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 speci-
fied order. Some options are subjected to %<letter> substitutions. For the sake of back-
wards compatibility with earlier versions, an "=" is permitted between keyword and value.
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 implemen-
tations. The severity option can be used to emphasize or to ignore specific events.
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
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.
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
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
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.
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.
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.
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.Make-
file) 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
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.
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 privi-
When a syntax error is found in an access control rule, the error is reported to the sys-
log daemon; further options will be ignored, and service is denied.
hosts_access(5), the default access control language
Wietse Venema (firstname.lastname@example.org)
Department of Mathematics and Computing Science
Eindhoven University of Technology
Den Dolech 2, P.O. Box 513,
5600 MB Eindhoven, The Netherlands