man > hosts_options(5)

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)