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.\"	@(#)resolver.5	5.9 (Berkeley) 12/14/89
.\"	$Id: resolver.5,v 8.6 1999/05/21 00:01:02 vixie Exp $
.\"
.\"     Revised by Mike MacCana
.\"
.Dd June 23, 2004
.Dt RESOLVER 5 
.Os BSD 4
.Sh NAME
.Nm resolver 
.Nd DNS client
.Sh SYNOPSIS
.Pa /etc/resolv.conf
.Sh DESCRIPTION
The
.Nm resolver
is the DNS client used on most Linux and BSD systems. It comes with glibc. 
Its configuration file 
.Pa /etc/resolv.conf 
(note the spelling) determines the DNS servers to use, and various other options - see below.
.Pp
Almost all machines have a DNS server set up in this file - if it
doesn't exist, the system will assume there's a DNS server running on the local 
machine, and work out the search path from the machines domain name.
.Pp
The config file is read the first time the DNS client is invoked by a process.
.Pp
The different configuration options are:
.Bl -tag -width "nameserver"
.It Li nameserver
IP address of a DNS server to use. Multiple name servers may be listed, each on their own line. The 
.Nm resolver 
will use them in order listed - if the first server times out answering the query, the next server will be tried, and so on. If the
.Li resolver
runs out out of name servers, the first server will be queried again, until a maximum number of retries are made. 
.Pp 
The maximum number of DNS servers to use is set by 
.Dv MAXNS 
(see
.Pa <resolv.h>
)
.It Li search
Domain(s) to use for DNS lookups when no domain is specified. List each domain 
following the
.Li search
keyword with spaces or tabs between them. Each possible domain will be checked in order until a match is found. Note that this process may be slow (queries will time out if no server is available for a domain) and will generate a lot of network traffic if the servers for the listed domains aren't local.
.Pp
The search list is currently limited to six domains with a total of 256 characters.
If
.Li search
isn't specified, the search list will be determined from the local domain name (whatever comes after the first dot). If the host name doesn't contain a domain, the root domain is used. 
.Pp
By default, it 
.Li search
contains only the local domain name.
.It Li domain
Local domain name. You can use this instead of the 
.Li search
option to specify a single domain to check if a hostname isn't specified. Most people just use 
.Li search
instead (that option lets you use multiple servers, 
.Li domain
doesn't). You can't use 
.Li domain
and
.Li search
at the same time - they're mutually exclusive. 
.Pp
If
.Li domain
isn't specified, the domain will be determined from the local domain name (whatever comes after the first dot). If the host name doesn't contain a domain, the root domain is used.
.It Li sortlist
Sorts addresses returned by the
.Li gethostbyname 
system call.
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 ip6-dotint / no-ip6-dotint
sets / clears the
.Dv RES_NOIP6DOTINT
bit in 
.Ft _res.options ,
which when set (ip6-dotint) will enable reverse IPv6 lookups to be made in the (deprecated) 
ip6.int zone; when clear (no-ip6-dotint), reverse IPv6 lookups are made in the ip6.arpa zone 
by default.
.It Li ip6-bytestring
sets
.Dv RES_USEBSTRING
in 
.Ft _res.options .
This causes reverse IPv6 lookups to be made using the bit-label 
format of RFC 2673; if not set, then nibble format is used.
.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 named 8 ,
.Xr resolver 3 , 
.Xr resolver 5 .
.Dq Name Server Operations Guide for Sy BIND