HOSTS_OPTIONS(5) File Formats Manual HOSTS_OPTIONS(5)
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 pro-
cessed 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. Fa-
cility names (such as mail) are optional, and are not supported
on systems with older syslog implementations. The severity op-
tion 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_ac-
cess(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_ac-
cess(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 regu-
lar "finger" command, to limit possible damage from data sent by
the finger server. The "safe_finger" command is part of the dae-
mon 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 de-
scribed in the hosts_access(5) manual page. Stdin, stdout and
stderr are connected to the client process. This option must ap-
pear 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 ban-
ners 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 commu-
nicate with the client process; UDP requires other I/O primi-
tives.
NETWORK OPTIONS
keepalive
Causes the server to periodically send a message to the client.
The connection is considered broken when the client does not re-
spond. 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 deliv-
ered 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 ser-
vices 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 spec-
ified 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 re-
placed 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 implementa-
tions 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
HOSTS_OPTIONS(5)