resolver
RESOLV.CONF(5) Linux Programmer's Manual RESOLV.CONF(5)
NAME
resolv.conf - resolver configuration file
SYNOPSIS
/etc/resolv.conf
DESCRIPTION
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. The configuration file is con-
sidered a trusted source of DNS information (e.g., DNSSEC AD-bit infor-
mation will be returned unmodified from this source).
If this file does not exist, only the name server on the local machine
will be queried, and the search list contains the local domain name de-
termined from the hostname.
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 ad-
dress 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 re-
solver 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.)
search Search list for host-name lookup.
By default, the search list contains one entry, the local domain
name. It is determined from the local hostname returned by
gethostname(2); the local domain name is taken to be everything
after the first '.'. Finally, if the hostname does not contain
a '.', the root domain is assumed as 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.
If there are multiple search directives, only the search list
from the last instance is used.
In glibc 2.25 and earlier, the search list is limited to six do-
mains with a total of 256 characters. Since glibc 2.26, the
search list is unlimited.
The domain directive is an obsolete name for the search direc-
tive that handles one search list entry only.
sortlist
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 sepa-
rated by slashes. Up to 10 pairs may be specified. Here is an
example:
sortlist 130.155.160.0/255.255.240.0 130.155.0.0
options
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)).
ndots:n
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.
timeout:n
Sets the amount of time the resolver will wait for a re-
sponse 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 sin-
gle timeout. Measured in seconds, the default is
RES_TIMEOUT (currently 5, see <resolv.h>). The value for
this option is silently capped to 30.
attempts:n
Sets the number of times the resolver will send a query
to its name servers before giving up and returning an er-
ror 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 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.
no-check-names
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 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).
Some programs behave strangely when this option is turned
on.
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 de-
scribed 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 2.24)
Clear/set RES_NOIP6DOTINT in _res.options. When this op-
tion 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 de-
fault. 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 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)
Sets RES_SNGLKUPREOP in _res.options. The resolver uses
the same socket for the A and AAAA requests. Some hard-
ware mistakenly sends back only one reply. When that
happens the client system will sit and wait for the sec-
ond reply. Turning this option on changes this behavior
so that if two requests from the same port are not han-
dled correctly it will close the socket and open a new
one before sending the second request.
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.
no-reload (since glibc 2.26)
Sets RES_NORELOAD in _res.options. This option disables
automatic reloading of a changed configuration file.
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 op-
tions.
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.
FILES
/etc/resolv.conf, <resolv.h>
SEE ALSO
gethostbyname(3), resolver(3), host.conf(5), hosts(5), nss-
witch.conf(5), hostname(7), named(8)
Name Server Operations Guide for BIND
COLOPHON
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4th Berkeley Distribution 2019-10-10 RESOLV.CONF(5)
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