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HOSTNAME(7) Linux Programmer's Manual HOSTNAME(7)
NAME
hostname - hostname resolution description
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 to z, the digits from 0 to 9, 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 gethostbyname(3).)
Hostnames are resolved by the NSS framework in glibc according to the hosts configuration
in nsswitch.conf. The DNS-based name resolver (in the dns NSS service module) resolves
them in the following fashion.
If the name consists of a single component, that is, contains no dot, and if the environ-
ment 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 ex-
ample, 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 re-
solver(5)).
SEE ALSO
getaddrinfo(3), gethostbyname(3), nsswitch.conf(5), resolver(5), mailaddr(7), named(8)
IETF RFC 1123 <http://www.ietf.org/rfc/rfc1123.txt>
IETF RFC 1178 <http://www.ietf.org/rfc/rfc1178.txt>
COLOPHON
This page is part of release 5.05 of the Linux man-pages project. A description of the
project, information about reporting bugs, and the latest version of this page, can be
found at https://www.kernel.org/doc/man-pages/.
Linux 2019-05-09 HOSTNAME(7)
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