We recently had a really nasty DOS attack that took down a large part of our network across several cell sites, from the infected client all the way to the Internet transit. Take note that we identified the problem quickly and cured it quickly. But.... This is the first time that this has occured in 5 years, as we have a good number of smart design characteristics that have limited the effects of most viruses on our network. We stopped the attack, by blocking SSH to the infected sub. The average amount of traffic crossing the entire network path from the client to the Internet was about 500 kbps on average. (This was a 20 mbps wireless link, and a 100mbps fiber trnasport link to the transit.). The two routers were a P4 2Ghz, and a Dual XEON 2.2Ghz w/ 10,000rpm SCSI3. The damage was that the CPU was nailed on both routers to about 99.9% using "TOP" to monitor stats. We varified that successful SSH sessions were not made directly to the protected routers themselves. Take note that the wireless links were barely effected, it was the router 2 hops away (Dual XEON) that got over loaded the most. Our routers have been tested to pass over 2 gbps of throughput easilly. And have been load tested to survive very small packets and high PPS adequately. The infected sub was bandwidth managed with HTB to 256k cir, 1 mbps mir, but not anything for PPS. So I'm looking for reasons that the CPU got overloaded. My theory is that the DOS attack resulted in a large number of disk writes, ( maybe logging?) causing the CPU saturation. I've had a hard time locating the cause. And have not discovered which virus yet, although I should have more info soon from my clients.

So my question....

What needs to be done on a Linux machine to harden it, to protect against CPU oversaturation, during DOS attacks?

What should and shouldn't be logged? Connection Tracking? Firewall logging? Traffic stats?

Tom DeReggi
RapidDSL & Wireless, Inc
IntAirNet- Fixed Wireless Broadband
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