RESOLV.CONF(5) Linux Programmer's Manual RESOLV.CONF(5)
resolv.conf - resolver configuration file
/etc/resolv.conf
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 considered a trusted source of DNS information
(e.g., DNSSEC AD-bit information 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 determined 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
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
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 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
domains 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
directive 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 separated 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
modified. 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
response 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
single 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
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 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).
ip6-bytestring (since glibc 2.3.4 to 2.24)
Sets RES_USEBSTRING in _res.options. This causes
reverse IPv6 lookups to be made using the bit-label
format described 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
option 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 default. 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
performs 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
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 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 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.
Lines that contain a semicolon (;) or hash character (#) in the first
column are treated as comments.
/etc/resolv.conf, <resolv.h>
gethostbyname(3), resolver(3), host.conf(5), hosts(5),
nsswitch.conf(5), hostname(7), named(8)
Name Server Operations Guide for BIND
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4th Berkeley Distribution 2020-08-13 RESOLV.CONF(5)
Pages that refer to this page: pmdanetcheck(1) , pmhostname(1) , dn_comp(3) , dn_expand(3) , res_init(3) , res_mkquery(3) , res_nclose(3) , res_ninit(3) , res_nmkquery(3) , res_nquery(3) , res_nquerydomain(3) , res_nsearch(3) , res_nsend(3) , resolver(3) , res_query(3) , res_querydomain(3) , res_search(3) , res_send(3) , hosts(5) , hostname(7)
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