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HOSTS_OPTIONS(5)                                              HOSTS_OPTIONS(5)

       hosts_options - host access control language extensions

       This  document  describes  optional  extensions  to the language described in the hosts_access(5) document. The
       extensions are enabled at program build time. For example, by editing the Makefile  and  turning  on  the  PRO-
       CESS_OPTIONS compile-time option.

       The extensible language uses the following format:

          daemon_list : client_list : option : option ...

       The first two fields are described in the hosts_access(5) manual page.  The remainder of the rules is a list of
       zero or more options.  Any ":" characters within options should be protected with a backslash.

       An option is of the form "keyword" or "keyword value". Options are  processed  in  the  specified  order.  Some
       options  are  subjected  to  %<letter> substitutions. For the sake of backwards compatibility with earlier ver-
       sions, an "=" is permitted between keyword and value.


       severity notice
              Change the severity level at which the event will be logged. Facility names (such as mail) are optional,
              and  are  not supported on systems with older syslog implementations. The severity option can be used to
              emphasize or to ignore specific events.


       deny   Grant (deny) service. These options must appear at the end of a rule.

       The allow and deny keywords make it possible to keep all access control rules within a single file, for example
       in the hosts.allow file.

       To permit access from specific hosts only:

          ALL: .friendly.domain: ALLOW
          ALL: ALL: DENY

       To permit access from all hosts except a few trouble makers:

          ALL: .bad.domain: DENY
          ALL: ALL: ALLOW

       Notice the leading dot on the domain name patterns.

       spawn shell_command
              Execute,  in  a  child  process,  the specified shell command, after performing the %<letter> expansions
              described in the hosts_access(5) manual page.  The command is executed with  stdin,  stdout  and  stderr
              connected to the null device, so that it won?t mess up the conversation with the client host. Example:

                 spawn (/some/where/safe_finger -l @%h | /usr/ucb/mail root) &

              executes,  in  a  background  child  process,  the  shell command "safe_finger -l @%h | mail root" after
              replacing %h by the name or address of the remote host.

              The example uses the "safe_finger" command instead of the regular "finger" command,  to  limit  possible
              damage  from  data  sent  by  the finger server. The "safe_finger" command is part of the daemon wrapper
              package; it is a wrapper around the regular finger command that filters the  data  sent  by  the  remote

       twist shell_command
              Replace  the  current process by an instance of the specified shell command, after performing the %<let-
              ter> expansions described in the hosts_access(5) manual page.  Stdin, stdout and stderr are connected to
              the client process. This option must appear at the end of a rule.

              To send a customized bounce message to the client instead of running the real ftp daemon:

                 in.ftpd : ... : twist /bin/echo 421 Some bounce message

              For an alternative way to talk to client processes, see the banners option below.

              To run /some/other/in.telnetd without polluting its command-line array or its process environment:

                 in.telnetd : ... : twist PATH=/some/other; exec in.telnetd

              Warning:   in  case  of  UDP  services,  do  not  twist  to  commands  that  use the standard I/O or the
              read(2)/write(2) routines to communicate with the client process; UDP requires other I/O primitives.

              Causes the server to periodically send a message to the client.  The  connection  is  considered  broken
              when  the  client does not respond. The keepalive option can be useful when users turn off their machine
              while it is still connected to a server.  The keepalive option is not useful  for  datagram  (UDP)  ser-

       linger number_of_seconds
              Specifies how long the kernel will try to deliver not-yet delivered data after the server process closes
              a connection.

       rfc931 [ timeout_in_seconds ]
              Look up the client user name with the RFC 931 (TAP, IDENT, RFC 1413) protocol.  This option is  silently
              ignored in case of services based on transports other than TCP.  It requires that the client system runs
              an RFC 931 (IDENT, etc.) -compliant daemon, and may cause noticeable delays with connections  from  non-
              UNIX clients.  The timeout period is optional. If no timeout is specified a compile-time defined default
              value is taken.

       banners /some/directory
              Look for a file in '/some/directory' with the same name as the daemon process  (for  example  in.telnetd
              for  the  telnet  service), and copy its contents to the client. Newline characters are replaced by car-
              riage-return newline, and %<letter> sequences are expanded (see the hosts_access(5) manual page).

              The tcp wrappers source code distribution provides a sample makefile (Banners.Makefile)  for  convenient
              banner maintenance.

              Warning: banners are supported for connection-oriented (TCP) network services only.

       nice [ number ]
              Change the nice value of the process (default 10).  Specify a positive value to spend more CPU resources
              on other processes.

       setenv name value
              Place a (name, value) pair into the process environment. The value is subjected to %<letter>  expansions
              and may contain whitespace (but leading and trailing blanks are stripped off).

              Warning: many network daemons reset their environment before spawning a login or shell process.

       umask 022
              Like the umask command that is built into the shell. An umask of 022 prevents the creation of files with
              group and world write permission.  The umask argument should be an octal number.

       user nobody

       user nobody.kmem
              Assume the privileges of the "nobody" userid (or user "nobody", group "kmem"). The first form is  useful
              with  inetd  implementations  that  run  all services with root privilege. The second form is useful for
              services that need special group privileges only.

       When a syntax error is found in an access control rule, the error is reported to  the  syslog  daemon;  further
       options will be ignored, and service is denied.

       hosts_access(5), the default access control language

       Wietse Venema (
       Department of Mathematics and Computing Science
       Eindhoven University of Technology
       Den Dolech 2, P.O. Box 513,
       5600 MB Eindhoven, The Netherlands