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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 proto-
     col 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 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 imple-
     mentations 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.

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 dae-
     mons 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 confi-
     guration 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  direc-
     tory  ("secret"  or otherwise), edit the inetd configuration
     file so that it specifies an absolute path name for the pro-
     cess 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 configura-
     tion 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 ([email protected]),
     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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