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 events.
  - 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.
  - 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 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. 
  - 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.
  - 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 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