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NAME
resolv.conf - resolver configuration file
SYNOPSIS
/etc/resolv.conf
DESCRIPTION
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
set to '.', the root domain is considered. 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.
Some programs behave strangely when this option is turned on.
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.
FILES
/etc/resolv.conf, <resolv.h>
SEE ALSO
gethostbyname(3), resolver(3), hostname(7), named(8)
Name Server Operations Guide for BIND
COLOPHON
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