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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

       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

                 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, 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


       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

       •      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 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 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

       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 unavailable.

       %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 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  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


       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 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.


       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  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

       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


       In this case, access is denied by default. Only explicitly authorized hosts are  permitted

       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 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   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

       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


       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