HOSTNAME(7)               (2019-05-09)                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 pro-
          grams, 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 accord-
          ing 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, con-
          tains 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 pro-
          cessing.

          If the input name ends with a trailing dot, the trailing dot
          is removed, and the remaining name is looked up with no fur-
          ther 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

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     HOSTNAME(7)               (2019-05-09)                HOSTNAME(7)

          local domain.  The search path can be changed from the
          default by a system-wide configuration file (see
          resolver(5)).

     SEE ALSO
          getaddrinfo(3), gethostbyname(3), nsswitch.conf(5),
          resolver(5), mailaddr(7), named(8)

          IETF RFC 1123

          IETF RFC 1178

     COLOPHON
          This page is part of release 5.10 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/.

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