Danny Guo | 郭亚东

My Cat Alerted Me to a DDoS Attack

 ·  509 words  ·  ~3 minutes to read

A few years ago, my cat gave me my most memorable middle of the night software engineering incident. I was working at a startup, and we didn’t have a formal on-call rotation yet. That was a deliberate decision, since being on-call is painful, and the team was good about just collectively keeping an eye out for urgent alerts. We eventually set up an on-call rotation, but before that happened, I had a fun night.

Around 3 a.m., I woke up because my cat was grooming my hair. The grooming itself wasn’t unusual. She did it occasionally, and I optimistically took it as a sign that she actually liked me and didn’t just tolerate me. Here’s the cat tax:

my cat in a Fancy Feast box

my cat grooming my hair

But in 9 years, that was the only time she did it while I was sleeping. I checked my phone to see what time it was, and I noticed that an AWS CloudWatch alert had gone off a couple minutes ago because of unhealthy targets for our load balancer.

I tried to go to our website, but it didn’t load. I groaned and went to log onto my work laptop. Our monitoring dashboard showed a massive number of requests coming from many IP addresses that were associated with different countries. And international traffic wasn’t typical for us anyway since our products were only available to people in the United States. It was a distributed denial-of-service (DDoS) attack.

My first and not great thought was to block IP addresses at the server level, which would have been tedious and possibly ineffective if the attacker had significantly more source IP addresses to use. But then I remembered that we had already set up AWS Web Application Firewall. To deal with the immediate outage, I set up a rule to block requests from other countries. It took effect and blocked hundreds of thousands of requests over the next hour or so. Our website started working again, and the flood of requests stopped.

Later that morning, we noticed that we had received an email to our customer support inbox around when the attack started. With horrible grammar, the sender claimed to have found a vulnerability with our website that crashed Apache, which we didn’t even use. They said they stopped all traffic to our website and could keep it that way for months. They generously offered to give us a “solution file” if we sent them $5,000 in Bitcoin. We didn’t reply, though in retrospect, it could have been fun to try to troll them.

I still find it hard to believe the perfect timing of my cat waking me up. You might guess that the AWS alert caused my phone to vibrate or make a sound, waking my cat up first. But I keep my phone in do not disturb mode during the night. So I just like to think that somehow, she sensed something was wrong that couldn’t wait until the morning. It was certainly a more pleasant way to be woken up than by a blaring PagerDuty alarm.

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