RESOLV.CONF(5) File Formats and Configurations RESOLV.CONF(5)

NAME

resolv.conf - resolver configuration file

SYNOPSIS

/etc/resolv.conf

DESCRIPTION

The resolver is a set of routines that provide access to the Internet
Domain Name System. See resolver(3RESOLV). resolv.conf is a
configuration file that contains the information that is read by the
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 resolver information.


The resolv.conf file contains the following configuration directives:

nameserver
Specifies the IPv4 or IPv6 Internet address of
a name server that the resolver is to query.
Up to MAXNS name servers may be listed, one
per keyword. See <resolv.h>. If there are
multiple servers, the resolver library queries
them in the order listed. If no name server
entries are present, the resolver library
queries the name server on the local machine.
The resolver library follows the algorithm to
try a name server until the query times out.
It then tries the name servers that follow,
until each query times out. It repeats all the
name servers until a maximum number of retries
are made.


domain
Specifies the local domain name. Most queries
for names within this domain can use short
names relative to the local domain. If no
domain entry is present, the domain is
determined from sysinfo(2) or from
gethostname(3C). (Everything after the first
`.' is presumed to be the domain name.) If the
host name does not contain a domain part, the
root domain is assumed. You can use the
LOCALDOMAIN environment variable to override
the domain name.


search
The 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. You can change
the default behavior by listing the desired
domain search path following the 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. This process may
be slow and will generate a lot of network
traffic if the servers for the listed domains
are not local. Queries will time out if no
server is available for one of the domains.

The search list is currently limited to six
domains and a total of 256 characters.


sortlistaddresslist
Allows addresses returned by the libresolv-
internal gethostbyname() to be sorted. A
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:

sortlist 130.155.160.0/255.255.240.0 130.155.0.0


options
Allows certain internal resolver variables to
be modified. The syntax is

options option ...


where option is one of the following:

debug
Sets RES_DEBUG in the
_res.options field.


ndots:n
Sets a threshold floor for
the number of dots which
must appear in a name given
to res_query() before an
initial absolute (as-is)
query is performed. See
resolver(3RESOLV). The
default value for n is 1,
which means that if there
are any dots in a name, the
name is tried first as an
absolute name before any
search list elements are
appended to it.


timeout:n
retrans:n
Sets the amount of time the
resolver will wait for a
response from a remote name
server before retrying the
query by means of a
different name server.
Measured in seconds, the
default is RES_TIMEOUT. See
<resolv.h>. The timeout and
retrans values are the
starting point for an
exponential back off
procedure where the timeout
is doubled for every
retransmit attempt.


attempts:n
retry: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
RES_DFLRETRY. See
<resolv.h>.


rotate
Sets RES_ROTATE in
_res.options. The name
servers are queried round-
robin from among those
listed. The query load is
spread among all listed
servers, rather than having
all clients try the first
listed server first every
time.


no-check-names
Sets RES_NOCHECKNAME in
_res.options. This disables
the modern BIND checking of
incoming host names and mail
names for invalid characters
such as underscore (_), non-
ASCII, or control
characters.


inet6
Sets RES_USE_INET6 in
_res.options. In the Solaris
BIND port, this has no
effect on
gethostbyname(3NSL). To
retrieve IPv6 addresses or
IPv4 addresses, use
getaddrinfo(3SOCKET) instead
of setting inet6.


The domain and search keywords are mutually exclusive. If more than
one instance of these keywords is present, the last instance takes
precedence.


You can override the search keyword of the system resolv.conf file on
a per-process basis by setting the environment variable LOCALDOMAIN
to a space-separated list of search domains.


You can amend the options keyword of the system resolv.conf file on a
per-process basis by setting the environment variable RES_OPTIONS to
a space-separated list of resolver options.


The keyword and value must appear on a single line. Start the line
with the keyword, for example, nameserver, followed by the value,
separated by white space.

FILES

/etc/resolv.conf

ATTRIBUTES

See attributes(7) for descriptions of the following attributes:


+--------------------+---------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+---------------------+
|Interface Stability | Standard BIND 8.3.3 |
+--------------------+---------------------+

SEE ALSO

sysinfo(2), gethostname(3C), gethostbyname(3NSL), resolver(3RESOLV),
getipnodebyname(3SOCKET), getnameinfo(3SOCKET), attributes(7),
domainname(8)


Vixie, Paul, Dunlap, Keven J., Karels, Michael J. Name Server
Operations Guide for BIND. Internet Software Consortium, 1996.

December 15, 2004 RESOLV.CONF(5)