NAME
hosts_options — host access control language extensions
DESCRIPTION
This manual page describes the optional extensions to the
language described in
hosts_access(5).
The extensible language uses the following format:
daemon_list : client_list :
option : option ...
The first two fields are described in
hosts_access(5).
Briefly,
daemon_list
is a list of one or more daemon process names or wildcards.
client_list
is a list of one or more host names, host addresses, patterns or
wildcards that will be matched against the client host
name or address.
The remainder of the rules is a list of zero or more options. Any ":"
characters within options must 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 equals sign "=" is permitted between keyword and value.
Logging Options
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. See
syslog(3C)
related to facilities. The severity option can be used
to emphasize or to ignore specific events.
Access Control Options
Grant or deny the service for
allow
and
deny
options respectively.
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.
Examples are as follows:
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, after
performing the
%letter
expansions described in
hosts_access(5).
The command is executed with
stdin,
stdout
and
stderr
connected to the null device, so that it
will not mess up the conversation with the client host.
For example:
spawn (/usr/bin/sffinger -l @%h | \
/usr/bin/mailx -s "alert" root) &
executes, in a background child process, the shell command
sffinger -l @%h | mail root
after replacing
%h
by the name or address of the
remote host.
The example uses the
sffinger
command instead of the regular
finger
command to limit possible damage from data sent by the finger
server. The
sffinger
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
hosts_access(5).
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:
ftpd : ... : twist /bin/echo 421 Some bounce message
For an alternative way to communicate with the client processes, see the
banners
option below.
To run /some/other/telnetd without polluting its command-line
array or its process environment:
telnetd : ... : twist PATH=/some/other; exec telnetd
WARNING: in case of UDP services, do not twist to commands that use
the standard I/O or the
read()/write()
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 undelivered
data after the server process closes a connection.
Username Lookup Options
- 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-compliant daemon (IDENT etc.) and may cause noticeable
delays with connections from non-UNIX clients. The timeout period is
tunable through configuration file
/etc/tcpd.conf.
If no or invalid timeout is specified, the user name lookup is disabled.
Miscellaneous Options
- banners /some/directory
Look for a file in
/some/directory
with the same name as the daemon
process (for example,
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
hosts_access(5)).
The banner option does not add any service-specific characters when
sending the text to the client as specified in the service protocol.
To use this option successfully, the file must contain the necessary
protocol parameters in addition to the actual text.
For example, in an
ftpd
service, the lines in the banners file are not
automatically prefixed by the status code
(220-)
as defined in FTP RFC 959.
Therefore, if you want to send the following text to the FTP client:
This is a sample Welcome text to demonstrate the banners
option in tcpd.
we recommend adding the protocol-specific response code as follows:
220-This is a sample Welcome text to demonstrate the banners
220-option in tcpd.
For the
rlogind
service, a null character
(\0)
must be placed at the beginning of the
rlogind
banner file as specified in the following example:
# echo "\0This is a sample Welcome text to demonstrate \
the banners" > rlogind
# echo "option in tcpd." >> rlogind
The
/usr/examples/tcpd/Banners.Makefile
file may be used to generate
banners for multiple services. For more information,
refer to
/usr/examples/tcpd/Banners.Makefile.
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. A
umask
of 022
prevents the creation of files with group and world write permission. The
umask
argument must be an octal number.
- user someuser or user someuser.somegroup
Assume the privileges of the "someuser" userid (or user "someuser", group
"somegroup"). 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
Problems are reported via
syslogd,
the
syslog
daemon, at
info, notice, warning
and
err
levels.
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.
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
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