There was bit of a duel (or a full-scale war) between Spamhaus and Cyberbunker. Spamhaus has published some information about their side here. All this dates back to 2011 when they first collided. The latest skirmish started with Spamhaus on the losing side, as allegedly Cyberbunker had some help with a botnet creating tons of DNS-requests to Spamnaus' servers. Cyberbunker naturally denies all this, but in reality they are simply coward liars! Who else would have the motivation to knock Spamhaus off the net. No need to think that any longer.
Since the enemy had some help, what happened next was Spamhaus joining forces with Cloudflare, a company specializing in mitigating the effects of a DDoS-attack. What happened at the end of March 2013 has been described as "The DDoS That Almost Broke the Internet" by Cloudflare blog.
The spam-blocking service Spamhaus is providing technically works on top of DNS. Anybody running a receiving mail-server can configure it to confirm the connecting client's IP-address with a simple DNS-query returning funny-but-pre-determined names as an answer to determine the "spamminess" level of connecting client. The judgement who is a spammer and who is not is made solely by the Spamhaus. That's what the dispute between them and Cyberbunker is all about.
As described by Cloudflare, technically Cyberbunker's (alleged) DDoS works by amplifying incoming 36 UDP-bytes containing a valid query for RIPE.net's zone into 100-fold. There are at least 30.000 open DNS-servers responding to recursive queries. All they have to do is spoof the original UDP-packet's sender's IP into Spamhaus and they have harnessed a huge Internet traffic amplification machine targeting a single IP-address.
Since I myself am running a couple of DNS-boxes, I wanted to re-verify my servers, that they cannot be used into such activity. I googled some and found The Measurement Factory's Open resolver test. That appears to be a piece of crap. You punch in an IP-address and get open/closed status as a response. You can enter any invented IP-address to get the closed-verdict. WTF?!
The second thing I found is much convincing: Open DNS Resolver Project. The problem with that one is, that they just browse The Net and try to find open DNS-servers. For example my boxes were not listed. Not as open, closed or existing. They don't publish information about properly configured DNS-servers. It still leaves the original question unanswered: Can my DNS-server be used for attacking innocent or not.
It caches the result of any query for 24 hours, and cannot be used for bullying somebody. That feature I simply stole copied from The Measurement Factory. Its fully written in Perl and even the source code is available for you to get.
Initial feedback after putting the thing on-line was to support FQDNs. The answer is NO. My thing won't do any unnecessary DNS-requests, if possible. But if you have any other suggestions, please drop a comment.