resolv.conf

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 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 (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; the domain name is determined from the hostname and
       the domain search path is constructed from the domain name.

       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.)

       domain Local domain name.
              Most queries for names within this domain can use short names
              relative to the local domain.  If set to '.', the root domain is
              considered.  If no domain entry is present, the domain is
              determined from the local hostname returned by gethostname(2);
              the domain part is taken to be everything after the first '.'.
              Finally, if the hostname 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
              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-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).

              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 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 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 versions up to 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_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
                     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 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.

       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 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

COLOPHON
       This page is part of release 5.01 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         2019-03-06                    RESOLV.CONF(5)