RESOLVER(5) BSD File Formats Manual RESOLVER(5)NAMEresolver — DNS client
The resolver is the DNS client used on most Linux and BSD systems. It
comes with glibc. Its configuration file /etc/resolv.conf (note the
spelling) determines the DNS servers to use, and various other options -
Almost all machines have a DNS server set up in this file - if it doesn't
exist, the system will assume there's a DNS server running on the local
machine, and work out the search path from the machines domain name.
The config file is read the first time the DNS client is invoked by a
The different configuration options are:
nameserver IP address of a DNS server to use. Multiple name servers may
be listed, each on their own line. The resolver will use them
in order listed - if the first server times out answering the
query, the next server will be tried, and so on. If the
resolver runs out out of name servers, the first server will
be queried again, until a maximum number of retries are made.
The maximum number of DNS servers to use is set by MAXNS (see
search Domain(s) to use for DNS lookups when no domain is specified.
List each domain following the search keyword with spaces or
tabs between them. Each possible domain will be checked in
order until a match is found. Note that this process may be
slow (queries will time out if no server is available for a
domain) and will generate a lot of network traffic if the
servers for the listed domains aren't local.
The search list is currently limited to six domains with a
total of 256 characters. If search isn't specified, the
search list will be determined from the local domain name
(whatever comes after the first dot). If the host name
doesn't contain a domain, the root domain is used.
By default, it search contains only the local domain name.
domain Local domain name. You can use this instead of the search
option to specify a single domain to check if a hostname
isn't specified. Most people just use search instead (that
option lets you use multiple servers, domain doesn't). You
can't use domain and search at the same time - they're mutu‐
If domain isn't specified, the domain will be determined from
the local domain name (whatever comes after the first dot).
If the host name doesn't contain a domain, the root domain is
sortlist Sorts addresses returned by the gethostbyname system call. A
sortlist is specified by IP address netmask pairs. The net‐
mask 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. For 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 ...
where option is one of the following:
debug sets RES_DEBUG in _res.options.
ndots:n 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 retry‐
ing the query via a different name server. Mea‐
sured in seconds, the default is RES_TIMEOUT (see
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 (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
sets RES_NOCHECKNAME in _res.options, which dis‐
ables 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.
ip6-dotint / no-ip6-dotint
sets / clears the RES_NOIP6DOTINT bit in
_res.options, which when set (ip6-dotint) will
enable reverse IPv6 lookups to be made in the (dep‐
recated) ip6.int zone; when clear (no-ip6-dotint),
reverse IPv6 lookups are made in the ip6.arpa zone
sets RES_USEBSTRING in _res.options. This causes
reverse IPv6 lookups to be made using the bit-label
format of RFC 2673; if not set, then nibble format
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.
SEE ALSOgethostbyname(3), hostname(7), named(8), resolver(3), resolver(5). “Name
Server Operations Guide for BIND”
4th Berkeley Distribution June 23, 2004 4th Berkeley Distribution