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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  pro‐
       tected 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  com‐
       patibility 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  implementa‐
              tions. 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  %<let‐
              ter> 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 re‐
              placing %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 %<let‐
              ter> 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 com‐
              mand 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  per‐
              forming 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 dae‐
              mon:

                 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  pe‐
              riod  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 ex‐
              ample 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  ex‐
              panded (see the hosts_access(5) manual page).

              The  tcp  wrappers  source code distribution provides a sample makefile (Banners.Make‐
              file) 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 cre‐
              ation 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 privi‐
              lege. 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 AT wzv.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)