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HOSTS_ACCESS(5)		      File Formats Manual	       HOSTS_ACCESS(5)

       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 ig-
	      nored.  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 be-
	      tween [] 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).

       client_list  is	a list of one or more host names, host addresses, pat-
       terns or	wildcards (see below) that will	be matched against the	client
       host name or address.

       The  more  complex forms	daemon@host and	user@host are explained	in the
       sections	on server endpoint patterns and	on  client  username  lookups,

       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 `'  matches  the  host
	      name `'.

       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  ad-
	      dress 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

       KNOWN  Matches any user whose name is known, and	matches	any host whose
	      name  and	 address  are  known. 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 soft-
	      ware cannot figure out what type of network it is	talking	to.

	      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  ta-
	      bles.   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.  In-
       stead, 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

       In order	to distinguish clients by the network address that  they  con-
       nect to,	use patterns of	the form:

	  process_name@host_pattern : client_list ...

       Patterns	like these can be used when the	machine	has different internet
       addresses with different	internet hostnames.  Service providers can use
       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, Free-
       BSD) can	have more than one internet address on one physical interface;
       with  other systems you may have	to resort to SLIP or PPP pseudo	inter-
       faces that live in a dedicated network address space.

       The host_pattern	obeys the same syntax rules  as	 host  names  and  ad-
       dresses 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  de-
       scendants  (TAP,	IDENT, RFC 1413) the wrapper programs can retrieve ad-
       ditional	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.

       A  flaw in the sequence number generator	of many	TCP/IP implementations
       allows intruders	to easily impersonate trusted hosts and	 to  break  in
       via,  for  example,  the	remote shell service.  The IDENT (RFC931 etc.)
       service can be used to detect such and other host address spoofing  at-

       Before  accepting a client request, the wrappers	can use	the IDENT ser-
       vice to find out	that the client	did not	send the request at all.  When
       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 al-
       low  table is scanned before the	deny table, that the search terminates
       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 ex-

	  ALL: LOCAL @some_netgroup

       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 domain (no-
       tice the	leading	dot), with the exception of

       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
       in the deny file. For example:

	  ALL:, .some.domain
	  ALL EXCEPT in.fingerd:, .other.domain

       The first rule denies some hosts	and domains all	services;  the	second
       rule still permits finger requests from other hosts and domains.

       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 in-
       etd(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-
       group lookups are case sensitive.

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



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