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resolv.conf - resolver configuration file
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.
On a normally configured system this file should not be necessary. The
only name server to be queried will be on the local machine; the domain
name is determined from the host name and the domain search path is
constructed from the domain name.
The different configuration options are:
nameserver Name server IP address
Internet address (in dot notation) of a name server that the
resolver should query. 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
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 host name returned by
gethostname(); the domain part is taken to be everything after
the first ‘.’. Finally, if the host name 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 allows addresses returned by gethostbyname 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. E.g.,
sortlist 188.8.131.52/255.255.240.0 184.108.40.206
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.
sets a threshold for the number of dots which must appear
in a name given to res_query() (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.
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>).
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>).
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.
sets RES_NOCHECKNAME in _res.options, which disables the
modern BIND checking of incoming host names 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() function, and of mapping IPv4 responses
in IPv6 ‘‘tunnelled form’’ if no AAAA records are found
but an A record set exists.
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
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.
gethostbyname(3), resolver(3), hostname(7), named(8)
Name Server Operations Guide for BIND