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       hosts_options - host access control language extensions


       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 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 "=" is permitted between keyword and value.



       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. The
              severity option can be used to emphasize or to  ignore  specific



       deny   Grant  (deny) service. These options must appear at the end of a

       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.


       spawn shell_command
              Execute,  in a child process, the specified shell command, after
              performing   the   %<letter>   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 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 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 daemon:

                 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


              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.


       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 period is optional. If no timeout
              is specified a compile-time defined default value is taken.


       banners /some/directory
              Look  for  a file in ‘/some/directory´ with the same name as the
              daemon process (for example 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
              expanded (see the 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)
              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 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  run all services with root privilege. The
              second form is useful  for  services  that  need  special  group
              privileges only.


       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.


       hosts_access(5), the default access control language


       Wietse Venema (
       Department of Mathematics and Computing Science
       Eindhoven University of Technology
       Den Dolech 2, P.O. Box 513,
       5600 MB Eindhoven, The Netherlands