Slashdot.org was unreachable for about 75 minutes this evening. Here is the post-mortem from Sourceforge's chief network engineer Uriah Welcome. "What we had was indeed a DoS, however it was not externally originating. At 8:55 PM EST I received a call saying things were horked, at the same time I had also noticed things were not happy. After fighting with our external management servers to login I finally was able to get in and start looking at traffic. What I saw was a massive amount of traffic going across the core switches; by massive I mean 40 Gbit/sec. After further investigation, I was able to eliminate anything outside our network as the cause, as the incoming ports from Savvis showed very little traffic. So I started poking around on the internal switch ports. While I was doing that I kept having timeouts and problems with the core switches. After looking at the logs on each of the core switches they were complaining about being out of CPU, the error message was actually something to do with multicast. As a precautionary measure I rebooted each core just to make sure it wasn't anything silly. After the cores came back online they instantly went back to 100% fabric CPU usage and started shedding connections again. So slowly I started going through all the switch ports on the cores, trying to isolate where the traffic was originating. The problem was all the cabinet switches were showing 10 Gbit/sec of traffic, making it very hard to isolate. Through the process of elimination I was finally able to isolate the problem down to a pair of switches... After shutting the downlink ports to those switches off, the network recovered and everything came back. I fully believe the switches in that cabinet are still sitting there attempting to send 20Gbit/sec of traffic out trying to do something — I just don't know what yet. Luckily we don't have any machines deployed on [that row in that cabinet] yet so no machines are offline. The network came back up around 10:10 PM EST."