This paper reports the most important techniques used by TCP port scanners . TCP port scanners are specialized programs used to determine what TCP ports of a host have processes listening on them for possible connections. Since these ports characterize, in part, the amount of exposure of the hosts to potential external attacks, knowing their existence is a fundamental matter for network and/or security administrators. Moreover, as scanners are also used by hackers, administrators need to know how they work and what possible weakness they exploit to be able to prevent unwanted scanning or at least to record each scanning attempt.

The Internet put the rest of the world at the reach of our computers. In the same way it also made our computers reachable by the rest of the world. Good news and bad news!. Over the last decade, the Internet has been subject to widespread security attacks. Besides the classical terms, new ones had to be found in order to designate a large collection of threats: Worms, break-ins, hackers, crackers, hijacking, phrackers, spoofing, man-in-the-middle, password-sniffing, denial-of-service, and so on.Since the Intemet was born of academic efforts to share information, it never strove for high security measures. In fact in some of its components, security was consciously traded for easiness in sharing.Although the advent of electronic commerce has pushed for "real security" in the Interact, there is yet a huge amount of users (including scientists) very vulnerable to attacks, mostly because they are not aware of the nature (and ease) of the attacks and still believe that a "good" password is all they need to be concerned about. We wrote this paper aiming for a better understanding of the subject. In the paper we report some of the major actual known attacks. Besides the description of each attack (the what), we also discuss the way they are carried on (the how) and, when possible, the related means of prevention, detection and/or defense.

The Internet put the rest of the world at the reach of our computers. In the same way it also made our computers reachable by the rest of the world. Good news and bad news! Over the last decade, the Internet has been subject to widespread security attacks. Besides the classical terms, new ones had to be found in order to designate a large collection of threats: Worms, break-ins, hackers, crackers, hijacking, phrackers, spoofing, man-in-the-middle, password-sniffing, denial-of-service , and so on.Since the Internet was born of academic efforts to share information, it never strove for high security measures. In fact in some of its components, security was consciously traded for easiness in sharing. Although the advent of electronic commerce has pushed for "real security" in the Internet, there are still a large number of users (including computer scientists) that are very vulnerable to attacks, mostly because they are not aware of the nature (and ease) of the attacks and still believe that a "good" password is all they need to be concerned about.Aiming for a better understanding of the subject, we wrote a first paper [1] in which we discussed several threats and attacks related to TCP/IP . The present work is an extension of the first one, and its main goal is to include T/TCP in the discussion. Additionally, in an effort to make this paper more comprehensive, we included some sections from the former.Besides the description of each attack (the what ), we also discuss the way they are carried out (the how ) and, when possible, the related means of prevention, detection and/or defense.