Tribblix: manual page: hosts

HOSTS_OPTIONS(5) File Formats and Configurations HOSTS_OPTIONS(5)

NAME

hosts_options - host access control language extensions

DESCRIPTION

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.

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

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.

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

ATTRIBUTES