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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 (e.g., DNSSEC AD-bit information will be returned unmodified from  this
       source).

       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)
                     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.

       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
       This  page  is  part of release 5.05 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                   2019-10-10                             RESOLV.CONF(5)

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