TCPD(8)                     System Manager's Manual                    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.

       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