resolv.conf — resolver configuration file
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. The configuration file is considered a trusted
source of DNS information (e.g., DNSSEC AD-bit information
will be returned unmodified from this source).
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:
nameserverName 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
> name servers may be listed, one
per keyword. If there are multiple servers, the
resolver library queries them in the order listed. If
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
domainLocal 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.
searchSearch 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
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.
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 220.127.116.11/255.255.240.0 18.104.22.168
Options allows certain internal resolver variables to be modified. The syntax is
- options option ...
optionis one of the following:
_res.options(effective only if glibc was built with debug support; see resolver(3)).
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
nis 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.
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
>The value for this option is silently capped to 30.
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
>The value for this option is silently capped to 5.
_res.options, which causes round-robin selection of name servers 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.
_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.
_res.options. This has the effect of trying an 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)
_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.
no-ip6-dotint(since glibc 2.3.4)
_res.options. When this option is clear (
ip6-dotint), reverse IPv6 lookups are made in the (deprecated)
ip6.intzone; when this option is set (
no-ip6-dotint), reverse IPv6 lookups are made in the
ip6.arpazone by default. This option is set by default.
edns0(since glibc 2.6)
_res.options. This enables support for the DNS extensions described in RFC 2671.
single-request(since glibc 2.10)
_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)
_res.options. 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.
no-tld-query(since glibc 2.14)
_res.options. This option causes
res_nsearch() to not attempt to resolve an unqualified name as if it were a top level domain (TLD). This option can cause problems if the site has ``localhost'' as a TLD rather than having localhost on one or more elements of the search list. This option has no effect if neither RES_DEFNAMES or RES_DNSRCH is set.
use-vc(since glibc 2.14)
_res.options. This option forces the use of TCP for DNS resolutions.
keywords are mutually exclusive. If more than one
instance of these keywords is present, the last
keyword of a system's
resolv.conf file can be
overridden on a per-process basis by setting the environment
LOCALDOMAIN to a
space-separated list of search domains.
keyword of a system's
resolv.conf file can be
amended on a per-process basis by setting the environment
RES_OPTIONS to a
space-separated list of resolver options as explained above
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
Lines that contain a semicolon (;) or hash character (#) in the first column are treated as comments.
Name Server Operations Guide for BIND
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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 $
Added ndots remark by Bernhard R. Link - debian bug #182886