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Me and varnish win against a DDOS attack.

This past month one of my servers experienced her first DDOS - a distributed denial of service attack. A denial of service attack (or DOS) just means an attempt to shut down an internet-based service by overwhelming it with requests. A simple DOS attack is usually relatively easy to deal with using the standard linux firewall called iptables.  The way iptables works is by filtering the traffic based on the incoming request source (i.e., the IP of the attacking machine). The attacking machine's IP can be added into your custom ip tables 'blacklist' to block all traffic from it, and it's quite scalable so the only thing that can be overwhelmed is your actual internet connection, which is hard to do.

The reason a distributed DOS is harder is because the attack is distributed from multiple machines. I first noticed an increase in my traffic about a day after it had started - it wasn't slowing down my machine, but it did show up as a spike in traffic. I quickly saw that a big chunk of traffic was all of the same form - a POST to a domain that wasn't actually in use except as a redirect. There were several requests per second, and each attacking machine would do the same request about 8 times. So it was coming from a lot of different machines, making it not feasible to manually keep adding in these ip's into my blacklist.

It certainly could have been a lot worse. Because it was attacking a domain that was being redirected, it was using up an apache process, but no php, so it was getting handled very easily without making a noticeable dent in regular services. But it was worrisome, in case the traffic picked up. It was also a curious attack - why make an attack on an old domain that wasn't even in use? My best guess is that it was either a mistake, or a way of keeping someone's botnet busy. I have heard that there are a number of these networks of "zombie" machines, presumably a kind of mercenary force for hire, and maybe if there are no contracts, they get sent out on scurrilous missions to keep them busy.

In any case, I also thought a bit about why Varnish wasn't being useful here. Varnish is my reverse-proxy protective bubble for my servers (yes, kind of like how a layer of varnish protects your furniture). The requests weren't getting cached by Varnish because in general, it's not possible to responsibly cache POST requests (which is presumably why a DDOS would favour this kind of traffic). To see why, just imagine a login request , which is a POST - each request will have a unique user/pass and the results of the request will need to get handled by the underlying CMS (Drupal in my case).

But, in this case, I wasn't getting any valid POST requests to that domain anyway, so that made it relatively easy to add the following stanza to my varnish configuration:

 if (req.http.host ~ "example.com" && req.request == "POST") {
   return (lookup);
 }

And indeed, now all the traffic is bouncing off my varnish and I'm not worrying. In case it was a domain that was actively in use, I could have added an extra path condition (since no one should be POST'ing to the front page of most of my domains anyway), but it would have started getting trickier. Which is why you won't find Varnish too helpful for DDOS POST attacks in general. As usual, the details matter, and in this case, since I was being attacked by a collection of mindless machines, the good guys won.

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