RESOLVER(5) File Formats Manual RESOLVER(5)

resolver — resolver configuration file format

resolv.conf

The 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.

macOS supports a DNS search strategy that may involve multiple DNS resolver clients. See the SEARCH STRATEGY section below for an overview of multi-client DNS search.

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 /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.

The different configuration options are:

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, 10.0.0.17.55 specifies that the nameserver at 10.0.0.17 uses port 55. 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).
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 nameserver above) to override the default or the port number specified as a value for this keyword.
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 /etc/resolver directory. In that case the file name is used as the domain name. However, 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 SEARCH STRATEGY section for more details.
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.

The search list is currently limited to six domains with a total of 256 characters.

Only required for those clients that share a domain name with other clients. Queries will be sent to these clients in order by ascending 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.
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
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.
Options allows certain internal resolver variables to be modified. The syntax is

options option ...

where is one of the following:

sets RES_DEBUG in _res.options.
sets RES_USEVC to use TCP instead of UDP for queries.
:n
sets a threshold for the number of dots which must appear in a name given to () (see resolver(3)) before an will be made. The default for 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.
: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 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 RES_TIMEOUT, the allowed maximum is RES_MAXRETRANS (see <resolv.h>).
: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>).
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.
: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.

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.

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 /etc/resolv.conf file or to the "primary" DNS configuration on the system are used for the query.

If multiple clients are available for the same domain name, the clients ordered according to a search_order value (see above). Queries are sent to these resolvers in sequence by ascending value of search_order.

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 /etc/resolv.conf file and in the files found in the /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.

/etc/resolv.conf
The file resolv.conf resides in /etc.
/etc/resolver/*
 

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 4.4.4.4

# Attach an OPT pseudo-RR for the EDNS0 extension,
# as specified in RFC 2671.
options edns0

gethostbyname(3), resolver(3), hostname(7)

The resolv.conf file format appeared in 4.3BSD.

November 23, 2022 Mac OS X 14