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HOSTS_ACCESS(5)               File Formats Manual              HOSTS_ACCESS(5)



NAME
       hosts_access, hosts.allow, hosts.deny - 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.

       Note that in a `stock' installation of the tcp_wrappers package, a
       program called tcpd is called from /etc/inetd.conf, and this program
       performs the wrapper checks and then executes the daemon.  In NetBSD
       inetd(8) has been modified to perform this check internally, and so
       tcpd is neither used nor supplied.

       Also note that libwrap under NetBSD uses the extensions to the access
       control language as described in the hosts_options(5).

       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.  WARNING:  The total length of an
              entry can be no more than 2047 characters long including the
              final newline.

       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, things
              between [] being optional:

              daemon_list : client_list : option : option ...

       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.  When a client_list item needs to include
       colon character (for IPv6 addresses), the item needs to be wrapped with
       square bracket.

       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.  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 `131.155.72.0/255.255.254.0' matches every
              address in the range `131.155.72.0' through `131.155.73.255'.
              Note that `m.m.m.m' portion must always be specified.

       o      An expression of the form `ipv6-addr/ipv6-mask' is interpreted
              as masked IPv6 address match, just like masked IPv4 address
              match (see above).  Note that `ipv6-mask' portion must always be
              specified.

       o      An expression of the form `ipv6-addr/prefixlen' is interpreted
              as masked IPv6 address match (with mask specified by numeric
              prefixlen), just like masked IPv4 address match (see above).
              Note that `prefixlen' portion must always be specified.

       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.

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.

       PARANOID
              Matches any host whose name does not match its address.  Note
              that unlike the default mode of tcpd, NetBSD inetd does not
              automatically drop these requests; you must explicitly drop them
              in your /etc/hosts.allow or /etc/hosts.deny file.

       {RBL}.domain
              Matches any host whose reversed address appears in the DNS under
              domain.  The primary such domain used for blocking unsolicited
              commercial e-mail (spam) is `.rbl.maps.vix.com'.

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))'.

% EXPANSIONS
       The following expansions are available within some options:

       %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 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, NetBSD) 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 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 (RFC 931 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 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
       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: spawn (/some/where/safe_finger -l @%h | \
               /usr/ucb/mail -s %d-%h root) &

       (The safe_finger command can be gotten from the tcp_wrappers package
       and 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 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), hosts_access(3)
       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.

       The total length of an entry can be no more than 2047 characters long,
       including the final newline.

AUTHOR
       Wietse Venema (wietseATwzv.nl)
       Department of Mathematics and Computing Science
       Eindhoven University of Technology
       Den Dolech 2, P.O. Box 513,
       5600 MB Eindhoven, The Netherlands



                                                               HOSTS_ACCESS(5)