HOSTS_ACCESS(5)               File Formats Manual              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.

       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.

       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
              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 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,

       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
              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 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
              software cannot figure out what type of network it is talking

              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> 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
              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

       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.

       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
       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 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
       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.

       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

          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: (/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.

       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.

       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)
       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