HOSTS_OPTIONS(5)

NAME
hosts_options - host access control language extensions

DESCRIPTION
This document describes optional extensions to the language described in the hosts_access(5) document. The extensions are enabled at program build time by editing the Makefile.

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. With some options, the value is subjected to %<character> substitutions.

OPTIONS

severity = mail.info
    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 completely ignore specific events.

allow (deny)
    Grant (deny) service, even when the matched rule was found in the hosts.deny (hosts.allow) file. These options must appear at the end of a rule.

With the allow and deny keywords it is possible to keep all access control rules within a single file, for example in the hosts.allow file. For example,

ALL: .friendly.domain: allow
ALL: ALL: deny

permits access from specific hosts only. On the other hand,

ALL: .trouble.makers: deny
ALL: ALL: allow

permits access from all hosts except a few trouble makers.

twist = shell_command
    Replace the current process by an instance of the specified shell command, after performing the %<character> expansions described in the hosts_access(5) manual page. Stdin, stdout and stderr are connected to the remote client process. This option must appear at the end of a rule.

    Examples:

    in.ftpd : ... : twist = /bin/echo 421 Some bounce message

    sends a customized bounce message to the remote client instead of running the real ftp daemon.

    in.telnetd : ... : twist = PATH=/some/other; exec in.telnetd

    would run /some/other/in.telnetd without polluting its command-line array or its process environment.

    Warning: in case of UDP services, do not twist into 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.

spawn = shell_command
    Execute the shell command in a child process, after performing the %<character> 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 remote 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.

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.

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.

nice = niceval

nice (no argument)
    Change the nice value of the process (default 10). Specify a positive value to spend more CPU resources on other processes.

user = nobody
    Assume the privileges of the "nobody" account. This is useful with inetd implementations that run all services with root privilege. It is good practice to run services such as "finger" at a reduced privilege level.

group = tty
    Assume the privileges of the "tty" group. This is useful mostly in combination with the "user" option. In order to switch both user and group ids, switch group id before switching user id.

setenv = name value
    Place a (name, value) pair into the process environment. The value is subjected to %<character> 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.

rfc931 = timeout_in_seconds

rfc931 (no argument)
    Look up the remote user name with the RFC 931 (IDENT, etc.) 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 hosts. The timeout period is optional. If no timeout is specified a default value is taken.

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