On 8/21, I noticed that internet connectity through our 4.7 FreeBSD gateway 
NAT box was getting REALLY slow. Checking with our T1 provider, there was 
only 128K of data stream (aprox) flowing out the T1. Ping times to the router 
on the external interface yielded times of up to 3 seconds!

This box is a Dell 2350 server with one 500mhz Pent 3 and 512 mg ram.

Running tcpdump on both the internal and external interfaces showed a very 
small number of ICMP packets flowing on either and virtually no IP.

My first conclusion...wrong..was that I had a bad ethernet card. Pulled 
server/gateway box off line and replaced the card. No difference.

It turned out that we had several Windows boxes in the building that had been 
infected with the Nachi worm. This causes some kind of DOS or ping probe out 
onto the internet and the local LAN.

Removing the inside interface's ethernet cable caused the ping times on the 
outside interface to go back to the normal .4 milliseconds to the router.

Apparently, the blast of packets coming from the infected boxes managed to 
cause a "live lock" condition in the server. I assume it was interrupt bound 
servicing the inside interface. The packets were ICMP requests to various 
addresses.

At one point, I substituted a Dell 2650 with 1 gig interfaces and 2 1800 mg 
Xeons at the gateway addresses and it bound up also. Speed seems not to be 
the answer 8-( .

My questions is.. what, if any, is a technique for preventing this condition? 
I know, fix the windows boxes, but  I can't continually check the status of 
the virus software and patch level of the Windows boxes. There are 250 plus 
of them and one of me. Users won't install upgrades even when warned this 
worm thing was coming. But, i'd like to prevent loss of service when one of 
Bill's boxes goes nuts!

The inside interface is the 'xl' driver on a 3Com 3C905. Can it be run in 
polling mode or given lower interrupt priority?

BTW, it seems to only take about 3 infected windows boxes to bring things to a 
halt.

-Jim

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