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TCPD(8) OpenBSD 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(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 onto 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 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 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.
HOST NAME VERIFICATION
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 ADDRESS SPOOFING
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.
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
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.
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)) al-
ways 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.
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 (firstname.lastname@example.org),
Department of Mathematics and Computing Science,
Eindhoven University of Technology
Den Dolech 2, P.O. Box 513,
5600 MB Eindhoven, The Netherlands
OpenBSD 2.3 June 23, 1997 2
Source: OpenBSD 2.6 man pages. Copyright: Portions are copyrighted by BERKELEY
SOFTWARE DESIGN, INC., The Regents of the University of California, Massachusetts
Institute of Technology, Free Software Foundation, FreeBSD Inc., and others.
(Corrections, notes, and links courtesy of RocketAware.com)
OpenBSD sources for tcpd(8)
Up to: Communication Filtering and Firewalls - Preventing certain types of communication. Communication security, encryption, et al. -
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