tcpd - access control facility for internet services


   The tcpd program can be set up to monitor incoming requests for telnet,
   finger, ftp, exec, rsh, rlogin, tftp, talk, comsat and  other  services
   that have a one-to-one mapping onto executable files.

   The  program  supports  both  4.3BSD-style sockets and System V.4-style
   TLI.  Functionality may be limited when the protocol underneath TLI  is
   not an internet protocol.

   There  are  two possible modes of operation: execution of tcpd before a
   service started by inetd, or linking a daemon with the  libwrap  shared
   library  as  documented  in  the hosts_access(3) manual page. Operation
   when started by inetd is as follows: whenever  a  request  for  service
   arrives,  the  inetd  daemon  is  tricked into running the tcpd program
   instead of the desired server. tcpd logs  the  request  and  does  some
   additional  checks.  When all is well, tcpd runs the appropriate server
   program and goes away.

   Optional features are: pattern-based access  control,  client  username
   lookups  with  the RFC 931 etc. protocol, protection against hosts that
   pretend to have someone elses host name, and protection  against  hosts
   that pretend to have someone elses network address.


   Connections  that  are  monitored  by  tcpd  are  reported  through the
   syslog(3) facility. Each record contains a time stamp, the client  host
   name  and  the  name  of the requested service.  The information can be
   useful  to  detect  unwanted  activities,   especially   when   logfile
   information from several hosts is merged.

   In  order  to  find  out  where your logs are going, examine the syslog
   configuration file, usually /etc/syslog.conf.


   Optionally, tcpd supports a simple form of access control that is based
   on  pattern  matching.   The access-control software provides hooks for
   the execution of shell commands when a pattern fires.  For details, see
   the hosts_access(5) manual page.


   The  authentication  scheme  of  some protocols (rlogin, rsh) relies on
   host names. Some implementations believe the host name  that  they  get
   from any random name server; other implementations are more careful but
   use a flawed algorithm.

   tcpd  verifies  the  client  host  name  that  is   returned   by   the
   address->name  DNS  server by looking at the host name and address that
   are returned by the name->address DNS server.  If  any  discrepancy  is
   detected,  tcpd  concludes that it is dealing with a host that pretends
   to have someone elses host name.

   If the sources  are  compiled  with  -DPARANOID,  tcpd  will  drop  the
   connection  in  case  of  a host name/address mismatch.  Otherwise, the
   hostname can  be  matched  with  the  PARANOID  wildcard,  after  which
   suitable action can be taken.


   Optionally,  tcpd  disables  source-routing  socket  options  on  every
   connection that it deals with. This will take care of most attacks from
   hosts  that  pretend  to  have an address that belongs to someone elses
   network. UDP services do not benefit from this protection. This feature
   must be turned on at compile time.

RFC 931

   When  RFC  931 etc. lookups are enabled (compile-time option) tcpd will
   attempt to establish the name of the client  user.  This  will  succeed
   only  if the client host runs an RFC 931-compliant daemon.  Client user
   name lookups will not work for datagram-oriented connections,  and  may
   cause noticeable delays in the case of connections from PCs.


   The  details  of  using  tcpd  depend  on pathname information that was
   compiled into the program.


   This example applies  when  tcpd  expects  that  the  original  network
   daemons will be moved to an "other" place.

   In  order  to  monitor  access to the finger service, move the original
   finger daemon to the "other" place and install tcpd in the place of the
   original finger daemon. No changes are required to configuration files.

        # mkdir /other/place
        # mv /usr/sbin/in.fingerd /other/place
        # cp tcpd /usr/sbin/in.fingerd

   The example assumes that the network daemons live in /usr/sbin. On some
   systems, network daemons live in /usr/sbin or in /usr/libexec, or  have
   no `in. prefix to their name.


   This  example  applies  when  tcpd expects that the network daemons are
   left in their original place.

   In order to monitor access to the finger service, perform the following
   edits on the inetd configuration file (usually /etc/inetd.conf):

        finger  stream  tcp  nowait  nobody  /usr/sbin/in.fingerd  in.fingerd


        finger  stream  tcp  nowait  nobody  /usr/sbin/tcpd     in.fingerd

   The example assumes that the network daemons live in /usr/sbin. On some
   systems, network daemons live in  /usr/sbin  or  in  /usr/libexec,  the
   daemons have no `in. prefix to their name, or there is no userid field
   in the inetd configuration file.

   Similar changes will be needed for the other services that  are  to  be
   covered  by  tcpd.   Send a `kill -HUP to the inetd(8) process to make
   the changes effective.


   In the case of daemons that do not live in a common directory ("secret"
   or  otherwise),  edit the inetd configuration file so that it specifies
   an absolute path name for the process name field. For example:

       ntalk  dgram  udp  wait  root  /usr/sbin/tcpd  /usr/local/lib/ntalkd

   Only the last component (ntalkd) of  the  pathname  will  be  used  for
   access control and logging.


   Some  UDP  (and  RPC) daemons linger around for a while after they have
   finished their work, in case another request comes in.   In  the  inetd
   configuration  file these services are registered with the wait option.
   Only the request that started such a daemon will be logged.

   The program does not work with RPC services over  TCP.  These  services
   are  registered  as  rpc/tcp  in the inetd configuration file. The only
   non-trivial service that is affected by this limitation is rexd,  which
   is  used by the on(1) command. This is no great loss.  On most systems,
   rexd is less secure than a wildcard in /etc/hosts.equiv.

   RPC broadcast requests (for example: rwall, rup, rusers) always  appear
   to  come  from  the  responding  host.  What happens is that the client
   broadcasts the request to all portmap  daemons  on  its  network;  each
   portmap  daemon  forwards  the request to a local daemon. As far as the
   rwall etc.  daemons know, the request comes from the local host.


   The default locations of the host access control tables are:



   hosts_access(3), functions provided by the libwrap library.
   hosts_access(5), format of the tcpd access control tables.
   syslog.conf(5), format of the syslogd control file.
   inetd.conf(5), format of the inetd control file.


   Wietse Venema (,
   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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