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man:resolver

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  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;  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 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.)
     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 deter-
            mined  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 follow-
            ing 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 avail-
            able 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  sepa-
            rated  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  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)).
            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  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.
            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  avail-
                   able 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 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.
     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),    nss-
     witch.conf(5), hostname(7), named(8)
     Name Server Operations Guide for BIND

COLOPHON

     This  page  is  part of release 4.16 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 2017-09-15 RESOLV.CONF(5)

/data/webs/external/dokuwiki/data/pages/man/resolver.txt · Last modified: 2019/05/17 09:32 by 127.0.0.1

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