.\"	$OpenBSD: resolv.conf.5,v 1.28 2006/06/05 16:13:55 ian Exp $
.\"	$NetBSD: resolv.conf.5,v 1.7 1996/03/06 18:22:16 scottr Exp $
.\"
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.\"     @(#)resolver.5	5.12 (Berkeley) 5/10/91
.\"
.Dd May 10, 1991
.Dt RESOLV.CONF 5
.Os
.Sh NAME
.Nm resolv.conf , resolv.conf.tail
.Nd resolver configuration files
.Sh DESCRIPTION
The
.Nm
file specifies how the
.Xr 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.
If the
.Nm resolv.conf
file does not exist, only the local host file
.Pa /etc/hosts
will be consulted,
i.e. the Domain Name System will not be used to resolve hosts.
.Pp
The file is designed to be human readable and contains a list of
keywords with values that provide various types of resolver information.
A resolv.conf file is not required for some setups, so this file is optional.
It can be created manually, and is also created as part of the
.Ox
install process
if use of the DHCP protocol is specified for any interface.
.Pp
If
.Xr dhclient 8
is used to configure the network,
the DHCP client back-end
.Xr dhclient-script 8
will normally overwrite the
.Nm resolv.conf
file with updated information such as nameserver addresses,
losing any previous values the file contained.
In order to force options to be passed to the
.Xr resolver 3
routines, the file
.Nm resolv.conf.tail
may be created manually.
This file will be appended to the generated
.Nm resolv.conf
file by
.Xr dhclient-script 8 ,
ensuring options remain.
.Pp
On a machine whose network connection does not change frequently (such as a desktop
machine on a local-area network), the
.Nm resolv.conf.tail
file should not be necessary.
However the
.Nm resolv.conf.tail
file may be useful on notebooks, to search multiple domains,
to refer to hard-coded information in local files, or otherwise
override the defaults.
.Pp
A hash mark
.Ql #
or semicolon
.Ql \&;
in the file 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.
.Pp
The configuration options (which may be placed in either file) are:
.Bl -tag -width nameserver
.It Sy nameserver
IPv4 address (in dot notation)
or IPv6 address (in hex-and-colon notation)
of a name server that the resolver should query.
Scoped IPv6 address notation is accepted as well
(see
.Xr inet6 4
for details).
Up to
.Dv MAXNS
(currently 3) name servers may be listed, one per line.
If there are multiple servers, the resolver library queries them in the
order listed.
If no
.Sy 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.)
.It Sy domain
Local domain name.
Most queries for names within this domain can use short names
relative to the local domain.
If no
.Sy domain
entry is present, the domain is determined
from the local host name returned by
.Xr gethostname 3 ;
the domain part is taken to be everything after the first
.Dq \&. .
Finally, if the host name does not contain a domain part, the root
domain is assumed.
.It Sy lookup
This keyword is used by the library routines
.Xr gethostbyname 3
and
.Xr gethostbyaddr 3 .
It specifies which databases should be searched, and the order to do so.
The legal space-separated values are:
.Pp
.Bl -tag -width bind -compact
.It Sy bind
Use the Domain Name server by querying
.Xr named 8 .
.It Sy file
Search for entries in
.Pa /etc/hosts .
.It Sy yp
Talk to the YP system if
.Xr ypbind 8
is running.
.El
.Pp
If the
.Sy lookup
keyword is not used in the system's
.Pa resolv.conf
file then the assumed order is
.Sy bind file .
Furthermore, if the system's
.Pa resolv.conf
file does not exist, then the only database used is
.Sy file .
.Pp
.It Sy 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
.Sy 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.
.Pp
The search list is currently limited to six domains
with a total of 1024 characters.
Only one
.Nm search
line should appear; if more than one is present, the last one found
overwrites any values found in earlier lines.
So if such a line appears in the
.Nm resolv.conf.tail
file, it should include all the domains that need to be searched.
.It Sy sortlist
Allows addresses returned by
.Xr gethostbyname 3
to be sorted.
A
.Sy 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.:
.Pp
.Sy sortlist 130.155.160.0/255.255.240.0 130.155.0.0
.It Sy options
Allows certain internal resolver variables to be modified.
The syntax is:
.Pp
.Sy options option ...
.Pp
where option is one of the following:
.Bl -tag -width insecure1
.It Sy debug
Sets RES_DEBUG in _res.options.
.It Sy edns0
Attach OPT pseudo-RR for EDNS0 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
non-EDNS0 DNS servers.
The option must be used only when all the DNS servers listed in
.Sy nameserver
lines are able to handle EDNS0 extension.
.It Sy inet6
Enables support for IPv6-only applications, by setting RES_USE_INET6 in
_res.options (see
.Xr resolver 3 ) .
Use of this option is discouraged, and meaningless on
.Ox .
.It Sy insecure1
Do not require IP source address on the reply packet to be equal to the
server's address.
.It Sy insecure2
Do not check if the query section of the reply packet is equal
to that of the query packet.
For testing purposes only.
.It Sy ndots:n
Sets a threshold for the number of dots which
must appear in a name given to res_query (see
.Xr resolver 3 )
before an initial absolute query will be made.
The default for
.Ar 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.
.El
.El
.Pp
The
.Sy domain
and
.Sy search
keywords are mutually exclusive.
If more than one instance of these keywords is present, the last instance
will override.
.Pp
The
.Sy search
keyword of a system's
.Nm resolv.conf
or
.Nm resolv.conf.tail
file can be overridden on a per-process basis by setting the
environment variable
.Ev LOCALDOMAIN
to a space-separated list of search domains.
.Pp
The
.Sy options
keyword of a system's
.Nm resolv.conf
or
.Nm resolv.conf.tail
file can be amended on a per-process basis by setting the
environment variable
.Ev RES_OPTIONS
to a space-separated list of resolver options as explained above.
.Pp
The keyword and value must appear on a single line, and the keyword (e.g.\&
.Sy nameserver )
must start the line.
The value follows the keyword, separated by whitespace.
.Sh FILES
.Bl -tag -width "/etc/resolv.conf.tailXX" -compact
.It Pa /etc/resolv.conf
.It Pa /etc/resolv.conf.tail
.El
.Sh SEE ALSO
.Xr gethostbyname 3 ,
.Xr resolver 3 ,
.Xr hosts 5 ,
.Xr hostname 7 ,
.Xr dhclient-script 8 ,
.Xr dhcp 8 ,
.Xr named 8
.Rs
.%T "Name Server Operations Guide for BIND"
.Re
.Sh HISTORY
The
.Nm
file format appeared in
.Bx 4.3 .
.Sh BUGS
Due to resolver internal issues,
.Xr getaddrinfo 3
may not behave as
.Sy lookup
suggests.
Consequently, userland programs that use
.Xr getaddrinfo 3
may behave differently from what
.Sy lookup
says.