Published Jan 15, 2022
[
resolv.conf - resolver configuration file
/etc/resolv.conf
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; see the trust-ad option below for details.
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 '.'. Finally, if the hostname
does not contain a '.', 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.