HOSTS_ACCESS(5)                                   HOSTS_ACCESS(5)

       hosts_access - format of host access control files

       This  manual  page  describes a simple access control lan-
       guage that is based on  client  (host  name/address,  user
       name),  and  server (process name, host name/address) pat-
       terns.  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

       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 pro-
       cess names are specified in the inetd configuration  file.

       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 (dae-
              mon,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.

       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 `#' charac-
              ter are ignored.  This permits you to  insert  com-
              ments  and whitespace so that the tables are easier
              to read.

       o      All other lines should satisfy the  following  for-
              mat, 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.

       The access control language implements the following  pat-

       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 pat-
              tern    `'    matches    the    host    name

       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

       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 sup-
              ported for daemon process names or for client  user

       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  `' matches every
              address  in  the   range   `'   through

       The access control language supports explicit wildcards:

       ALL    The universal wildcard, always matches.

       LOCAL  Matches  any host whose name does not contain a dot

              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.

              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 paren-
              theses,  `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> 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

       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  state-

       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 com-

       %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

       %p     The daemon process id.

       %s     Server information: daemon@host, daemon@address, or
              just  a daemon name, depending on how much informa-
              tion 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.

       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 host-
       names.  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 sys-
       tems (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-ori-
       ented services.

       When the client host supports the RFC 931 protocol or  one
       of its descendants (TAP, IDENT, RFC 1413) the wrapper pro-
       grams can retrieve additional information about the  owner
       of  a connection. Client username information, when avail-
       able, 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  user-
       name  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

       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

       o      A well-known UNIX kernel bug may cause loss of ser-
              vice  when  username lookups are blocked by a fire-
              wall. The wrapper README document describes a  pro-
              cedure 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.

       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 triv-
       ial deny file:

          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:

          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 lead-
       ing dot), with the exception of

       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:

          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: (/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  possi-
       ble 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  com-

       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 ser-
       vices  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  dae-

       /etc/hosts.allow, (daemon,client) pairs that are granted access.
       /etc/hosts.deny, (daemon,client) pairs that are denied access.

       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 (for-
       merly 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