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.Dd November 23, 2022
.Dt RESOLVER 5
.Os
.Sh NAME
.Nm resolver
.Nd resolver configuration file
.Sh SYNOPSIS
.Nm resolv.conf
.Sh DESCRIPTION
The
.Xr 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.
.Pp
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.
.Pp
The different configuration options are:
.Bl -tag -width nameserver
.It Sy nameserver
IPv4 or IPv6 address of a name server
that the resolver should query.
Up to
.Dv 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
.Sy 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).
.It Sy domain
Local domain name.
Most queries for names within this domain can use short names
relative to the local domain.
If no
.Sy domain
entry is present, the domain is determined
from the local host name returned by
.Xr gethostname 3 ;
the domain part is taken to be everything after the first
.Ql \&. .
Finally, if the host name does not contain a domain part, the root
domain is assumed.
.It Sy 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
.Sy 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.
.Pp
The search list is currently limited to six domains
with a total of 256 characters.
.It Sy 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.,
.Pp
.Dl "sortlist 10.9.1.0/255.255.240.0 10.9.0.0/255.255.0.0"
.It Sy options
Options allows certain internal resolver variables to be modified.
The syntax is
.Pp
\fBoptions\fP \fIoption\fP \fI...\fP
.Pp
where
.Sy option
is one of the following:
.Bl -tag -width no_tld_query
.It Sy debug
sets
.Dv RES_DEBUG
in _res.options.
.It Sy usevc
sets
.Dv RES_USEVC
to use TCP instead of UDP for queries.
.It Sy ndots : Ns Ar n
sets a threshold for the number of dots which must appear in a name given to
.Fn res_query
(see
.Xr resolver 3 )
before an
.Em initial absolute query
will be made.
The default for
.Em n
is
.Dq 1 ,
meaning that if there are any dots in a name, the name
will be tried first as an absolute name before any
.Em search list
elements are appended to it.
.It Sy timeout : Ns Ar 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
.Dv RES_TIMEOUT ,
the allowed maximum is
.Dv RES_MAXRETRANS
(see
.In resolv.h ) .
.It Sy attempts : Ns Ar 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
.Dv RES_DFLRETRY ,
the allowed maximum is
.Dv RES_MAXRETRY
(see
.In resolv.h ) .
.It Sy 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
.Sy domain
and
.Sy search
rules with the given name.
.It Sy reload-period : Ns Ar n
The resolver checks the modification time of
.Pa /etc/resolv.conf
every
.Ar n
seconds.
If
.Pa /etc/resolv.conf
has changed, it is automatically reloaded.
The default for
.Ar n
is two seconds.
Setting it to zero disables the file check.
.El
.Pp
Options may also be specified as a space or tab separated list using the
.Dv RES_OPTIONS
environment variable.
.El
.Pp
The
.Sy domain
and
.Sy search
keywords are mutually exclusive.
If more than one instance of these keywords is present,
the last instance will override.
.Pp
The keyword and value must appear on a single line, and the keyword
.Pq for example, Sy nameserver
must start the line.
The value follows the keyword, separated by white space.
.Sh FILES
.Bl -tag -width /etc/resolv.conf -compact
.It Pa /etc/resolv.conf
The file
.Nm resolv.conf
resides in
.Pa /etc .
.El
.Sh EXAMPLES
A basic resolv.conf file could be in the following form.
.Bd -literal -offset indent
# 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 4.4.4.4

# Attach an OPT pseudo-RR for the EDNS0 extension,
# as specified in RFC 2671.
options edns0
.Ed
.Sh SEE ALSO
.Xr gethostbyname 3 ,
.Xr resolver 3 ,
.Xr hostname 7 ,
.Xr resolvconf 8
.Sh HISTORY
The
.Nm resolv.conf
file format appeared in
.Bx 4.3 .