HOSTS_ACCESS(5)                                                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 pro-
       gram  called tcpd is called from /etc/inetd.conf, and this program per-
       forms 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 con-
       figuration 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 back-
              slash 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] val-
       ues) 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.  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.

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  soft-
              ware 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  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))'.

% 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  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
       underscores.

SERVER ENDPOINT PATTERNS
       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,
       FreeBSD, NetBSD) can have more than one internet address on one  physi-
       cal 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  user-
       name  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 exam-
       ple, 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 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.

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 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
       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.foo-
       bar.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 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.

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) net-
       group 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 (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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