Last week we discussed SYN flood attacks, a devious way that miscreants can cause trouble. In essence, a source machine sends connection requests (usually from a false address so the requests are hard to trace) that the destination machine responds to. As the source machine never completes the connection request and sends many requests quickly, the destination machine can be overwhelmed.

Central to this 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.

One of the biggest information giveaways for hackers is for you to have machines with ports that aren't in use but respond anyway. Windows, unfortunately, makes it horribly easy to leave your machine open for information to be discovered - see Gibson Research's ShieldsUp! site for details.

Also see the discussion of something called NanoProbe technology, also from Gibson Research, which makes port scanning faster. Cool stuff.

Anyway, it is the hacking threat that is the reason you use a firewall - to prevent someone outside your network from connecting to things they shouldn't know about and to prevent them from even finding out about those things in the first place.

There are many reasons you might want to use tools for testing and exploring ports. We talked about the killer tool for this work a couple of weeks ago, Nmap, but erroneously said it was not available for Windows NT. We found out it is - go to eEye Digital Security (thanks to reader Brett Hiscock for letting us know).

This implementation has a few limitations compared with the Unix version but nothing you can't live with.

» posted by ITworld staff

Network World

• Email
• Print
• Share
  • Slashdot
  • Digg
  • Stumble
  • Reddit
  • Newsvine