|RESOLV.CONF(5)||File Formats Manual||RESOLV.CONF(5)|
resolv.conf.tail — resolver
resolv.conf file specifies how the
resolver 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
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
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 is normally used to specify 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
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:
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.)
domainentry 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.
lookup keyword is not used in
resolv.conf file then the assumed
bind file. Furthermore, if the system's
resolv.conf file does not exist, then the only
database used is
searchkeyword 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
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
resolv.conf.tail file, it should include
all the domains that need to be searched.
sortlistis 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:
sortlist 126.96.36.199/255.255.240.0 188.8.131.52
A maximum of two families can be specified, where family can be any of:
If only one family is specified, only that family is tried.
Where option is one of the following:
DEBUG. By default on OpenBSD this option does nothing.
nameserverlines are able to handle the extension.
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
search keywords are mutually exclusive. If more than
one instance of these keywords is present, the last instance will
resolv.conf file format appeared in
|April 25, 2020||OpenBSD-current|