man > hosts.allow(5)

HOSTS_ACCESS(5)                          File Formats Manual                         HOSTS_ACCESS(5)

NAME
       hosts_access - format of host access control files

DESCRIPTION
       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) doc‐
       ument. 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 speci‐
       fied in the inetd configuration file.

ACCESS CONTROL FILES
       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 con‐
       trol can be turned off by providing no access control files.

ACCESS CONTROL RULES
       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 per‐
              mits 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 end‐
       point 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 insensi‐
       tive.

PATTERNS
       The access control language implements the following patterns:

       •      A string that begins with a `.´ character. A host name is matched if the  last  compo‐
              nents  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  nu‐
              meric  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.  Net‐
              group matches are not supported for daemon process names or for client user names.  On
              Debian  systems,  support  for  NIS  netgroups has been disabled since package version
              7.6.q-33.

       •      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  `131.155.72.0/255.255.254.0´  matches
              every address in the range `131.155.72.0´ through `131.155.73.255´.  `255.255.255.255´
              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 be‐
              ginning with `.´ or IP address matching ending with `.´.

WILDCARDS
       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.

       UNKNOWN
              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 soft‐
              ware 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  soft‐
              ware cannot figure out what type of network it is talking to.

       PARANOID
              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.

OPERATORS
       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 dae‐
              mon_lists  and in client_lists. The EXCEPT operator can be nested: if the control lan‐
              guage would permit the use of parentheses, `a EXCEPT b EXCEPT c´ would  parse  as  `(a
              EXCEPT (b EXCEPT c))´.

SHELL COMMANDS
       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.

% EXPANSIONS
       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,  depend‐
              ing 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.

SERVER ENDPOINT PATTERNS
       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 dif‐
       ferent 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 con‐
       text. Usually, server endpoint information is available only  with  connection-oriented  ser‐
       vices.

CLIENT USERNAME LOOKUP
       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 connec‐
       tion. 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.

DETECTING ADDRESS SPOOFING ATTACKS
       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.

EXAMPLES
       The  language  is  flexible  enough  that different types of access control policy can be ex‐
       pressed 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 net‐
       work/netmask information, to reduce the impact of temporary name server lookup failures.

MOSTLY CLOSED
       In this case, access is denied by default. Only explicitly authorized hosts are permitted ac‐
       cess.

       The default policy (no access) is implemented with a trivial deny file:

       /etc/hosts.deny:
          ALL: ALL

       This  denies all service to all hosts, unless they are permitted access by entries in the al‐
       low file.

       The explicitly authorized hosts are listed in the allow file.  For example:

       /etc/hosts.allow:
          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 terminalserver.foo‐
       bar.edu.

MOSTLY OPEN
       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:

       /etc/hosts.deny:
          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 fin‐
       ger requests from other hosts and domains.

BOOBY TRAPS
       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.

       /etc/hosts.allow:
          in.tftpd: LOCAL, .my.domain

       /etc/hosts.deny:
          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 sys‐
       tem.

DIAGNOSTICS
       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.

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

SEE ALSO
       hosts_options(5) extended syntax.
       tcpd(8) tcp/ip daemon wrapper program.
       tcpdchk(8), tcpdmatch(8), test programs.

BUGS
       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.

AUTHOR
       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


                                                                                     HOSTS_ACCESS(5)