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HOSTS_ACCESS(5)                                                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 exten-
       sions 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 daemon  process,  and  client  is  the  name
       and/or address of a host requesting service. Network daemon process names are specified in the inetd configura-
       tion 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

       ?      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 appear-
       ance. The search terminates when a match is found.

       ?      A  newline  character is ignored when it is preceded by a backslash 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] values) or wildcards (see below).

       client_list is a list of one or more host names, host addresses, patterns 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, 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.

       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 dae-
              mon 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

       ?      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 format 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 conjunction with 'net/mask? matching, hostname matching beginning with '.? or IP address match-
              ing 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 talk-
              ing to.

       KNOWN  Matches  any  user whose name is known, and matches any host whose name and address are known. This pat-
              tern 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 talk-
              ing 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 construct 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> sub-
       stitutions (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 differ-
       ent 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 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 daemon name, depending on how much informa-
              tion 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.

       In order to distinguish clients by the network address that they connect 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 host-
       names.   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,  FreeBSD) can have more than one internet address on one physical interface; with other sys-
       tems 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.

       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 member-
       ship 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 suit-
              able 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 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 attacks.

       Before  accepting  a client request, the wrappers can use the IDENT service 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 spoofing 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 spoof-
       ing 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 implemented 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 ta-
       ble, that the search terminates when a match is found, and that access is granted when no  match  is  found  at

       The examples use host and domain names. They can be improved by including address and/or network/netmask infor-
       mation, 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:

          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:

          ALL: LOCAL @some_netgroup
          ALL: EXCEPT

       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: spawn (/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 pos-
       sible  damage  from data sent by the remote finger server.  It gives better protection than the standard finger

       The expansion of the %h (client host) and %d (service name) sequences is described in the section on shell com-

       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 exam-
       ple. The result is an excellent early-warning system.

       An  error  is reported when a syntax error is found in a host access control rule; when the length of an access
       control rule exceeds the capacity of an internal buffer; when an access control rule is  not  terminated  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) netgroup 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