HOSTS_OPTIONS
Section: File Formats (5)
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NAME
hosts_options - host access control language extensions
DESCRIPTION
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.
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 makers:
ALL: .bad.domain: DENY
ALL: ALL: ALLOW
Notice the leading dot on the domain name patterns.
RUNNING OTHER COMMANDS
- 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.
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
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.
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
Index
- NAME
-
- DESCRIPTION
-
- LOGGING
-
- ACCESS CONTROL
-
- RUNNING OTHER COMMANDS
-
- NETWORK OPTIONS
-
- USERNAME LOOKUP
-
- MISCELLANEOUS
-
- DIAGNOSTICS
-
- SEE ALSO
-
- AUTHOR
-
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