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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:

       ·      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.

       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.

       ·      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 [ : 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:

       ·      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 `´.

       ·      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	 `´  matches	 every
	      address in the range `´ through `´.

       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
	      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

       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
       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.

       ·      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

       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

       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
       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

       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
       (notice	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  comes	 with  the  tcpd wrapper 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 stan‐
       dard 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.

       /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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