HOSTS_OPTIONS(5) Standards, Environments, and Macros HOSTS_OPTIONS(5)


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 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 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.



deny Grant (deny) service. These options must appear at the end of a

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.


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


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 (
Department of Mathematics and Computing Science
Eindhoven University of Technology
Den Dolech 2, P.O. Box 513,
5600 MB Eindhoven, The Netherlands


See attributes(7) for descriptions of the following attributes:

|Interface Stability | Committed |

September 15, 2011 HOSTS_OPTIONS(5)