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resolver(5): Overhaul

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* Modernize the markup

* Describe the comment syntax

* Drop obsolete advice

* Capitalize sentences

* Improve the language

* Replace no_tld_query with no-tld-query; both are supported, but all the
  other multi-word options use hyphens rather than underscores.

* Add missing ENVIRONMENT section

* Redo the example

MFC after:	1 week
Reviewed by:	markj
Differential Revision:	https://reviews.freebsd.org/D57921
This commit is contained in:
Dag-Erling Smørgrav committed 2026-07-06 14:23:20 +02:00
1 parent 24576c3978
commit 4319e4bf42
1 file changed
+116 -111
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share/man/man5/resolver.5
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@@ -1,5 +1,6 @@
.\" Copyright (c) 1986, 1991, 1993
.\" The Regents of the University of California. All rights reserved.
.\" Copyright (c) 2026 Dag-Erling Smørgrav
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
@@ -25,7 +26,7 @@
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.Dd March 15, 2026
.Dd July 6, 2026
.Dt RESOLVER 5
.Os
.Sh NAME
@@ -40,57 +41,56 @@ 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.
The file is read line by line.
Anything after a
.Sq #
or
.Sq \&;
character is considered a comment and discarded.
Blank lines are ignored.
The first word on each non-blank line is a keyword which determines
how the rest of the line is interpreted.
Available keywords are:
.Bl -tag -width indent
.It Cm nameserver Ar address
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.
(currently 3) name servers may be listed on separate lines.
If there are multiple servers, the resolver library queries them in
the order listed, unless
.Dv RES_ROTATE
is in effect
.Po
See the
.Cm rotate
option below
.Pc .
If no
.Sy nameserver
.Cm 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.
.It Cm domain Ar name
Set both the local domain name and the search list to
.Ar 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
If no domain name has been set, it is derived 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.
.It Cm search Ar name1 Op name2 ...
Set the domain search list.
Additionally, sets the local domain to
.Ar name1 .
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
@@ -101,7 +101,7 @@ for one of the domains.
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.
Sortlist allows addresses returned by the resolver 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;
@@ -113,26 +113,21 @@ 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
.It Cm options Ar option1 Op Ar option2 ...
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
The following options are supported:
.Bl -tag -width indent
.It Cm debug
Set
.Dv RES_DEBUG
in _res.options.
.It Sy usevc
sets
in
.Va _res.options .
.It Cm usevc
Set
.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
.It Cm ndots Ns Li : Ns Ar n
Set a threshold for the number of dots which must appear in a name given to
.Fn res_query
(see
.Xr resolver 3 )
@@ -147,8 +142,8 @@ 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
.It Cm timeout Ns Li : Ns Ar n
Set 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
@@ -160,8 +155,8 @@ 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
.It Cm attempts Ns Li : Ns Ar n
Set 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
@@ -170,118 +165,128 @@ the allowed maximum is
.Dv RES_MAXRETRY
(see
.In resolv.h ) .
.It Sy edns0
Sets
.It Cm edns0
Set
.Dv 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.
.It Sy inet6
Sets
.It Cm inet6
Set
.Dv RES_USE_INET6 .
Causes
.Xr gethostbyname 3
Causes the resolver
to look up AAAA records before A records
and to map IPv4 responses into IPv6 addresses.
The use of this option is discouraged.
.It Sy insecure1
Sets
.It Cm insecure1
Set
.Dv RES_INSECURE1 .
Disables the check that the response was received from the
This 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.
.It Sy insecure2
Sets
.It Cm insecure2
Set
.Dv RES_INSECURE2 .
Disables the check that the response contains a query
This 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.
.It Sy no-check-names
Sets
.It Cm no-check-names
Set
.Dv RES_NOCHECKNAME .
Disables the check of incoming host names for invalid characters
This disables the check of incoming host names for invalid characters
such as underscore, non-ASCII, or control characters.
.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.
.It Cm no-tld-query
Set
.Dv RES_NOTLDQUERY .
This 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
.Cm domain
and
.Sy search
.Cm search
rules with the given name.
.It Sy rotate
Sets
.It Cm rotate
Set
.Dv 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.
.It Sy reload-period : Ns Ar n
This causes the resolver to round-robin among the configured name
servers, distributing the query load instead of always trying the
first listed server.
.It Cm reload-period Ns Li : Ns Ar n
The resolver checks the modification time of
.Pa /etc/resolv.conf
every
if more than
.Ar n
seconds.
If
.Pa /etc/resolv.conf
has changed, it is automatically reloaded.
seconds have elapsed since the last check.
If the file 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
Options may also be passed as a space- or tab-separated list in the
.Dv RES_OPTIONS
environment variable.
This environment variable is applied after the configuration file has
been read.
.El
.Pp
The
.Sy domain
.Cm domain
and
.Sy search
.Cm search
keywords are mutually exclusive.
If more than one instance of these keywords is present,
the last instance will override.
the last instance will override all previous instances.
.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 ENVIRONMENT
.Bl -tag -width indent
.It Dv LOCALDOMAIN
If set, overrides the local domain and search list set in the resolver
configuration file.
.It Dv RES_OPTIONS
If set, its value is split into words and interpreted as resolver
options as described for the
.Cm options
keyword in the resolver configuration file.
.El
.Sh FILES
.Bl -tag -width /etc/resolv.conf -compact
.Bl -tag -width indent
.It Pa /etc/resolv.conf
The file
.Nm resolv.conf
resides in
.Pa /etc .
resolver configuration file
.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
.Bd -literal
# Set the local domain to example.com and the search list to
# example.com, example.net, example.org
search example.com example.net example.org
# In case you a running a local dns server or caching name server
# like local-unbound(8) for example.
# Set the local domain and search list to example.com,
# completely overriding the previous line
domain example.com
# Use this if you are running a local caching forwarding (or
# recursing) resolver like local-unbound(8)
nameserver 127.0.0.1
# IP address of the local or ISP name service
nameserver 192.168.2.1
nameserver 198.51.100.53
nameserver 203.0.113.53
# 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.
# Enable EDNS0, which tells the name server that we are capable of
# handling large responses. This is required for DNSSEC.
options edns0
.Ed
.Sh SEE ALSO
.Xr getaddrinfo 3 ,
.Xr gethostbyname 3 ,
.Xr gethostname 3 ,
.Xr resolver 3 ,
.Xr hostname 7 ,
.Xr resolvconf 8