HOSTS_ACCESS(5)                                   HOSTS_ACCESS(5)

     NAME
          hosts_access - 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.

          The extended version of the access control language is
          described in the hosts_options(5) document. Note that this
          language supersedes the meaning 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 configuration 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
               backslash character. This permits you to break up long
               lines so that they are easier to edit.

          +o    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.

          +o    All other lines should satisfy the following format,

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     HOSTS_ACCESS(5)                                   HOSTS_ACCESS(5)

               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 server port numbers 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.

     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. 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 `131.155.72.0/255.255.254.0' matches every
               address in the range `131.155.72.0' through
               `131.155.73.255'.  `255.255.255.255' is not a valid
               mask value, so a single host can be matched just by its
               IP.

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     HOSTS_ACCESS(5)                                   HOSTS_ACCESS(5)

          +o    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.

          +o    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'.

          +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 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.

          +o    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 matching
               ending with `.'.

     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 software cannot
               figure out what type of network it is talking to.

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          PARANOID
               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.

     OPERATORS
          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))'.

     SHELL COMMANDS
          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.

     % EXPANSIONS
          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)

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     HOSTS_ACCESS(5)                                   HOSTS_ACCESS(5)

               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 daemon 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 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
          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, FreeBSD) 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 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

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     HOSTS_ACCESS(5)                                   HOSTS_ACCESS(5)

          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 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.

     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 (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.

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     HOSTS_ACCESS(5)                                   HOSTS_ACCESS(5)

          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 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 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 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.foobar.edu.

     MOSTLY OPEN

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     HOSTS_ACCESS(5)                                   HOSTS_ACCESS(5)

          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: (/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 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 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

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     HOSTS_ACCESS(5)                                   HOSTS_ACCESS(5)

          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) extended syntax.
          tcpd(8) tcp/ip daemon wrapper program.
          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) netgroup lookups are case sensitive.

     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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