MAN page from CentOS 7 tcp_wrappers-libs-7.6-77.el7.x86_64.rpm
Section: File Formats (5)Index
hosts_options - host access control language extensions
This document describes optional extensions to the language describedin the hosts_access
(5) document. The extensions are enabled at programbuild 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 areprocessed in the specified order. Some options are subjected to%<letter> substitutions. For the sake of backwards compatibility withearlier versions, an "=" is permitted between keyword and value.
- severity mail.info
- severity notice
- Change the severity level at which the event will be logged. Facilitynames (such as mail) are optional, and are not supported on systemswith older syslog implementations. The severity option can be usedto emphasize or to ignore specific events.
- Grant (deny) service. These options must appear at the end of a rule.
The allow and deny keywords make it possible to keep allaccess control rules within a single file, for example in thehosts.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
- spawn shell_command
- Execute, in a child process, the specified shell command, afterperforming the %<letter> expansions described in the hosts_access(5)manual page. The command is executed with stdin, stdout and stderrconnected to the null device, so that it won't mess up theconversation 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 theremote host.
The example uses the "safe_finger" command instead of the regular"finger" command, to limit possible damage from data sent by the fingerserver. The "safe_finger" command is part of the daemon wrapperpackage; it is a wrapper around the regular finger command that filtersthe data sent by the remote host.
- twist shell_command
- Replace the current process by an instance of the specified shellcommand, after performing the %<letter> expansions described in thehosts_access(5) manual page. Stdin, stdout and stderr are connected tothe client process. This option must appear at the end of a rule.
To send a customized bounce message to the client instead ofrunning the real ftp daemon:
in.ftpd : ... : twist /bin/echo 421 Some bounce message
For an alternative way to talk to client processes, see thebanners option below.
To run /some/other/in.telnetd without polluting its command-linearray 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 usethe standard I/O or the read(2)/write(2) routines to communicate withthe client process; UDP requires other I/O primitives.
- Causes the server to periodically send a message to the client. Theconnection is considered broken when the client does not respond. Thekeepalive option can be useful when users turn off their machine whileit is still connected to a server. The keepalive option is not usefulfor datagram (UDP) services.
- linger number_of_seconds
- Specifies how long the kernel will try to deliver not-yet delivereddata 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 ontransports other than TCP. It requires that the client system runs anRFC 931 (IDENT, etc.) -compliant daemon, and may cause noticeabledelays with connections from non-UNIX clients. The timeout period isoptional. If no timeout is specified a compile-time defined defaultvalue is taken.
- banners /some/directory
- Look for a file in `/some/directory' with the same name as the daemonprocess (for example in.telnetd for the telnet service), and copy itscontents to the client. Newline characters are replaced bycarriage-return newline, and %<letter> sequences are expanded (seethe 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) networkservices only.
- nice [ number ]
- Change the nice value of the process (default 10). Specify a positivevalue to spend more CPU resources on other processes.
- setenv name value
- Place a (name, value) pair into the process environment. The value issubjected to %<letter> expansions and may contain whitespace (butleading and trailing blanks are stripped off).
Warning: many network daemons reset their environment before spawning alogin or shell process.
- umask 022
- Like the umask command that is built into the shell. An umask of 022prevents 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 runall services with root privilege. The second form is useful forservices that need special group privileges only.
When a syntax error is found in an access control rule, the erroris reported to the syslog daemon; further options will be ignored,and service is denied.
(5), the default access control language
Wietse Venema (wietseAATTwzv.win.tue.nl)Department of Mathematics and Computing ScienceEindhoven University of TechnologyDen Dolech 2, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
- ACCESS CONTROL
- RUNNING OTHER COMMANDS
- NETWORK OPTIONS
- USERNAME LOOKUP
- SEE ALSO
This document was created byman2html,using the manual pages.