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HOSTS_OPTIONS(5)					      HOSTS_OPTIONS(5)

NAME
       hosts_options - host access control language extensions

DESCRIPTION
       This  document  describes  extensions  to the language described in the
       hosts_access(5) document.

       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.

LOGGING
       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  sup‐
	      ported  on systems with older syslog implementations. The sever‐
	      ity option can be	 used  to  emphasize  or  to  ignore  specific
	      events.

ACCESS CONTROL
       allow

       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.

RUNNING OTHER COMMANDS
       aclexec 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:

		 smtp : ALL : aclexec checkdnsbl %a

	      executes,	 in  a	background  child  process,  the shell command
	      "checkdnsbl %a" after replacing %a by the address of the	remote
	      host.

	      The  connection  will be allowed or refused depending on whether
	      the command returns a true or false exit status.

       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 (/usr/sbin/safe_finger -l @%h | /usr/bin/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‐
	      tives.

NETWORK OPTIONS
       keepalive
	      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.

USERNAME LOOKUP
       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.

MISCELLANEOUS
       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.

DIAGNOSTICS
       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.

SEE ALSO
       hosts_access(5), the default access control language

AUTHOR
       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

							      HOSTS_OPTIONS(5)
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