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HOSTS_ACCESS(5)                        File Formats Manual                        HOSTS_ACCESS(5)

       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 pro‐
       cessed 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 insen‐

       The access control language implements the following patterns:

       ·      A  string that begins with a `.´ character. A host name is matched if the last com‐
              ponents of its  name  match  the  specified  pattern.   For  example,  the  pattern
              `.tue.nl´ matches the host name `wzv.win.tue.nl´.

       ·      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)  net‐
              group  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.

       ·      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]/pre‐
              fixlen´ 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

       ·      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 match‐
              ing 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 any‐
              thing 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 sub‐
       jected 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 com‐
       mand 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 organiza‐
       tions. 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 dedi‐
       cated 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  dae‐
       mon_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  user‐
       name 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 time‐
              out  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  ser‐
       vice.   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  fail‐

       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: .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 termi‐

       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 omit‐
       ted.  The explicitly non-authorized hosts are listed in the deny file. For example:

          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.

       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 con‐
       trol software, even though the host is registered.

       Domain name server lookups are case insensitive; NIS (formerly YP)  netgroup  lookups  are
       case sensitive.

       Wietse Venema (wietse AT wzv.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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