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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.

       An  extended version of the access control language is described in the
       hosts_options(5) document. The extensions  are  turned  on  at  program
       build  time  by  building with -DPROCESS_OPTIONS.  These extensions are
       enabled on Mac OS X.

       In the following text, daemon is the the process name of a network dae-
       mon 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.

       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 [ : shell_command ]

       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.

       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'.


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.  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 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))'.


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)
              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) can have more than one internet address on one physical inter-
       face;  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 (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 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 includ-
       ing 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: (/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 possible damage from data sent
       by the remote finger server.  It gives better protection than the stan-
       dard 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

       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) net-
       group 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




                                                               hosts_access(5)

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