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RESOLV.CONF(5) File Formats Manual RESOLV.CONF(5)

resolv.conf, resolv.conf.tailresolver configuration files

The resolv.conf file specifies how the resolver(3) routines in the C library (which provide access to the Internet Domain Name System) should operate. The resolver configuration file contains information that is read by the resolver routines the first time they are invoked by a process. If the resolv.conf file does not exist, only the local host file /etc/hosts will be consulted, i.e. the Domain Name System will not be used to resolve hosts.

The file is designed to be human readable and contains a list of keywords with values that provide various types of resolver information. A resolv.conf file is not required for some setups, so this file is optional. It can be created manually, and is also created as part of the OpenBSD install process if use of the DHCP protocol is specified for any interface or if any DNS nameservers are configured.

If dhclient(8) is used to configure an interface it will overwrite resolv.conf whenever the interface becomes the default gateway. The information written is generated from the DHCP options domain-name-servers, domain-name and domain-search and the contents of the file resolv.conf.tail are appended to the generated information. If dhclient has no domain-name-servers, no domain-name and no domain-search information then it will not overwrite the existing resolv.conf, even if resolv.conf.tail exists. resolv.conf.tail is normally used to specify resolver(3) options that are not available via DHCP. e.g. lookup or family.

A keyword and its values must appear on a single line, and the keyword (e.g. nameserver) must start the line. The value follows the keyword, separated by whitespace. A hash mark (#) or semicolon (;) in the file indicates the beginning of a comment; subsequent characters up to the end of the line are not interpreted by the routines that read the file.

The configuration options (which may be placed in either file) are:

IPv4 address (in dot notation) or IPv6 address (in hex-and-colon notation) of a name server that the resolver should query. Scoped IPv6 address notation is accepted as well (see inet6(4) for details).

Up to ASR_MAXNS (currently 5) name servers may be listed, one per line. 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 name servers until a maximum number of retries are performed.)

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 dot. Finally, if the host name does not contain a domain part, the root domain is assumed.
This keyword is used by the library routines gethostbyname(3) and gethostbyaddr(3). It specifies which databases should be searched, and the order to do so. The legal space-separated values are:

Query a domain name server.
Search for entries in /etc/hosts.

If the lookup keyword is not used in the system's resolv.conf file then the assumed order is bind file. Furthermore, if the system's resolv.conf file does not exist, then the only database used is file.

Search list for hostname lookup. The search list is normally determined from the local domain name; by default, it begins with the local domain name, then successive parent domains that have at least two components in their names. 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 1024 characters. Only one search line should appear; if more than one is present, the last one found overwrites any values found in earlier lines. So if such a line appears in the resolv.conf.tail file, it should include all the domains that need to be searched.

Allows addresses returned by gethostbyname(3) to be sorted. A sortlist is specified by IP address netmask pairs. The netmask is optional and defaults to the natural netmask of the net. The IP address and optional network pairs are separated by slashes. Up to 10 pairs may be specified. For example:

Specify which type of Internet protocol family to prefer, if a host is reachable using different address families. By default IPv4 addresses are queried first, and then IPv6 addresses. The syntax is:
family family [family]

A maximum of two families can be specified, where family can be any of:

IPv4 queries.
IPv6 queries.

If only one family is specified, only that family is tried.

Allows certain internal resolver variables to be modified. The syntax is:
options option ...

Where option is one of the following:

Print debugging messages, if libc is compiled with DEBUG. By default on OpenBSD this option does nothing.
Attach an OPT pseudo-RR for the EDNS0 extension, as specified in RFC 2671. This informs DNS servers of a client's receive buffer size, allowing them to take advantage of a non-default receive buffer size, and thus send larger replies. DNS query packets with the EDNS0 extension are not compatible with non-EDNS0 DNS servers, so the option must be used only when all the servers listed in nameserver lines are able to handle the extension. On OpenBSD this option does nothing.

To verify whether a server supports EDNS, query it using the dig(1) query option +edns=0: the reply indicates compliance (EDNS version 0) and whether a UDP packet larger than 512 bytes can be used. Note that EDNS0 can cause the server to send packets large enough to require fragmentation. Other factors such as packet filters may impede these, particularly if there is a reduced MTU, as is often the case with pppoe(4) or with tunnels.

Enables support for IPv6-only applications, by setting RES_USE_INET6 in _res.options (see resolver(3)). On OpenBSD this option does nothing.
Do not require IP source address on the reply packet to be equal to the server's address.
Do not check if the query section of the reply packet is equal to that of the query packet. For testing purposes only.
Sets a threshold for the number of dots which must appear in a name given to res_query(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.
Forces the use of TCP for queries. Normal behaviour is to query via UDP but fall back to TCP on failure.

The domain and search keywords are mutually exclusive. If more than one instance of these keywords is present, the last instance will override.

A space-separated list of search domains, overriding the search keyword of a system's resolv.conf or resolv.conf.tail file.
A space-separated list of resolver options, overriding the options keyword of a system's resolv.conf or resolv.conf.tail file.


gethostbyname(3), resolver(3), hosts(5), hostname(7), dhclient(8), nsd(8), unbound(8)

The resolv.conf file format appeared in 4.3BSD.

August 13, 2017 OpenBSD-6.3