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hosts(4)                         File Formats                         hosts(4)

       hosts - host name database



       The  hosts  file is a local database that associates the names of hosts
       with their Internet Protocol (IP) addresses. The hosts file can be used
       in  conjunction  with,  or instead of, other hosts databases, including
       the Domain Name System (DNS), the NIS hosts map, the NIS+ hosts  table,
       or  information from an LDAP server. Programs use library interfaces to
       access information in the hosts file.

       The hosts file has one entry for each IP address of  each  host.  If  a
       host  has more than one IP address, it will have one entry for each, on
       consecutive lines. The format of each line is:

              IP-address official-host-name nicknames...

       Items are separated by any number of SPACE and/or TAB  characters.  The
       first  item on a line is the host's IP address. The second entry is the
       host's official name. Subsequent entries on the same line are  alterna-
       tive  names  for  the  same  machine,  or  "nicknames."  Nicknames  are

       For a host with more than one IP address, consecutive entries for these
       addresses  may contain the same or differing nicknames. Different nick-
       names are useful for assigning distinct names to different addresses.

       A call to gethostbyname(3NSL) returns a  hostent  structure  containing
       the  union  of  all addresses and nicknames from each line containing a
       matching official name or nickname.

       A `#' indicates the beginning of a comment; characters up to the end of
       the line are not interpreted by routines that search the file.

       Network  addresses  are written in the conventional "decimal dot" nota-
       tion and interpreted using the  inet_addr  routine  from  the  Internet
       address manipulation library, inet(3SOCKET).

       This interface supports host names as defined in Internet RFC 952 which

              A "name" (Net, Host, Gateway, or Domain name) is a  text  string
              up to 24 characters drawn from the alphabet (A-Z), digits (0-9),
              minus sign (-), and period  (.).  Note  that  periods  are  only
              allowed  when  they serve to delimit components of "domain style
              names".  (See RFC 921, "Domain Name System Implementation Sched-
              ule,"  for background). No blank or space characters are permit-
              ted as part of a name. No distinction is made between  uppercase
              and  lowercase.  The  first character must be an alpha character
              [or a digit. (RFC 1123 relaxed RFC 952's limitation of the first
              character  to  only  alpha characters.)] The last character must
              not be a minus sign or period.

       Although the interface accepts host names longer than 24 characters for
       the  host  portion  (exclusive of the domain component), choosing names
       for hosts that adhere to the 24 character restriction will insure maxi-
       mum interoperability on the Internet.

       A  host  which  serves  as a GATEWAY should have "-GATEWAY" or "-GW" as
       part of its name. Hosts which do not serve as Internet gateways  should
       not  use "-GATEWAY" and "-GW" as part of their names. A host which is a
       TAC should have "-TAC" as the last part of its host name, if  it  is  a
       DoD host. Single character names or nicknames are not allowed.

       Example 1: Example of a typical line from the hosts file.

       Here is a typical line from the hosts file:        gaia                        # John Smith

       gethostbyname(3NSL), inet(3SOCKET), nsswitch.conf(4), resolv.conf(4)

       /etc/inet/hosts  is  the official SVR4 name of the hosts file. The sym-
       bolic link /etc/hosts exists for BSD compatibility.

SunOS 5.10                        15 Dec 2004                         hosts(4)