Ubuntu Manpage: hosts_options - host access control language extensions

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 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.

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 supported on systems with older syslog implementations. The severity 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 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
primitives.

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 delivered 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 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.

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 implementations 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.

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)