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\"      @(#)resolver.5  5.9 (Berkeley) 12/14/89
\"      Id: resolver.man5,v 1.2 2009/01/21 00:12:34 each Exp
\"
Dd November 11, 1993
Dt RESOLVER 5
Os BSD 4
Sh NAME
Nm resolver
Nd resolver configuration file
Sh SYNOPSIS
Pa /etc/resolv.conf
Sh DESCRIPTION
The
Nm resolver
is a set of routines in the C library
Pq Xr resolve 3
that provide access to the Internet Domain Name System.
The
Nm resolver
configuration file contains information that is read
by the
Nm 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
Nm resolver
information.
Pp
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.
Pp
The different configuration directives are:
Bl -tag -width "nameser"
It Li nameserver
Internet address (in dot notation) of a name server that the
Nm resolver
should query.  Up to
Dv MAXNS
(see
Pa <resolv.h> )
name servers may be listed, one per keyword.
If there are multiple servers, the
Nm resolver
library queries them in the order listed.
If no
Li 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).
It Li domain
Local domain name.
Most queries for names within this domain can use short names
relative to the local domain.
If no
Li domain
entry is present, the domain is determined from the local host name returned by
Xr gethostname  ;
the domain part is taken to be everything after the first
Sq \&. .
Finally, if the host name does not contain a domain part, the root
domain is assumed.
It Li 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
Li search
keyword with spaces or tabs separating the names.
Most
Nm 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.
Pp
The search list is currently limited to six domains
with a total of 256 characters.
It Li sortlist
Allows addresses returned by gethostbyname to be sorted.
A
Li 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.  For example:
Bd -literal -offset indent
sortlist 130.155.160.0/255.255.240.0 130.155.0.0
Ed
It Li options
Allows certain internal
Nm resolver
variables to be modified.
The syntax is
D1 Li options Ar option ...
where
Ar option
is one of the following:
Bl -tag -width "ndots:n "
It Li debug
sets
Dv RES_DEBUG
in
Ft _res.options .
It Li ndots: Ns Ar n
sets a threshold for the number of dots which
must appear in a name given to
Fn res_query
(see
Xr resolver 3 )
before an
Em initial absolute query
will be made.  The default for
Ar n
is
Dq 1 ,
meaning that if there are
Em any
dots in a name, the name will be tried first as an absolute name before any
Em search list
elements are appended to it.
It Li timeout: Ns Ar 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.  Measured in
seconds, the default is
Dv RES_TIMEOUT
(see
Pa <resolv.h> ) .
It Li attempts: Ns Ar 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
Dv RES_DFLRETRY
(see
Pa <resolv.h> ) .
It Li rotate
sets
Dv RES_ROTATE
in
Ft _res.options ,
which causes round robin selection of nameservers 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.
It Li no-check-names
sets
Dv RES_NOCHECKNAME
in
Ft _res.options ,
which disables the modern BIND checking of incoming host names and mail names
for invalid characters such as underscore (_), non-ASCII, or control characters.
It Li inet6
sets
Dv RES_USE_INET6
in
Ft _res.options .
This has the effect of trying a AAAA query before an A query inside the
Ft gethostbyname
function, and of mapping IPv4 responses in IPv6 ``tunnelled form'' if no
AAAA records are found but an A record set exists.
It Li no-tld-query
sets
Dv RES_NOTLDQUERY
in
Ft _res.options .
This option causes
Fn res_nsearch
to not attempt to resolve a 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
Dv RES_DEFNAMES
or
Dv RES_DNSRCH
is set.
El
El
Pp
The
Li domain
and
Li search
keywords are mutually exclusive.
If more than one instance of these keywords is present,
the last instance wins.
Pp
The
Li search
keyword of a system's
Pa resolv.conf
file can be
overridden on a per-process basis by setting the environment variable
Dq Ev LOCALDOMAIN
to a space-separated list of search domains.
Pp
The
Li options
keyword of a system's
Pa resolv.conf
file can be amended on a per-process basis by setting the environment variable
Dq Ev RES_OPTIONS to a space-separated list of
Nm resolver
options as explained above under
Li options .
Pp
The keyword and value must appear on a single line, and the keyword
(e.g.,
Li nameserver )
must start the line.  The value follows the keyword, separated by white space.
Sh FILES
Pa /etc/resolv.conf
Pa <resolv.h>
Sh SEE ALSO
Xr gethostbyname 3 ,
Xr hostname 7 ,
Xr resolver 3 ,
Xr resolver 5 .
Dq Name Server Operations Guide for Sy BIND

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