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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
       PROCESS_OPTIONS compile-time option.  These extensions are  enabled  on
       Mac OS X.

       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 pro-
       cessed 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 notice
              Change the severity level at which the  event  will  be  logged.
              Facility  names  (such  as  mail) are optional, and are not sup-
              ported on systems with older syslog implementations. The  sever-
              ity  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. 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 regu-
              lar "finger" command, to limit possible damage from data sent by
              the finger server. The "safe_finger" command is part of the dae-
              mon 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 ban-
              ners 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 commu-
              nicate with the client process; UDP requires  other  I/O  primi-


              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 deliv-
              ered 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 ser-
              vices 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 spec-
              ified 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 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 implementa-
              tions 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


Mac OS X 10.6 - Generated Thu Sep 17 20:25:08 CDT 2009
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