When I hosted, occasionally people would try to connect, and their client would send lots of punch packets, and I'd see them in console, but they would never make it in. The client sends lots of them in a row before giving up. It only happened to some people. Something to do with our network configurations.
I'd like to point out the difference between what is usually meant by "distributed denial of service attack" and what this appears to me to be, "six people trying to connect."