resolver configuration files
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.
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
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:
- 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).
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
nameserverentries 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
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.
- This keyword is used by the library routines
gethostbyaddr(3). It specifies which databases should be
searched, and the order to do so. The legal space-separated values are:
lookupkeyword is not used in the system's
resolv.conffile then the assumed order is
bind file. Furthermore, if the system's
resolv.conffile does not exist, then the only database used is
- 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
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
searchline 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.tailfile, it should include all the domains that need to be searched.
- Allows addresses returned by
gethostbyname(3) to be sorted. A
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 18.104.22.168/255.255.240.0 22.214.171.124
- 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:
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.
- Allows certain internal resolver variables to be modified. The syntax is:
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
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 with tunnels.
- On OpenBSD this option does nothing. On some operating systems, this option enables IPv6 support in gethostbyname(3) by setting RES_USE_INET6 in _res.options (see res_init(3)).
- 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.
search keywords are mutually exclusive. If more than
one instance of these keywords is present, the last instance will
- A space-separated list of search domains, overriding the
searchkeyword of a system's
- A space-separated list of resolver options, overriding the
optionskeyword of a system's
gethostbyname(3), res_init(3), hosts(5), hostname(7), dhclient(8), nsd(8), unbound(8), unwind(8)
resolv.conf file format appeared in