Central to [the SYN] attack is the ability of the miscreant to find an "open" port - that is, a port on the destination machine that responds to connection requests. If a hacker is trying to find your weaknesses, he will . . .
Central to [the SYN] attack is the ability of the miscreant to find an "open" port - that is, a port on the destination machine that responds to connection requests. If a hacker is trying to find your weaknesses, he will usually begin by trying to find out what your network looks like. The obvious way to start is to ping all of the possible addresses in your subnet to find "live" machines.

But you might already be on the lookout for such ping surveys. A number of tools are available to watch for such activity, and they fall into the realm of packet sniffers - tools that watch passing packets and filter out suspicious events.

A tool that's great for spotting hack attempts is Computer Associates' eTrust Internet Defense - Intrusion Detection (previously was SessionWall from AbirNet until CA swallowed it up). This is an excellent product for detecting ping surveys as well as SYN flood attacks and a whole catalog of other hacking techniques.

Once the hacker has a live IP address, by using the stack fingerprinting technique he can build a detailed map of your network and figure out what is where. Even more suspicious than a ping survey is a port scan, the process of attempting to make connections to a range of ports on a machine or to a range of ports on a range of machines.

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