A network intruder will look for security weaknesses at every point in your network architecture. If you have adequately locked down the Physical, Data Link, Network, and Transport layers of yournetwork, the wily hacker will simply move up to those protocols and services your network does expose to the Internet. These application-specific protocols are actually much easier to exploit, so many hackers start there and drop down to the Network or Transport level when they need tocircumvent a protocol's security mechanisms. In this article, we'll go over each of the most commonly used Internet services, briefly examining each for their weaknesses and abuse potential. First, however, we'll discuss sockets and services in general, identifying typical service vulnerabilities so you can identify potential problems when you need to install services on your own network.

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