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HOSTS_ACCESS(5)		      File Formats Manual	       HOSTS_ACCESS(5)

NAME
       hosts_access,  hosts.allow,  hosts.deny - format	of host	access control
       files

DESCRIPTION
       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.

       Note that in a `stock' installation of the tcp_wrappers package,	a pro-
       gram  called tcpd is called from	/etc/inetd.conf, and this program per-
       forms the wrapper checks	and then executes the daemon.  In  NetBSD  in-
       etd(8)  has been	modified to perform this check internally, and so tcpd
       is neither used nor supplied.

       Also note that libwrap under NetBSD uses	the extensions to  the	access
       control language	as described in	the hosts_options(5).

       In  the	following text,	daemon is the process name of a	network	daemon
       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.

ACCESS CONTROL FILES
       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.

ACCESS CONTROL RULES
       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.  WARNING:	The total length of an
	      entry can	be no more than	2047 characters	long including the fi-
	      nal newline.

       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 : option : option ...

       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.	When a client_list item	needs to include colon
       character (for IPv6 addresses), the  item  needs	 to  be	 wrapped  with
       square bracket.

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

       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.

PATTERNS
       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 `131.155.72.0/255.255.254.0' matches every  ad-
	      dress  in	 the  range  `131.155.72.0'  through `131.155.73.255'.
	      Note that	`m.m.m.m' portion must always be specified.

       o      An expression of the form	`ipv6-addr/ipv6-mask'  is  interpreted
	      as  masked  IPv6	address	 match,	 just like masked IPv4 address
	      match (see above).  Note that `ipv6-mask'	portion	must always be
	      specified.

       o      An  expression  of the form `ipv6-addr/prefixlen'	is interpreted
	      as masked	IPv6 address match (with  mask	specified  by  numeric
	      prefixlen),  just	 like  masked  IPv4 address match (see above).
	      Note that	`prefixlen' portion must always	be specified.

       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.

WILDCARDS
       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.

       UNKNOWN
	      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
	      to.

       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.

       PARANOID
	      Matches any host whose name does not match  its  address.	  Note
	      that  unlike the default mode of tcpd, NetBSD inetd does not au-
	      tomatically drop these requests; you must	explicitly  drop  them
	      in your /etc/hosts.allow or /etc/hosts.deny file.

       {RBL}.domain
	      Matches any host whose reversed address appears in the DNS under
	      domain.  The primary such	domain used for	 blocking  unsolicited
	      commercial e-mail	(spam) is `.rbl.maps.vix.com'.

OPERATORS
       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))'.

% EXPANSIONS
       The following expansions	are available within some options:

       %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
	      unavailable.

       %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
       underscores.

SERVER ENDPOINT	PATTERNS
       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, NetBSD) can	have more than one internet address  on	 one  physical
       interface;  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  ad-
       dresses	in  client_list	context.  Usually, server endpoint information
       is available only with connection-oriented services.

CLIENT USERNAME	LOOKUP
       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
	      users.

       Selective  username  lookups can	alleviate the last problem.  For exam-
       ple, 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.

DETECTING ADDRESS SPOOFING ATTACKS
       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	(RFC 931 etc.)
       service can be used to detect such and other host address spoofing  at-
       tacks.

       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.

EXAMPLES
       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 in-
       cluding address and/or network/netmask information, to reduce  the  im-
       pact of temporary name server lookup failures.

MOSTLY CLOSED
       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:

       /etc/hosts.deny:
	  ALL: ALL

       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-
       ample:

       /etc/hosts.allow:
	  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 (no-
       tice the	leading	dot), with the exception of terminalserver.foobar.edu.

MOSTLY OPEN
       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:

       /etc/hosts.deny:
	  ALL: some.host.name, .some.domain
	  ALL EXCEPT in.fingerd: other.host.name, .other.domain

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

BOOBY TRAPS
       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.

       /etc/hosts.allow:
	  in.tftpd: LOCAL, .my.domain

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

       (The safe_finger	command	can be gotten from  the	 tcp_wrappers  package
       and 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.

DIAGNOSTICS
       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.

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

SEE ALSO
       hosts_options(5), hosts_access(3)
       tcpdchk(8), tcpdmatch(8), test programs.

BUGS
       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.

       The  total length of an entry can be no more than 2047 characters long,
       including the final newline.

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_ACCESS(5)

NAME | DESCRIPTION | ACCESS CONTROL FILES | ACCESS CONTROL RULES | PATTERNS | WILDCARDS | OPERATORS | % EXPANSIONS | SERVER ENDPOINT PATTERNS | CLIENT USERNAME LOOKUP | DETECTING ADDRESS SPOOFING ATTACKS | EXAMPLES | MOSTLY CLOSED | MOSTLY OPEN | BOOBY TRAPS | DIAGNOSTICS | FILES | SEE ALSO | BUGS | AUTHOR

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