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Our security gateway to the Internet, research.att.com, provides only a limited set of services. Most of the standard servers have been replaced by a variety of trap programs that look for attacks. Using these, we have detected a wide variety of pokes, ranging from simple doorknob-twisting to determined assaults. The attacks range from simple attempts to log in as guest to forged NFS packets. We believe that many other sites are being probed but are unaware of it: the standard network daemons do not provide administrators with either appropriate controls and filters or with the logging necessary to detect attacks.
Steven M. Bellovin (Wed,) studied this question.