Introducing PSDN and X25

The PSDN or Public Switched Data Network uses traditional, analog telephone lines to transmit data packets. Although it can be used to describe other systems, we're using it to refer to X.25 networks that communicate via normal telephone lines.

In the 1970s the TLC market wanted a set of protocols to provide companies with wide area network (WAN) connectivity across public data networks (PDNs). The result of this development effort—led by a United Nations agency called the International Telecommunications Union or ITU—was a group of protocols, the most popular being X.25.

The International Telecommunication Union-Telecommunication Standards Sector (ITU-T) (formerly CCITT) is the ITU committee responsible for voice and data communications. ITU-T members include the FCC, the European Postal Telephone and Telegraph organizations, the common carriers, and many computer and data communication companies. As a direct result, X.25 was developed by the common carriers (the telephone companies acting as a monopoly, essentially, since most of them were ITU members) rather than by any single commercial enterprise. The specification is, therefore, designed to work well regardless of a user's system type or manufacturer. As a result, X.25 is truly a global standard.

X.25 networks are often erroneously seen as "old, retired networks." However, in the past decade, these "dead" networks were the victims of an incredible number of highlevel attacks launched toward finance systems, multinationals, telcos, civil and military aeronautical networks, and governmental infrastructures. In fact, hackers use X.25 networks to attack computer systems around the world. Usually, this is a side effect of the security approach used by corporate companies—especially telcos—where they invest a lot of money in the security on the TCP/IP connection side but neglect their X.25 access points. Major corporations are still linked to X.25 networks, for instance, Alcatel, Digital (now Compaq), KPMG, E&Y, and so on. Moreover, X.25 networks are widely used (as they exploded much later) in Africa, the Middle East, and Central Asia, resulting in government and military computer systems being linked to these networks.

Many Internet users seem to view X.25 networks as mysterious. They view X.25 networks as an alien invention used only by telecommunications carriers to achieve international connectivity. Another common mistake is to think that X.25 networks aren't used anymore; this is completely wrong! X.25 technology has been used to construct the most pervasive data network—the global public data network formed by the PTTs connects at least 95 different countries.

Internet administrators may assume that tracing attackers across an X.25 network is almost impossible. The descriptions given in Clifford Stoll's book, The Cuckoo's Egg, reinforce this impression. In a chapter of the book the author describes the process of contacting Ron Vivier at Telenet/SprintNet, who then contacts Steve White, and so on, back to Hannover in Germany. In reality, tracing attacks across an X.25 network is as easy (or as difficult) as on a TCP/IP network.

This quick overview ends with a mention of the Société Internationale de Télécommunications Aéronautiques (SITA), established in 1949 ( SITA is a worldwide company that manages flight connections for many airlines. In airports all over the world, you'll find computer terminals with SITA logon banners. SITA has its own X.25 network and decided to "share" the network, forcing the first three digits of the Network User Address (NUA) to become the identifiers for the country.

Remember that it is not just the global public data network that uses X.25; many private and corporate networks also use X.25. Some of the techniques described here are equally applicable to private networks. Dealing with attacks that take place across an X.25 network requires the ability to

• Monitor the traffic

• Check the system logs

• Identify the origin and target of calls

The last section of this chapter will explain the key differences between TCP/IP and X.25 security testing, including a technical overview of the PSDN ITU standard protocols.

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