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 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 provid‐
       ing 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, 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 patterns:

       •      A string that begins with a `.´ character. A host name is matched if the last components of its name match the speci‐
              fied 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 consec‐
              utive `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]/pre‐
              fixlen   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 ad‐
              dress pattern can be used.

       •      Wildcards `*´ and `?´ can be used to match hostnames or IP addresses.  This method of matching cannot be used in con‐
              junction 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 un‐
              available 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  un‐
              available 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 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 incom‐
       patible 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

       %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 avail‐

       %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.   Ser‐
       vice  providers  can use this facility to offer FTP, GOPHER or WWW archives with internet names that may even belong to dif‐
       ferent 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  to‐
       gether 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 inter‐
       rogate 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 sup‐
       ported).  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  com‐
              promised.  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 sec‐
              onds: 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

       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 re‐
       quest at all.  When the client host provides IDENT service,  a  negative  IDENT  lookup  result  (the  client  matches  `UN‐
       KNOWN@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  connec‐
       tion. 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. Al‐
       though 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 re‐
       duce 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 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  excep‐
       tion 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

          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