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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