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
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(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