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 TCPD(8)                                                             TCPD(8)

      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.

      Operation 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

      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.

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 TCPD(8)                                                             TCPD(8)

      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

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

      The example assumes that the network daemons live in /usr/etc. 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 or /etc/inet/inetd.conf):

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


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 TCPD(8)                                                             TCPD(8)

           finger  stream  tcp  nowait  nobody  /some/where/tcpd     in.fingerd

      The example assumes that the network daemons live in /usr/etc. 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. AIX users may also have to execute the
      `inetimp' command.

      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

          ntalk  dgram  udp  wait  root  /some/where/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(5), format of the tcpd access control tables.

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 TCPD(8)                                                             TCPD(8)

      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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