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TCPD(8)								       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 (,
       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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