Ubuntu Manpage: resolv.conf - resolver configuration file

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 considered a trusted
source of DNS information; see the trust-ad option below for details.

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).

Some programs behave strangely when this option is turned on.

ip6-bytestring (since glibc 2.3.4 to glibc 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 glibc 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 up to glibc
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_EDNS0 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 glibc 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.

trust-ad (since glibc 2.31)
Sets RES_TRUSTAD in _res.options. This option controls the AD bit behavior
of the stub resolver. If a validating resolver sets the AD bit in a
response, it indicates that the data in the response was verified according
to the DNSSEC protocol. In order to rely on the AD bit, the local system
has to trust both the DNSSEC-validating resolver and the network path to it,
which is why an explicit opt-in is required. If the trust-ad option is
active, the stub resolver sets the AD bit in outgoing DNS queries (to enable
AD bit support), and preserves the AD bit in responses. Without this
option, the AD bit is not set in queries, and it is always removed from
responses before they are returned to the application. This means that
applications can trust the AD bit in responses if the trust-ad option has
been set correctly.

In glibc 2.30 and earlier, the AD is not set automatically in queries, and
is passed through unchanged to applications in responses.

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.

FILES

       /etc/resolv.conf, <resolv.h>

NAME

SYNOPSIS

DESCRIPTION

FILES

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