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NAME    [Toc]    [Back]

     resolv.conf - resolver configuration file

DESCRIPTION    [Toc]    [Back]

     The resolv.conf file specifies how the resolver(3)  routines
in the C library
 (which provide access to the Internet Domain Name System) should
     operate.  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

     A hash mark `#' or semicolon `;' indicates the beginning  of
a comment;
     subsequent  characters up to the end of the line are not interpreted by
     the routines that read the file.

     On a normally configured system this file should not be necessary.  The
     only name server to be queried will be on the local machine,
the domain
     name is determined from the host name, and the domain search
path is constructed
 from the domain name.

     The different configuration options are:

     nameserver   IPv4  address (in dot notation) or IPv6 address
(in hex-andcolon
 notation) of a name server  that  the  resolver should
                 query.  Scoped IPv6 address notation is accepted
as well (see
                 inet6(4) for details).  Up to  MAXNS  (currently
3) 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
                 name  servers  until a maximum number of retries
are performed.)

     domain      Local domain name.  Most queries for names within this domain
                 can  use  short  names relative to the local domain.  If no
                 domain entry is present, the  domain  is  determined from the
                 local  host name returned by gethostname(3); the
domain part
                 is taken to be everything after the first ``.''.
Finally, if
                 the  host  name  does not contain a domain part,
the root domain
                 is assumed.

     lookup      This keyword is used  by  the  library  routines
                 and   gethostbyaddr(3).    It   specifies  which
databases should be
                 searched, and the order to  do  so.   The  legal
                 values are:

                 bind   Use  the  Domain  Name server by querying
                 file  Search for entries in /etc/hosts.
                 yp    Talk to the YP system if ypbind(8) is running.

                 If  the  lookup  keyword is not used in the system's resolv.conf
                 file then the assumed order is bind file.   Furthermore, if
                 the  system's  resolv.conf  file does not exist,
then the only
                 database used is file.

     search      Search list for  hostname  lookup.   The  search
list is normally
                 determined  from  the  local domain name; by default, it begins
                 with the local domain name, then successive parent domains
                 that  have  at  least  two  components  in their
names.  This may
                 be changed by listing the desired domain  search
path following
 the search keyword with spaces or tabs separating the
                 names.  Most resolver queries will be  attempted
using each
                 component  of  the  search  path in turn until a
match is found.
                 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 1024 characters.

     sortlist    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, e.g.:


     options     Allows certain internal resolver variables to be
                 The syntax is:

                 options option ...

                 where option is one of the following:

                 debug      Sets RES_DEBUG in _res.options.

                 edns0      attach OPT pseudo-RR for ENDS0 extension specified
                            in RFC 2671, to inform DNS server  of
our receive
                            buffer  size.   The option will allow
DNS servers to
                            take advantage of non-default receive
buffer size,
                            and  to  send  larger  replies.   DNS
query packets
                            with EDNS0 extension are not compatible with nonEDNS0
  DNS  servers.  The option must
be used only
                            when all the DNS  servers  listed  in
                            lines are able to handle EDNS0 extension.

                 inet6      Enables support for IPv6-only  applications, by
                            setting RES_USE_INET6 in _res.options
                            resolver(3)).  Use of this option  is
                            and meaningless on OpenBSD.

                 insecure1   Do  not require IP source address on
the reply
                            packet to be equal  to  the  server's

                 insecure2   Do not check if the query section of
the reply
                            packet is equal to that of the  query
packet.  For
                            testing purposes only.

                 ndots:n     Sets  a  threshold for the number of
dots which must
                            appear in a name given  to  res_query
                            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  domain  and search keywords are mutually exclusive.  If
more than one
     instance of these keywords is  present,  the  last  instance
will override.

     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
     space-separated list of resolver options as explained above.

     The keyword and value must appear on a single line, and  the
     (e.g.,  nameserver)  must start the line.  The value follows
the keyword,
     separated by whitespace.

FILES    [Toc]    [Back]


SEE ALSO    [Toc]    [Back]

     gethostbyname(3),   resolver(3),   hosts(5),    hostname(7),

     Name Server Operations Guide for BIND.

HISTORY    [Toc]    [Back]

     The resolv.conf file format appeared in 4.3BSD.

BUGS    [Toc]    [Back]

     Due  to resolver internal issues, getaddrinfo(3) may not behave as lookup
     suggests.  Consequently, userland programs that  use  getaddrinfo(3) may
     behave differently from what lookup says.

OpenBSD      3.6                           May      10,      1991
[ Back ]
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