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).
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_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 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.
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 version 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.
