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.\" Portions Copyright (c) 2003 by Apple Computer, Inc.
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.\"     @(#)resolver.5	8.1 (Berkeley) 6/5/93
.\" $FreeBSD$
.\"
.Dd November 23, 2022
.Dt RESOLVER 5
.Os
.Sh NAME
.Nm resolver
.Nd resolver configuration file format
.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 (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.
.Pp
macOS supports a DNS search strategy that may involve multiple
DNS resolver clients.
See the
.Sx SEARCH STRATEGY
section below for an overview of multi-client DNS
search.
.Pp
Each DNS client is configured using the contents of a single configuration
file of the format described below, or from a property list supplied from
some other system configuration database.
Note that the 
.Pa /etc/resolv.conf
file, which contains configuration for the default (or "primary") DNS resolver client,
is maintained automatically by macOS and should not be edited manually.
Changes to the DNS configuration should be made by using the Network
Preferences panel.
.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 address may optionally have a trailing dot followed by a port number.
For example, 
.Li 10.0.0.17.55
specifies that the nameserver at 10.0.0.17
uses port 55.
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 port
IP port number to be used for this resolver.
The default port is 53.
The port number for an individual nameserver may be specified as
part of the nameserver address (see 
.Sx nameserver 
above) to override the default 
or the port number specified as a value for this keyword.
.It Sy domain
Domain name associated with this resolver configuration.
This option is normally not required by the macOS DNS search system
when the resolver configuration is read from a file in the
.Pa /etc/resolver
directory.
In that case the file name is used as the domain name.
However, 
.Sx domain
must be provided when there are 
multiple resolver clients for the same domain name, since multiple
files may not exist having the same name.
See the 
.Sx SEARCH STRATEGY
section for more details.
.It Sy search
Search list for host-name lookup.
This parameter is only used by the "Super" DNS resolver, which
manages the DNS search strategy amongst multiple DNS resolver clients.
Unqualified 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 search_order
Only required for those clients that share a domain name with other clients.
Queries will be sent to these clients in order by ascending
.Sx search_order
value.
For example, this allows two clients for the ".local"
domain, which is used by Apple's multicast DNS, but which may
also be used at some sites as private DNS domain name.
.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 timeout
Specifies the total amount of time allowed for a name resolution.
This time interval is divided by the number of nameservers and the number
of retries allowed for each nameserver.
.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 total timeout allowed for a query depends on the number of retries and the
number of nameservers.  This value is ignored if a total timeout is specified
using the
.Sx timeout
keyword (see above).
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 SEARCH STRATEGY
macOS uses a DNS search strategy that supports multiple DNS
client configurations.
Each DNS client has its own set of nameserver
addresses and its own set of operational parameters.
Each client can perform DNS queries and searches independent of other clients.
Each client has a symbolic name which is of the same format as a
domain name, e.g. "apple.com".
A special meta-client, known as the
"Super" DNS client acts as a router for DNS queries.
The Super client chooses among all available clients by finding a best match
between the domain name given in a query and the names of all known clients.
.Pp
Queries for qualified names
are sent using a client configuration
that best matches the domain name given in the query.
For example, if there is a client named "apple.com", a search for
"www.apple.com" would use the resolver configuration specified for that client.
The matching algorithm chooses the client with the maximum number of matching
domain components.
For example, if there are clients named "a.b.c", and "b.c", a search for
"x.a.b.c" would use the "a.b.c" resolver configuration, while a search
for "x.y.b.c" would use the "b.c" client.
If there are no matches, the configuration settings in the default client,
generally corresponding to the
.Pa /etc/resolv.conf
file or to the "primary" DNS
configuration on the system are used for the query.
.Pp
If multiple clients are available for the same domain name, the clients ordered
according to a
.Sx search_order
value (see above).
Queries are sent to these resolvers in sequence by ascending value of
search_order.
.Pp
The configuration for a particular client may be read from a file
having the format described in this man page.
These are at present located by the system in the 
.Pa /etc/resolv.conf
file and
in the files found in the 
.Pa /etc/resolver
directory.
However, client configurations are not limited to file storage.
The implementation of the DNS multi-client search strategy may also locate
client configuratins in other data sources, such as the System Configuration
Database.
Users of the DNS system should make no assumptions about the
source of the configuration data.
.Sh FILES
.Bl -tag -width /etc/resolv.conf -compact
.It Pa /etc/resolv.conf
The file
.Nm resolv.conf
resides in
.Pa /etc .
.It Pa /etc/resolver/*
.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 hostname 7 ,
.\"Xr resolvconf 8
.Sh HISTORY
The
.Nm resolv.conf
file format appeared in
.Bx 4.3 .