Aliases: resolver(5), resolver(5), resolver(5), resolver(5), resolver(5), resolver(5), resolver(5), resolver(5), resolver(5), resolver(5)
resolv.conf - resolver configuration file
The resolver is a set of routines in the C library that provide access to the Internet Domain Name System (DNS). 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. The configuration file is considered a trusted source of DNS information (e.g., DNSSEC AD-bit information will be returned unmodified from this source).
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.
If this file does not exist, only the name server on the local machine will be queried, and the search list contains the local domain name determined from the hostname.
The different configuration options are:
|nameserver Name server IP address|
|Internet address of a name server that the resolver should query, either an IPv4 address (in dot notation), or an IPv6 address in colon (and possibly dot) notation as per RFC 2373. Up to MAXNS (currently 3, see <resolv.h>) 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.)|
|search Search list for host-name lookup.|
|By default, the search list contains one entry, the local domain name. It is determined from the local hostname returned by gethostname(2); the local domain name is taken to be everything after the first \(aq.\(aq. Finally, if the hostname does not contain a \(aq.\(aq, the root domain is assumed as the local domain name.|
|This may be changed by listing the desired domain search path following the search keyword with spaces or tabs separating the names. Resolver queries having fewer than ndots dots (default is 1) in them will be attempted using each component of the search path in turn until a match is found. For environments with multiple subdomains please read options ndots:n below to avoid man-in-the-middle attacks and unnecessary traffic for the root-dns-servers. 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.|
|If there are multiple search directives, only the search list from the last instance is used.|
|In glibc 2.25 and earlier, the search list is limited to six domains with a total of 256 characters. Since glibc 2.26, the search list is unlimited.|
|The domain directive is an obsolete name for the search directive that handles one search list entry only.|
|This option 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. Here is an example:|
|sortlist 18.104.22.168/255.255.240.0 22.214.171.124|
|Options allows certain internal resolver variables to be modified. The syntax is
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 under options.
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.
Lines that contain a semicolon (;) or hash character (#) in the first column are treated as comments.