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RESOLV.CONF(5) Linux Programmer's Manual RESOLV.CONF(5)
resolv.conf - resolver configuration file
/etc/resolv.conf
The resolver is a set of routines in the C library that provide
access to the Internet Domain Name System (DNS). The resolver
configuration file contains 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.
If this file does not exist, only the name server on the local
machine will be queried; the domain name is determined from the
hostname and the domain search path is constructed from the domain
name.
The different configuration options are:
nameserver Name server IP address
Internet address of a name server that the resolver should
query, either an IPv4 address (in dot notation), or an IPv6
address in colon (and possibly dot) notation as per RFC 2373.
Up to MAXNS (currently 3, see <resolv.h>) name servers may be
listed, one per keyword. If there are multiple servers, the
resolver library queries them in the order listed. If no
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.)
domain 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 the local hostname returned by
gethostname(2); the domain part is taken to be everything
after the first '.'. Finally, if the hostname does not
contain a domain part, the root domain is assumed.
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 search keyword with spaces or tabs separating
the names. Resolver queries having fewer than ndots dots
(default is 1) in them will be attempted using each component
of the search path in turn until a match is found. For
environments with multiple subdomains please read options
ndots:n below to avoid man-in-the-middle attacks and
unnecessary traffic for the root-dns-servers. 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.
The search list is currently limited to six domains with a
total of 256 characters.
sortlist
This option allows addresses returned by gethostbyname(3) 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.
Here is an example:
sortlist 130.155.160.0/255.255.240.0 130.155.0.0
options
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 _res.options (effective only if glibc
was built with debug support; see resolver(3)).
ndots:n
sets a threshold for the number of dots which must
appear in a name given to res_query(3) (see
resolver(3)) before an initial absolute query will be
made. The default for 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. The value for this option is silently
capped to 15.
timeout: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 RES_TIMEOUT (currently 5, see
<resolv.h>). The value for this option is silently
capped to 30.
attempts: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 (currently 2, see <resolv.h>). The value
for this option is silently capped to 5.
rotate sets RES_ROTATE in _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.
no-check-names
sets RES_NOCHECKNAME in _res.options, which disables
the modern BIND checking of incoming hostnames and mail
names for invalid characters such as underscore (_),
non-ASCII, or control characters.
inet6 sets RES_USE_INET6 in _res.options. This has the
effect of trying a AAAA query before an A query inside
the gethostbyname(3) function, and of mapping IPv4
responses in IPv6 "tunneled form" if no AAAA records
are found but an A record set exists.
ip6-bytestring (since glibc 2.3.4)
sets RES_USE_BSTRING in _res.options. This causes
reverse IPv6 lookups to be made using the bit-label
format described in RFC 2673; if this option is not
set, then nibble format is used.
ip6-dotint/no-ip6-dotint (since glibc 2.3.4)
Clear/set RES_NOIP6DOTINT in _res.options. When this
option is clear (ip6-dotint), reverse IPv6 lookups are
made in the (deprecated) ip6.int zone; when this option
is set (no-ip6-dotint), reverse IPv6 lookups are made
in the ip6.arpa zone by default. This option is set by
default.
edns0 (since glibc 2.6)
sets RES_USE_EDNSO in _res.options. This enables
support for the DNS extensions described in RFC 2671.
single-request (since glibc 2.10)
sets RES_SNGLKUP in _res.options. By default, glibc
performs IPv4 and IPv6 lookups in parallel since
version 2.9. Some appliance DNS servers cannot handle
these queries properly and make the requests time out.
This option disables the behavior and makes glibc
perform the IPv6 and IPv4 requests sequentially (at the
cost of some slowdown of the resolving process).
single-request-reopen (since glibc 2.9)
The resolver uses the same socket for the A and AAAA
requests. Some hardware mistakenly sends back only one
reply. When that happens the client system will sit
and wait for the second reply. Turning this option on
changes this behavior so that if two requests from the
same port are not handled correctly it will close the
socket and open a new one before sending the second
request.
The domain and search keywords are mutually exclusive. If more than
one instance of these keywords is present, the last instance wins.
The search keyword of a system's resolv.conf file can be overridden
on a per-process basis by setting the environment variable
LOCALDOMAIN to a space-separated list of search domains.
The options keyword of a system's resolv.conf file can be amended on
a per-process basis by setting the environment variable RES_OPTIONS
to a space-separated list of resolver options as explained above
under options.
The keyword and value must appear on a single line, and the keyword
(e.g., nameserver) must start the line. The value follows the
keyword, separated by white space.
Lines that contain a semicolon (;) or hash character (#) in the first
column are treated as comments.
/etc/resolv.conf, <resolv.h>
gethostbyname(3), resolver(3), hostname(7), named(8)
Name Server Operations Guide for BIND
This page is part of release 3.51 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
4th Berkeley Distribution 2013-03-05 RESOLV.CONF(5)
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