hostname - hostname resolution description
Hostnames are domains, where a domain is a hierarchical, dot-separated list of
subdomains; for example, the machine "monet", in the
"example" subdomain of the "com" domain would be
represented as "monet.example.com".
Each element of the hostname must be from 1 to 63 characters long and the entire
hostname, including the dots, can be at most 253 characters long. Valid
characters for hostnames are ASCII(7)
letters from a
, the digits from 0
, and the hyphen (-). A hostname
may not start with a hyphen.
Hostnames are often used with network client and server programs, which must
generally translate the name to an address for use. (This task is generally
performed by either getaddrinfo(3)
or the obsolete
.) Hostnames are resolved by the Internet name resolver
in the following fashion.
If the name consists of a single component, that is, contains no dot, and if the
environment variable HOSTALIASES
is set to the name of a file, that
file is searched for any string matching the input hostname. The file should
consist of lines made up of two white-space separated strings, the first of
which is the hostname alias, and the second of which is the complete hostname
to be substituted for that alias. If a case-insensitive match is found between
the hostname to be resolved and the first field of a line in the file, the
substituted name is looked up with no further processing.
If the input name ends with a trailing dot, the trailing dot is removed, and the
remaining name is looked up with no further processing.
If the input name does not end with a trailing dot, it is looked up by searching
through a list of domains until a match is found. The default search list
includes first the local domain, then its parent domains with at least 2 name
components (longest first). For example, in the domain cs.example.com, the
name lithium.cchem will be checked first as lithium.cchem.cs.example and then
as lithium.cchem.example.com. lithium.cchem.com will not be tried, as there is
only one component remaining from the local domain. The search path can be
changed from the default by a system-wide configuration file (see
This page is part of release 4.16 of the Linux man-pages
description of the project, information about reporting bugs, and the latest
version of this page, can be found at