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RESOLVER(5) BSD File Formats Manual RESOLVER(5)
NAME
resolver - DNS client
SYNOPSIS
/etc/resolv.conf
DESCRIPTION
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 - see below.
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 process.
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 <resolv.h>
)
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 deter-
mined 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 spec-
ify 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 mutually exclusive.
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 used.
sortlist Sorts addresses returned by the gethostbyname system call. 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.
For example:
sortlist 130.155.160.0/255.255.240.0 130.155.0.0
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.
timeout:n
sets the amount of time the resolver will wait for a response
from a remote name server before retrying the query via a dif-
ferent name server. Measured in seconds, the default is
RES_TIMEOUT (see <resolv.h> ).
attempts:n
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 time.
no-check-names
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 char-
acters.
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 (deprecated) ip6.int zone; when clear (no-
ip6-dotint), reverse IPv6 lookups are made in the ip6.arpa zone
by default.
ip6-bytestring
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 is used.
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 options.
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.
FILES
/etc/resolv.conf <resolv.h>
SEE ALSO
gethostbyname(3), hostname(7), named(8), resolver(3), resolver(5). “Name Server
Operations Guide for BIND”
4th Berkeley Distribution June 23, 2004 4th Berkeley Distribution
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