RESOLV.CONF(5)File Formats ManualRESOLV.CONF(5)
resolv.conf — resolver configuration file
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. The file is designed to be human readable and contains a list of keywords with values that provide various types of resolver information.
On a normally configured system this file should not be necessary. The only name server to be queried will be on the local machine, the domain name is determined from the host name, and the domain search path is constructed from the domain name.
The different configuration options 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 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).
- 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.
- This keyword is now ignored: its function has been superseded by features of
- Search list for host-name 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.
- Sortlist allows addresses returned by gethostbyname 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, ie. sortlist 126.96.36.199/255.255.240.0 188.8.131.52
- Options allows certain internal resolver variables to be modified. The syntax is: options option ... where option is one of the following:
- enable debugging information, by setting RES_DEBUG in _res.options (see
- 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.
- sets the amount of time the resolver will wait for a response from a remote name server before retrying the query via a different name server. Measured in seconds, the default is RES_TIMEOUT (see <resolv.h>).
- sets the number of times the resolver will send a query to its name servers before giving up and returning an error to the calling application. The default is RES_DFLRETRY (see <resolv.h>).
- sets RES_ROTATE in_res.options , which causes round robin selection of nameservers from among those listed. This has the effect of spreading the query load among all listed servers, rather than having all clients try the first listed server first every time.
- sets RES_NOCHECKNAME in_res.options , which disables the modern BIND checking of incoming host names and mail names for invalid characters such as underscore ('_'), non-ASCII, or control characters. This is the default.
- clears RES_NOCHECKNAME in_res.options , which enables the modern BIND checking of incoming host names and mail names as described above.
- attach OPT pseudo-RR for ENDS0 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 is 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.
- enable support for IPv6-only applications, by setting RES_USE_INET6 in _res.options (see
resolver(3)). The option is meaningful with certain kernel configuration only and use of this option is discouraged.
- Do not require IP source address on the reply packet to be equal to the servers' 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 RES_NOTLDQUERY in_res.options . This option causes res_nsearch() to not attempt to resolve a unqualified name as if it were a top level domain (TLD). This option can cause problems if the site has “localhost” as a TLD rather than having localhost on one or more elements of the search list. This option has no effect if neither RES_DEFNAMES or RES_DNSRCH is set.
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 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 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 white space.
- The file resolv.conf resides in /etc.
Name Server Operations Guide for BIND, Release 4.9.4, CSRG,, Department of Electrical Engineering and Computer Sciences,, University of California, Berkeley, July 16, 1996, http://www.dns.net/dnsrd/docs/bog/bog.html., , ,
The resolv.conf file format appeared in 4.3BSD.
NetBSDSeptember 9, 2012NetBSD