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HOSTS_ACCESS(5)						       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
       inetd(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:

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

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

       ·      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.

       ·      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
	      final newline.

       ·      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.

       ·      All  other  lines	 should	 satisfy  the following format, things
	      between [] 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:

       ·      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´.

       ·      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).

       ·      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.

       ·      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
	      address  in  the	range `131.155.72.0´ through `131.155.73.255´.
	      Note that `m.m.m.m´ portion must always be specified.

       ·      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.

       ·      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.

       ·      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
	      automatically 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
       addresses 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
       descendants  (TAP,  IDENT,  RFC 1413) the wrapper programs can retrieve
       additional information about the owner of a connection.	 Client	 user‐
       name  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.

       ·      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.

       ·      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".

       ·      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.

       ·      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
       attacks.

       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
       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
       including address and/or network/netmask	 information,  to  reduce  the
       impact 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
       example:

       /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
       (notice	the  leading  dot),  with the exception of terminalserver.foo‐
       bar.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

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