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TCPD(8)                 OpenBSD System Manager's Manual                TCPD(8)

     tcpd - tcp wrappers access control facility for internet services

     The tcpd program can be set up to monitor incoming requests for
     telnet(1), finger(1), ftp(1), rexecd(8), rsh(1), rlogin(1), tftp(1),
     talk(1), comsat(8), and other services that have a one-to-one mapping on-
     to executable files.

     Operation is as follows: whenever a request for service arrives, the in-
     etd(8) daemon is tricked into running the tcpd program instead of the de-
     sired 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 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 de-
     tect unwanted activities, especially when logfile information from sever-
     al 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(1), rsh(1)) 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 con-
     cludes that it is dealing with a host that pretends to have someone elses
     host name.

     Optionally, tcpd disables source-routing socket options on every connec-
     tion 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 at-
     tempt 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.

     This example applies when tcpd expects that the network daemons are left
     in their original place, as it is configured within OpenBSD.

     In order to monitor access to the finger(1) service, perform the follow-
     ing edits on the inetd(8) configuration file, /etc/inetd.conf:

           finger  stream  tcp  nowait  nobody  /usr/libexec/fingerd  fingerd


           finger  stream  tcp  nowait  nobody  /usr/libexec/tcpd     fingerd

     Similar changes will be needed for the other services that are to be cov-
     ered 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(8) configuration file so that it specifies
     an absolute path name for the process name field.  For example:

         ntalk  dgram  udp  wait  root  /usr/libexec/tcpd  /usr/local/lib/ntalkd

     Only the last component (ntalkd) of the pathname will be used for access
     control and logging.

     The default locations of the host access control tables are:

     /etc/hosts.allow  Access control table (allow list)
     /etc/hosts.deny   Access control table (deny list)

     hosts_access(5), inetd.conf(5), syslog.conf(5).

           Wietse Venema (,
           Department of Mathematics and Computing Science,
           Eindhoven University of Technology
           Den Dolech 2, P.O. Box 513,
           5600 MB Eindhoven, The Netherlands

     Some UDP (and RPC) daemons linger around for a while after they have fin-
     ished their work, in case another request comes in.  In the inetd config-
     uration file these services are registered with the wait option.  Only
     the request that started such a daemon will be logged.

     RPC broadcast requests (for example: rwall(1), rup(1), rusers(1)) always
     appear to come from the responding host.  What happens is that the client
     broadcasts the request to all portmap(8) daemons on its network; each
     portmap(8) daemon forwards the request to a local daemon.  As far as the
     rwall(8) etc. daemons know, the request comes from the local host.

OpenBSD 2.8                      June 23, 1997                               2