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HOSTS_OPTIONS(5)              File Formats Manual             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
       PROCESS_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 versions, an "=" is permitted between keyword and value.

       severity mail.info

       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


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

       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.

                 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 host.

       twist shell_command
              Replace the current process by an instance of the specified
              shell command, after performing the %<letter> 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

              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) services.

       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

       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 carriage-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(3) hosts_access(5), the default access control language

       Wietse Venema (wietse@wzv.win.tue.nl)
       Department of Mathematics and Computing Science
       Eindhoven University of Technology
       Den Dolech 2, P.O. Box 513,
       5600 MB Eindhoven, The Netherlands