hosts_options - host access control language extensions
This document describes 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.
- 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.
- aclexec 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:
smtp : ALL : aclexec checkdnsbl %a
executes, in a background child process, the shell command "checkdnsbl
%a" after replacing %a by the address of the remote host.
The connection will be allowed or refused depending on whether the command
returns a true or false exit status.
- 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 (/usr/sbin/safe_finger -l @%h | /usr/bin/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(5), the default access control language
Wietse Venema ([email protected])
Department of Mathematics and Computing Science
Eindhoven University of Technology
Den Dolech 2, P.O. Box 513,
5600 MB Eindhoven, The Netherlands