HOSTS_OPTIONS(5) OpenBSD Programmer's Manual HOSTS_OPTIONS(5)NAMEhosts_options - tcp wrapper host access control language extensions
DESCRIPTION
This document describes optional 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 /etc/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:
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/telnetd without polluting its
command-line array or its process environment:
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) or
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 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).
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.
SEE ALSOhosts_access(5)AUTHORS
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
OpenBSD 4.9 May 31, 2007 OpenBSD 4.9