HOSTS_OPTIONS(5)
NAME
hosts_options - host access control language extensions
DESCRIPTION
This document describes optional extensions to the lan-
guage 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 mak-
ers:
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 car-
riage-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-ori-
ented (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 envi-
ronment. The value is subjected to %<letter> expan-
sions and may contain whitespace (but leading and
trailing blanks are stripped off).
Warning: many network daemons reset their environ-
ment 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 ser-
vices with root privilege. The second form is use-
ful 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