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       hosts_access - format of host access control files


       This  manual  page  describes  a simple access control language that is
       based on client (host name/address, user  name),  and  server  (process
       name,  host name/address) patterns.  Examples are given at the end. The
       impatient reader is encouraged to skip to the EXAMPLES  section	for  a
       quick introduction.

       An  extended version of the access control language is described in the
       hosts_options(5) document. The extensions  are  turned  on  at  program
       build time by building with -DPROCESS_OPTIONS.

       In the following text, daemon is the the process name of a network dae-
       mon process, and client is the name and/or address of a host requesting
       service.  Network  daemon process names are specified in the inetd con-
       figuration file.


       The access control software consults two files. The search stops at the
       first match:

       o      Access  will  be	granted when a (daemon,client) pair matches an
	      entry in the /etc/hosts.allow file.

       o      Otherwise, access will be denied	when  a  (daemon,client)  pair
	      matches an entry in the /etc/hosts.deny file.

       o      Otherwise, access will be granted.

       A  non-existing	access	control file is treated as if it were an empty
       file. Thus, access control can be turned off  by  providing  no	access
       control files.


       Each access control file consists of zero or more lines of text.  These
       lines are processed in order of appearance. The search terminates  when
       a match is found.

       o      A  newline  character  is ignored when it is preceded by a back-
	      slash character. This permits you to break up long lines so that
	      they are easier to edit.

       o      Blank  lines  or	lines  that  begin  with  a  `#' character are
	      ignored.	This permits you to insert comments and whitespace  so
	      that the tables are easier to read.

       o      All  other  lines  should  satisfy  the following format, things
	      between [] being optional:

		 daemon_list : client_list [ : shell_command ]

       daemon_list is a list of one or more daemon process names (argv[0] val-
       ues) or wildcards (see below).

       List elements should be separated by blanks and/or commas.

       With  the  exception  of  NIS (YP) netgroup lookups, all access control
       checks are case insensitive.


       The access control language implements the following patterns:

       o      A string that begins with  a  `.'  character.  A	host  name  is
	      matched  if  the last components of its name match the specified
	      pattern.	For example, the pattern `.tue.nl'  matches  the  host
	      name `wzv.win.tue.nl'.

       o      A  string  that  ends  with  a  `.' character. A host address is
	      matched if its first numeric fields match the given string.  For
	      example,	the pattern `131.155.' matches the address of (almost)
	      every host on the Eindhoven University network (131.155.x.x).

       o      A string that begins with an `@' character is treated as an  NIS
	      (formerly  YP)  netgroup name. A host name is matched if it is a
	      host member of the specified netgroup. Netgroup matches are  not
	      supported for daemon process names or for client user names.

       o      An  expression of the form `n.n.n.n/m.m.m.m' is interpreted as a
	      `net/mask' pair. A host address is matched if `net' is equal  to
	      the  bitwise AND of the address and the `mask'. For example, the
	      net/mask	pattern  `'  matches	 every
	      address in the range `' through `'.

       o      An  expression  of the form `[n:n:n:n:n:n:n:n]/m' is interpreted
	      as a `[net]/prefixlen' pair. A IPv6 host address is  matched  if
	      `prefixlen'  bits  of  `net' is equal to the `prefixlen' bits of
	      the  address.   For   example,   the   [net]/prefixlen   pattern
	      `[3ffe:505:2:1::]/64'   matches	every  address	in  the  range
	      `3ffe:505:2:1::' through `3ffe:505:2:1:ffff:ffff:ffff:ffff'.

       o      A string that begins with a `/' character is treated as  a  file
	      name.  A	host name or address is matched if it matches any host
	      name or address pattern listed in the named file. The file  for-
	      mat is zero or more lines with zero or more host name or address
	      patterns separated by whitespace.  A file name  pattern  can  be
	      used anywhere a host name or address pattern can be used.


       The access control language supports explicit wildcards:

       ALL    The universal wildcard, always matches.

       LOCAL  Matches any host whose name does not contain a dot character.

	      Matches  any  user  whose  name is unknown, and matches any host
	      whose name or address are unknown.  This pattern should be  used
	      with  care:  host names may be unavailable due to temporary name
	      server problems. A network address will be unavailable when  the
	      software	cannot	figure	out what type of network it is talking
	      Matches any host whose name does not match  its  address.   When
	      tcpd  is built with -DPARANOID (default mode), it drops requests
	      from such clients even before  looking  at  the  access  control
	      tables.	Build  without	-DPARANOID  when you want more control
	      over such requests.


       EXCEPT Intended use is of the form: `list_1 EXCEPT list_2';  this  con-
	      struct  matches  anything  that matches list_1 unless it matches
	      list_2.  The EXCEPT operator can be used in daemon_lists and  in
	      client_lists.  The EXCEPT operator can be nested: if the control
	      language would permit the use of parentheses, `a EXCEPT b EXCEPT
	      c' would parse as `(a EXCEPT (b EXCEPT c))'.


       If the first-matched access control rule contains a shell command, that
       command is subjected to %<letter>  substitutions  (see  next  section).
       The  result is executed by a /bin/sh child process with standard input,
       output and error connected to /dev/null.  Specify an `&' at the end  of
       the command if you do not want to wait until it has completed.

       Shell  commands	should	not  rely  on  the  PATH setting of the inetd.
       Instead, they should use absolute path names, or they should begin with
       an explicit PATH=whatever statement.

       The  hosts_options(5)  document	describes an alternative language that
       uses the shell command field in a different and incompatible way.


       The following expansions are available within shell commands:

       %a (%A)
	      The client (server) host address.

       %c     Client information: user@host, user@address,  a  host  name,  or
	      just an address, depending on how much information is available.

       %d     The daemon process name (argv[0] value).

       %h (%H)
	      The client (server) host name or address, if the	host  name  is

       %n (%N)
	      The client (server) host name (or "unknown" or "paranoid").

       %p     The daemon process id.

       %s     Server  information: daemon@host, daemon@address, or just a dae-
	      mon name, depending on how much information is available.

       %u     The client user name (or "unknown").

       %%     Expands to a single `%' character.

       Characters in % expansions that may confuse the shell are  replaced  by

       this facility to offer FTP, GOPHER or WWW archives with internet  names
       that  may  even belong to different organizations. See also the `twist'
       option  in  the	hosts_options(5)  document.  Some  systems   (Solaris,
       FreeBSD) can have more than one internet address on one physical inter-
       face; with other systems you may have to resort to SLIP or  PPP	pseudo
       interfaces that live in a dedicated network address space.

       The  host_pattern  obeys  the  same  syntax  rules  as  host  names and
       addresses in client_list context. Usually, server endpoint  information
       is available only with connection-oriented services.


       When  the  client  host	supports  the  RFC  931 protocol or one of its
       descendants (TAP, IDENT, RFC 1413) the wrapper  programs  can  retrieve
       additional information about the owner of a connection. Client username
       information, when available, is logged together with  the  client  host
       name, and can be used to match patterns like:

	  daemon_list : ... user_pattern@host_pattern ...

       The  daemon wrappers can be configured at compile time to perform rule-
       driven username lookups (default) or to always interrogate  the	client
       host.   In  the	case  of  rule-driven username lookups, the above rule
       would cause username lookup only when  both  the  daemon_list  and  the
       host_pattern match.

       A  user pattern has the same syntax as a daemon process pattern, so the
       same wildcards apply  (netgroup	membership  is	not  supported).   One
       should not get carried away with username lookups, though.

       o      The  client  username  information  cannot be trusted when it is
	      needed most, i.e. when the client system has  been  compromised.
	      In  general,  ALL  and (UN)KNOWN are the only user name patterns
	      that make sense.

       o      Username lookups are possible only with TCP-based services,  and
	      only  when  the client host runs a suitable daemon; in all other
	      cases the result is "unknown".

       o      A well-known UNIX kernel bug may	cause  loss  of  service  when
	      username	lookups  are blocked by a firewall. The wrapper README
	      document describes a procedure to find out if  your  kernel  has
	      this bug.

       o      Username lookups may cause noticeable delays for non-UNIX users.
	      The default timeout for username	lookups  is  10  seconds:  too
	      short to cope with slow networks, but long enough to irritate PC

       Selective username lookups can alleviate the last problem. For example,
       a rule like:

	  daemon_list : @pcnetgroup ALL@ALL

       would  match members of the pc netgroup without doing username lookups,
       but would perform username lookups with all other systems.


       the client host provides IDENT service, a negative IDENT lookup	result
       (the client matches `UNKNOWN@host') is strong evidence of a host spoof-
       ing attack.

       A positive IDENT lookup result (the  client  matches  `KNOWN@host')  is
       less  trustworthy.  It  is  possible  for an intruder to spoof both the
       client connection and the IDENT	lookup,  although  doing  so  is  much
       harder  than spoofing just a client connection. It may also be that the
       client's IDENT server is lying.

       Note: IDENT lookups don't work with UDP services.


       The language is flexible enough that different types of access  control
       policy  can  be expressed with a minimum of fuss. Although the language
       uses two access control tables, the most common policies can be	imple-
       mented with one of the tables being trivial or even empty.

       When  reading  the  examples  below it is important to realize that the
       allow table is scanned before the deny table, that  the	search	termi-
       nates  when  a match is found, and that access is granted when no match
       is found at all.

       The examples use host and domain names. They can be improved by includ-
       ing address and/or network/netmask information, to reduce the impact of
       temporary name server lookup failures.


       In this case, access is denied by default. Only	explicitly  authorized
       hosts are permitted access.

       The default policy (no access) is implemented with a trivial deny file:


       This denies all service to all hosts, unless they are permitted	access
       by entries in the allow file.

       The  explicitly	authorized  hosts  are	listed in the allow file.  For

	  ALL: LOCAL @some_netgroup
	  ALL: .foobar.edu EXCEPT terminalserver.foobar.edu

       The first rule permits access from hosts in the local domain (no `.' in
       the  host  name)  and  from members of the some_netgroup netgroup.  The
       second rule permits access from all  hosts  in  the  foobar.edu	domain
       (notice	the  leading  dot),  with the exception of terminalserver.foo-


       Here, access is granted by default; only explicitly specified hosts are
       refused service.

       The  default  policy (access granted) makes the allow file redundant so
       that it can be omitted.	The explicitly non-authorized hosts are listed


       The next example permits tftp requests from hosts in the  local	domain
       (notice	the  leading  dot).  Requests from any other hosts are denied.
       Instead of the requested file, a finger probe is sent to the  offending
       host. The result is mailed to the superuser.

	  in.tftpd: LOCAL, .my.domain

	  in.tftpd: ALL: (/some/where/safe_finger -l @%h | \
	       /usr/ucb/mail -s %d-%h root) &

       The  safe_finger  command  is  intended	for  use in back-fingering and
       should be installed in a suitable place. It limits possible damage from
       data sent by the remote finger server.  It gives better protection than
       the standard finger command.

       The expansion of the %h (client host) and %d (service  name)  sequences
       is described in the section on shell commands.

       Warning:  do not booby-trap your finger daemon, unless you are prepared
       for infinite finger loops.

       On network firewall systems this trick can  be  carried	even  further.
       The typical network firewall only provides a limited set of services to
       the outer world. All other services can be "bugged" just like the above
       tftp example. The result is an excellent early-warning system.


       An error is reported when a syntax error is found in a host access con-
       trol rule; when the length of an access control rule exceeds the capac-
       ity  of	an  internal buffer; when an access control rule is not termi-
       nated by a newline character; when the result  of  %<letter>  expansion
       would  overflow	an  internal  buffer;  when  a	system call fails that
       shouldn't.  All problems are reported via the syslog daemon.


       Some operating systems are distributed with TCP Wrappers as part of the
       base  system. It is common for such systems to build wrapping function-
       ality  into  networking	utilities.  Notably,  some  systems  offer  an
       inetd(8) which does not require the use of the tcpd(8). Check your sys-
       tem's documentation for details.


       /etc/hosts.allow, (daemon,client) pairs that are granted access.
       /etc/hosts.deny, (daemon,client) pairs that are denied access.


       tcpd(8) tcp/ip daemon wrapper program.
       tcpdchk(8), tcpdmatch(8), test programs.


       If a name server lookup times out, the host name will not be  available
       to the access control software, even though the host is registered.

       Domain name server lookups are case insensitive; NIS (formerly YP) net-



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