RESOLV.CONF(5)             Linux Programmer's Manual            RESOLV.CONF(5)

       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
       various types of resolver information.  The configuration file is  con-
       sidered a trusted source of DNS information (e.g., DNSSEC AD-bit infor-
       mation 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 de-
       termined 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 ad-
              dress 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 re-
              solver  library  queries  them in the order listed.  If no name-
              server 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 do-
              mains 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 direc-
              tive that handles one search list entry only.

              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 sepa-
              rated by slashes.  Up to 10 pairs may be specified.  Here is  an


              Options  allows  certain internal resolver variables to be modi-
              fied.  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)).

                     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.

                     Sets  the amount of time the resolver will wait for a re-
                     sponse 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 sin-
                     gle  timeout.   Measured  in  seconds,  the  default   is
                     RES_TIMEOUT (currently 5, see <resolv.h>).  The value for
                     this option is silently capped to 30.

                     Sets the number of times the resolver will send  a  query
                     to its name servers before giving up and returning an er-
                     ror  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.

                     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

              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 de-
                     scribed 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

              ip6-dotint/no-ip6-dotint (glibc 2.3.4 to 2.24)
                     Clear/set RES_NOIP6DOTINT in _res.options.  When this op-
                     tion is clear (ip6-dotint), reverse IPv6 lookups are made
                     in the (deprecated) zone; when this option is set
                     (no-ip6-dotint),  reverse  IPv6  lookups  are made in the
            zone by default.  These options are available in
                     glibc versions up to 2.24, where no-ip6-dotint is the de-
                     fault.  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_EDNSO 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 per-
                     forms 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 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  hard-
                     ware  mistakenly  sends  back  only one reply.  When that
                     happens the client system will sit and wait for the  sec-
                     ond  reply.  Turning this option on changes this behavior
                     so that if two requests from the same port are  not  han-
                     dled  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 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  op-

       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.

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

       gethostbyname(3),    resolver(3),    host.conf(5),    hosts(5),    nss-
       witch.conf(5), hostname(7), named(8)

       Name Server Operations Guide for BIND

       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

4th Berkeley Distribution         2019-10-10                    RESOLV.CONF(5)
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