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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.

       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  sys-
       log(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 informa-
       tion from several hosts is merged.

       In order to find out where your logs are going, examine the syslog con-
       figuration 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 connec-
       tion 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  con-
       nection  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  com-
       piled into the program.

       This  example  applies when tcpd expects that the original network dae-
       mons 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.

       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

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


            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 dae-
       mons 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'

       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.

       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.
       syslog.conf(5), format of the syslogd control file.
       inetd.conf(5), format of the inetd control file.

       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