RESOLV.CONF(5)

NAME
resolv.conf, resolv.conf.tail - resolver configuration files

DESCRIPTION
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 the network, the DHCP client back-end dhclient-script(8) will normally overwrite the resolv.conf file with updated information such as nameserver addresses, losing any previous values the file contained. In order to force options to be passed to the resolver(3) routines, the file resolv.conf.tail may be created manually. This file will be appended to the generated resolv.conf file by dhclient-script(8), ensuring options remain.

On a machine whose network connection does not change frequently (such as a desktop machine on a local-area network), the resolv.conf.tail file should not be necessary. However the resolv.conf.tail file may be useful on notebooks, to search multiple domains, to refer to hard-coded information in local files, or otherwise override the defaults.

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:

nameserver
    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). A non-standard port may be specified using [host]:port syntax. When a non-standard port is specified the host address must be enclosed in square brackets. For example:

        nameserver [10.0.0.1]:5353
        nameserver [::1]:5353

    Up to MAXNS (currently 3) 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.)

domain
    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 “”.. Finally, if the host name does not contain a domain part, the root domain is assumed.

lookup
    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:

    bind
        Use the Domain Name server by querying named(8).
    file
        Search for entries in /etc/hosts.
    yp  Talk to the YP system if ypbind(8) is running.

    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
    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.

sortlist
    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, e.g.:

    sortlist 130.155.160.0/255.255.240.0 130.155.0.0

family
    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 family1 [family2]

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

    inet4
        IPv4 queries.
    inet6
        IPv6 queries.

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

    options option ...

    where option is one of the following:

    debug
        Sets RES_DEBUG in _res.options.

    edns0
        Attach OPT pseudo-RR for EDNS0 extension specified in RFC 2671, to inform DNS server of our receive buffer size. The option will allow DNS servers to take advantage of non-default receive buffer size, and to send larger replies. DNS query packets with EDNS0 extension are not compatible with non-EDNS0 DNS servers. The option must be used only when all the DNS servers listed in nameserver lines are able to handle EDNS0 extension.

    inet6
        Enables support for IPv6-only applications, by setting RES_USE_INET6 in _res.options (see resolver(3)). Use of this option is discouraged, and meaningless on OpenBSD.

    insecure1
        Do not require IP source address on the reply packet to be equal to the server's address.

    insecure2
        Do not check if the query section of the reply packet is equal to that of the query packet. For testing purposes only.

    ndots:n
        Sets a threshold for the number of dots which must appear in a name given to res_query (see resolver(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.

    tcp 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.

The search keyword of a system's resolv.conf or resolv.conf.tail file can be overridden on a per-process basis by setting the environment variable LOCALDOMAIN to a space-separated list of search domains.

The options keyword of a system's resolv.conf or resolv.conf.tail file can be amended on a per-process basis by setting the environment variable RES_OPTIONS to a space-separated list of resolver options as explained above.

The keyword and value must appear on a single line, and the keyword (e.g. nameserver) must start the line. The value follows the keyword, separated by whitespace.

FILES
/etc/resolv.conf
/etc/resolv.conf.tail

SEE ALSO
gethostbyname(3), resolver(3), hosts(5), hostname(7), dhclient-script(8), dhcp(8), named(8) Name Server Operations Guide for BIND.

HISTORY
The resolv.conf file format appeared in 4.3BSD.

BUGS
Due to resolver internal issues, getaddrinfo(3) may not behave as lookup suggests. Consequently, userland programs that use getaddrinfo(3) may behave differently from what lookup says.