Originally Posted by KRosh

Open DNS resolvers are recursive DNS servers that are configured to accept queries from any computers on the Internet. These act as relays between users and authoritative DNS servers; they receive queries for ANY domain name, find the authoritative name server responsible for it and relay the information received from that server back to the user.

Authoritative name servers, such as Dyn, will only respond to queries concerning the domain names they serve.

The attackers would have to do a lot of homework and know exactly what domains are hosted by Dyn to launch their attack, making it much harder...of course not impossible.