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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.

       The extended version of the access control language is described in the
       hosts_options(5) document. Note that this language supersedes the mean‐
       ing of shell_command as documented below.

       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.

       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 server port numbers 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. An IPv4 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
	      `´.	 `´ is not a valid mask	value,
	      so a single host can be matched just by its IP.

       ·      An  expression  of  the  form  `n.n.n.n/mm'  is interpreted as a
	      `net/masklength' pair, where `mm' is the number  of  consecutive
	      `1' bits in the netmask applied to the `n.n.n.n' address.

       ·      An  expression  of the form `[n:n:n:n:n:n:n:n]/m´ is interpreted
	      as a `[net]/prefixlen´ pair. An 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´.

       ·      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	 `*´  and  `?´	can  be	 used to match hostnames or IP
	      addresses.  This method of matching cannot be used  in  conjunc‐
	      tion  with `net/mask´ matching, hostname matching beginning with
	      `.´ or IP address matching ending with `.´.

       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").

       %r (%R)
	      The clients (servers) port number (or "0").

       %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: (/usr/sbin/safe_finger -l @%h | \
	       /usr/bin/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.

       hosts_options(5) extended syntax.
       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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