TCPD(8)                                                                                                                  TCPD(8)



NAME
       tcpd - access control facility for internet services

DESCRIPTION
       The  tcpd program can be set up to monitor incoming requests for telnet, finger, ftp, exec, rsh, rlogin, tftp, talk, com-
       sat 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 pro-
       gram 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.

LOGGING
       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.

ACCESS CONTROL
       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.

HOST NAME VERIFICATION
       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.  Oth-
       erwise, the hostname can be matched with the PARANOID wildcard, after which suitable action can be taken.

HOST ADDRESS SPOOFING
       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  bene-
       fit 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.

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

EXAMPLE 1
       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/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.

EXAMPLE 2
       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

       becomes:

            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.

EXAMPLE 3
       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  /some/where/tcpd  /usr/local/lib/ntalkd


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

BUGS
       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.

FILES
       The default locations of the host access control tables are:

       /etc/hosts.allow
       /etc/hosts.deny

SEE ALSO
       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.

AUTHORS
       Wietse Venema (wietseATwzv.nl),
       Department of Mathematics and Computing Science,
       Eindhoven University of Technology
       Den Dolech 2, P.O. Box 513,
       5600 MB Eindhoven, The Netherlands




                                                                                                                         TCPD(8)