resolv.conf - resolver configuration file
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
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
- 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 188.8.131.52/255.255.240.0 184.108.40.206
- Options allows certain internal resolver variables to be modified. The
- options option ...
is one of the following:
- Sets RES_DEBUG in _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 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.
- 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.
This may not be the total time taken by any resolver API call and
there is no guarantee that a single resolver API call maps to a single
timeout. Measured in seconds, the default is RES_TIMEOUT (currently
5, see <resolv.h>). 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
<resolv.h>). The value for this option is silently capped to
- Sets RES_ROTATE in _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.
- 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.
- Sets RES_USE_INET6 in _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. Since glibc 2.25,
this option is deprecated; applications should use getaddrinfo(3),
rather than gethostbyname(3).
- ip6-bytestring (since glibc 2.3.4)
- Sets RES_USEBSTRING 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 (which is the default), then
nibble format is used. This option was removed in glibc 2.25, since it
relied on a backward-incompatible DNS extension that was never deployed on
- ip6-dotint/no-ip6-dotint (glibc 2.3.4 to 2.24)
- 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. These options are available in glibc
versions up to 2.24, where no-ip6-dotint is the default. Since
ip6-dotint support long ago ceased to be available on the Internet,
these options were removed in glibc 2.25.
- 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
- single-request-reopen (since glibc 2.9)
- Sets RES_SNGLKUPREOP in _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)
- Sets RES_NOTLDQUERY in _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)
- Sets RES_USEVC in _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 instance wins.
keyword of a system's resolv.conf
file can be
overridden on a per-process basis by setting the environment variable
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 variable RES_OPTIONS
to a space-separated list of resolver options as explained above under
The keyword and value must appear on a single line, and the keyword (e.g.,
) 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.
Name Server Operations Guide for BIND
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