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; 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.
/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