responses if the trust-ad option has been
                     set correctly.

                     In glibc version 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  do-
       mains.

       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  op-
       tions 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

COLOPHON
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4th Berkeley Distribution                   2020-08-13                             RESOLV.CONF(5)

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