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       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