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 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/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 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.

       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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