This was sent to me by a friend is an interesting read about a recent massive DDoS (distributed denial-of-service) attack that was aimed at Spamhaus.
The attack was a type of DNS-recursion amplification attack [pdf] that uses bogus queries to DNS servers to massively amplify the amount of bandwidth consumed by the attackers to over 300Gbps at times.
Cloudflare have another great explanation of this type of attack here.

The website at http://openresolverproject.org/ can be used to help you identify if you have DNS servers configured to allow recursion, and provides some configuration suggestions (such as rate limiting)
.