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hosts_access (5)
  • hosts_access (3) ( Solaris man: Библиотечные вызовы )
  • hosts_access (3) ( FreeBSD man: Библиотечные вызовы )
  • hosts_access (3) ( Linux man: Библиотечные вызовы )
  • >> hosts_access (5) ( Solaris man: Форматы файлов )
  • hosts_access (5) ( FreeBSD man: Форматы файлов )
  • hosts_access (5) ( Русские man: Форматы файлов )
  • hosts_access (5) ( Linux man: Форматы файлов )
  • 
    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.
    
         In the following text, daemon is the 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,
              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 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  pat-
         terns:
    
         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  net-
              group.  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  inter-
              preted  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  unavail-
              able  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  unavail-
              able  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.
              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> substitu-
         tions (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  com-
         mands:
    
         %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 infor-
              mation 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  dif-
         ferent 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 end-
         point 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  pat-
         tern,  so  the  same wildcards apply (netgroup membership is
         not supported).  One should not get carried away with  user-
         name 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  ser-
              vices,  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  user-
         name  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  ser-
         vice,  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  per-
         mitted 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  speci-
         fied 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 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 lim-
         ited 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 charac-
         ter; 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  (form-
         erly 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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