RESOLVER(5) FreeBSD File Formats Manual RESOLVER(5)
NAME
resolver - resolver configuration file
SYNOPSIS
resolv.conf
DESCRIPTION
The resolver(3) is a set of routines in the C library which provide
access to the Internet Domain Name System. 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.
On a normally configured system, setting this file manually should not be
necessary. The only name server(s) to be queried will be on the local
machine or automatically configured using DHCP or a similar mechanism,
the domain name is determined from the host name, and the domain search
path is constructed from the domain name.
The different configuration options are:
nameserver IPv4 or IPv6 address of a name server that the resolver
should query. Up to MAXNS (currently 3) 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).
domain Local domain name. Most queries for names within this domain
can use short names relative to the local domain. If no
domain entry is present, the domain is determined from the
local host name returned by gethostname(3); the domain part
is taken to be everything after the first `.'. Finally, if
the host name does not contain a domain part, the root domain
is assumed.
search Search list for host-name lookup. The search list is
normally determined from the local domain name; by default,
it contains only 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.
Most resolver queries will be attempted using each component
of the search path in turn until a match is found. 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.
The search list is currently limited to six domains with a
total of 256 characters.
sortlist Sortlist allows addresses returned by gethostbyname to be
sorted. A sortlist is specified by IP address netmask pairs.
If the netmask is not specified, it defaults to the
historical Class A/B/C netmask of the net; this usage is
deprecated. The IP address and network pairs are separated
by slashes. Up to 10 pairs may be specified. E.g.,
sortlist 10.9.1.0/255.255.240.0 10.9.0.0/255.255.0.0
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.
usevc sets RES_USEVC to use TCP instead of UDP for
queries.
ndots:n sets a threshold for the number of dots which
must appear in a name given to res_query() (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.
timeout:n sets the initial amount of time the resolver
will wait for a response from a remote name
server before retrying the query via a
different name server. The resolver may wait
longer during subsequent retries of the current
query since an exponential back-off is applied
to the timeout value. Measured in seconds, the
default is RES_TIMEOUT, the allowed maximum is
RES_MAXRETRANS (see <resolv.h>).
attempts:n sets the number of times the resolver will send
a query to each of its name servers before
giving up and returning an error to the calling
application. The default is RES_DFLRETRY, the
allowed maximum is RES_MAXRETRY (see
<resolv.h>).
edns0 Sets RES_USE_EDNS0. Attach an OPT pseudo-RR
for the EDNS0 extension, as specified in RFC
2671. This allows the resolver to advertise a
larger UDP receive buffer size, permitting
responses larger than the original 512-byte
limit.
inet6 Sets RES_USE_INET6. Causes gethostbyname(3) to
look up AAAA records before A records and to
map IPv4 responses into IPv6 addresses. The
use of this option is discouraged.
insecure1 Sets RES_INSECURE1. Disables the check that
the response was received from the same server
to which the query was sent. Use of this
option is a security risk and is not
recommended.
insecure2 Sets RES_INSECURE2. Disables the check that
the response contains a query matching the one
that was sent. Use of this option is a
security risk and is not recommended.
no-check-names
Sets RES_NOCHECKNAME. Disables the check of
incoming host names for invalid characters such
as underscore, non-ASCII, or control
characters.
no_tld_query tells the resolver not to attempt to resolve a
top level domain name, that is, a name that
contains no dots. Use of this option does not
prevent the resolver from obeying the standard
domain and search rules with the given name.
rotate Sets RES_ROTATE. Causes the resolver to round-
robin among the configured name servers,
distributing the query load instead of always
trying the first listed server.
reload-period:n
The resolver checks the modification time of
/etc/resolv.conf every n seconds. If
/etc/resolv.conf has changed, it is
automatically reloaded. The default for n is
two seconds. Setting it to zero disables the
file check.
Options may also be specified as a space or tab separated
list using the RES_OPTIONS environment variable.
The domain and search keywords are mutually exclusive. If more than one
instance of these keywords is present, the last instance will override.
The keyword and value must appear on a single line, and the keyword (for
example, nameserver) must start the line. The value follows the keyword,
separated by white space.
FILES
/etc/resolv.conf The file resolv.conf resides in /etc.
EXAMPLES
A basic resolv.conf file could be in the following form.
# The domain directive is only necessary, if your local
# router advertises something like localdomain and you have
# set up your hostnames via an external domain.
domain localdomain.tld
# In case you a running a local dns server or caching name server
# like local-unbound(8) for example.
nameserver 127.0.0.1
# IP address of the local or ISP name service
nameserver 192.168.2.1
# Fallback nameservers, in this case these from Google.
nameserver 8.8.8.8
nameserver 8.8.4.4
# Attach an OPT pseudo-RR for the EDNS0 extension,
# as specified in RFC 2671.
options edns0
SEE ALSO
gethostbyname(3), resolver(3), hostname(7), resolvconf(8)
HISTORY
The resolv.conf file format appeared in 4.3BSD.
FreeBSD 15.1-STABLE-HBSD March 15, 2026 RESOLVER(5)