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 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
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 ...
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
- 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 the Internet.
- ip6-dotint/no-ip6-dotint (glibc 2.3.4 to
- 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 resolving process).
- 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
- 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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