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, 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.

LOGGING
       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.

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 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.

HOST ADDRESS SPOOFING
       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.

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

EXAMPLE 1
       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.

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):

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

       becomes:

	    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.

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  /usr/sbin/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(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.

AUTHORS
       Wietse Venema (wietse@wzv.win.tue.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)