hosts_options

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 protected with a backslash.

       An option is of the form "keyword" or "keyword value". Options are pro-
       cessed  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.

LOGGING
       severity mail.info

       severity notice
              Change the severity level at which the event will be logged. Fa-
              cility names (such as mail) are optional, and are not  supported
              on  systems  with older syslog implementations. The severity op-
              tion 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  %<letter> expansions described in the hosts_ac-
              cess(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 replacing %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 %<letter> expansions described in  the  hosts_ac-
              cess(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 regu-
              lar "finger" command, to limit possible damage from data sent by
              the finger server. The "safe_finger" command is part of the dae-
              mon 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  de-
              scribed  in  the hosts_access(5) manual page.  Stdin, stdout and
              stderr are connected to the client process. This option must ap-
              pear 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 ban-
              ners 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 commu-
              nicate with the client process; UDP requires  other  I/O  primi-
              tives.

NETWORK OPTIONS
       keepalive
              Causes  the server to periodically send a message to the client.
              The connection is considered broken when the client does not re-
              spond.  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 deliv-
              ered 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 ser-
              vices 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 spec-
              ified 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 example in.telnetd for the telnet service),
              and copy its contents to the client. Newline characters are  re-
              placed  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 implementa-
              tions that run all services with root privilege. 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@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

                                                              HOSTS_OPTIONS(5)