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.
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 name-
server 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 deter-
mined 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 follow-
ing 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 avail-
able for one of the domains.
sortlist 184.108.40.206/255.255.240.0 220.127.116.11
Options allows certain internal resolver variables to be modi-
fied. The syntax is
options option ...
where option is one of the following:
debug 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. 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 5.
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 hostnames 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 geth-
ostbyname(3) function, and of mapping IPv4 responses in
IPv6 "tunneled form" if no AAAA records are found but an
A record set exists.
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. This option is set by
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 per-
forms 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)
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
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.
Lines that contain a semicolon (;) or hash character (#) in the first
column are treated as comments.
gethostbyname(3), resolver(3), hostname(7), named(8)
Name Server Operations Guide for BIND
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