control access to internet services
The tcpd program can be set up to monitor incoming requests for
telnet, finger, ftp,
rlogin, tftp, talk
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 away.
Optional features are: pattern-based access control, client username
lookups with the RFC 931 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
facility. Each record contains
a time stamp, the client host name and the name of the requested
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 configuration file, /etc/syslog.conf.
supports a simple form of access control that is based on pattern
The access-control software provides hooks for the execution
of shell commands when a pattern matches.
For details, see
Host name verification
The authentication scheme of some protocols
(such as rlogin and 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.
When looking up the client host name corresponding to an IP
verifies the name that is returned by the DNS
server by comparing it with the host name and address that are returned
when the name is used to look up the corresponding IP address.
This is know as a ``double reverse lookup''.
If any discrepancy is detected, tcpd
concludes that it is dealing with a host that is pretending to have someone
else's host name.
The supplied version of tcpd
was compiled with PARANOID defined,
so that it will drop the connection in case of a host name/address mismatch.
Host address spoofing
disables source-routing socket options on every connection 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 was turned on at compile time.
RFC 931 lookups
RFC 931 lookups are enabled (a compile-time option),
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 some machines.
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.
are registered as rpc/tcp in the
inetd configuration file.
The only non-trivial service that is affected by this limitation is
RPC broadcast requests
(from rwall or rusers, for example) always
appear to come from the responding host. What happens is that the
client broadcasts the request to all rpcbind daemons on its
network; each rpcbind daemon forwards the request to a local
daemon. As far as the rwall and similar daemons know,
the request comes from the local host.
Wietse Venema (email@example.com),
Department of Mathematics and Computing Science,
Eindhoven University of Technology
Den Dolech 2, P.O. Box 513,
5600 MB Eindhoven, The Netherlands
In order to monitor access to the finger service,
the following entry must exist uncommented
in the inetd configuration file,
finger stream tcp nowait nobody /usr/sbin/in.tcpd in.fingerd
Similar changes will be needed for the other services that are to be
covered by tcpd.
In the case of daemons that do not live in /usr/sbin,
specify an absolute path name for the process name field. For example:
ntalk dgram udp wait root /usr/sbin/in.tcpd /usr/local/lib/ntalkd
Only the last component (/usr/local/lib/ntalkd)
of the pathname will be used for access control and logging.
Stop and restart inetd to have the changes take effect:
sacadm -k -p inetd
sacadm -s -p inetd
© 2005 The SCO Group, Inc. All rights reserved.
SCO OpenServer Release 6.0.0 - 01 June 2005