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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