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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 ad-
       dress->name DNS server by looking at the	host name and address that are
       returned	 by  the  name->address	DNS server.  If	any discrepancy	is de-
       tected, 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 ac-
       cess 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 (,
       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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