IT Pro confession: How I helped in the BIGGEST DDoS OF ALL TIME

Oh Trevor, how could you? Like this

Some wires and a switch

Sysadmin blog I contributed to the massive DDoS attack against Spamhaus. What flowed through my network wasn't huge - it averaged 500Kbit/sec – but it contributed. This occurred because I made a simple configuration error when setting up a DNS server; it's fixed now, so let's do an autopsy.

The problem

I should start off by apologizing to CloudFlare and Spamhaus; my lapse contributed to a DDoS against their infrastructure. More damning than merely having been an unwitting participant is that I knew enough about this sort of attack to have set up rudimentary protections against it and yet I still forgot the critical component: actually disabling recursive lookups.

The way a DNS amplification attack works is simple. DNS servers can be configured in one of two basic ways. In one possible configuration a DNS server serves only domains for which it is responsible (authoritative). In the other configuration the DNS server serve those domains and goes looking on the wider internet for any domains it isn't personally set up to manage (recursive).

Your DNS server at the office might be configured to be authoritative for If you want to go to then in a strictly authoritative configuration your DNS server won't be able to provide an answer: it doesn't know where is.

In a recursive configuration the DNS server asks the list of root servers it has preconfigured who owns the ".com" domain. It then asks the .com servers who owns "". It then asks "" who owns "" and delivers that address back to you.

As you can see, recursive DNS servers are what allow the internet to work. They are also an attack vector. Let's say that you leave your recursive server open to the internet. Now not only can you ask your DNS server for information about other DNS servers on the internet, so can anyone else. If someone asks your server "where is" a whole bunch of times then your server starts flooding's DNS servers.

For every 1 byte of data sent to your DNS server 50 bytes of traffic end up directed at the target. This is a DNS amplification attack. The issue has been around for ages, but it has taken this latest over-the-top attack to get most DNS administrators to sit up and notice. Things aren't great right now for Cloudflare or Spamhaus, but open recursive servers are finally starting to close. What follows is nerd detail on the problem.

My server

The server in question is what I term an "edge scrubber." The system itself is nothing particularly special. It is an Intel Atom D510 with 2GB of RAM, 2 1Gbit Intel NICs running CentOS 5.9 with a real time kernel.

The scrubber sits on the very edge of my network and does what most Cisco networking folks would use a router for. It has an external IP address given to me by my ISP and a gateway on the ISP's network.. I have a /27 block of IP addresses assigned me by my ISP and this device hands those out to servers and routers within the datacenter.

The scrubber also serves various "scut work" functions on behalf of all the other devices on the network. It is the datacenter-local network time server, external DNS server, IDS, edge firewall and bandwidth limiter. Every single packet in or out of the datacenter passes through this box; it handles 30Mbit symmetrical with some heavy deep packet inspection just fine. 45 Mbit seems to be the maximum it will do reliably.

The bad DNS configuration

The DNS server in question was one I had gotten halfway through setting up to "scrub" bad addresses. I hadn't tested the setup as it had been copied directly from a testlab system but did not have any production servers yet pointed to it.

The goal is to use the malwaredomains DNS blackhole list to make sure that nothing in the datacenter can access the worst of the known "bad guy" websites out there. Cutting off DNS access to these sites effectively neuters a whole host of potential browser-based attacks and even helps thwart any botnets that might take hold. The config consists of three files:

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