Long has been the list of failures in ISPs and carriers to force borken DNS servers on their customers, thereby manipulating their customers traffic, or outright censoring what their customers can see. To combat such manipulations, and also to make it harder to observe their customers' behaviour, it has been a pet project for some, also for me at some time, to run an open resolver, that allows random people on the Internet to query your DNS server for an arbitrary name. Unfortunately, the evil guys developed an attack  that makes it impractical to run an open resolver. So, while politically desirable, it is unfeasible to run an open resolver, and network operators around the globe strive for shutting them down.
Now, these attacks all rely on the simple fact that, with UDP, you do not have any kind of assurance that the source address in a packet in fact belongs to the sending host. In my opinion, if you are willing to take the effort, there is one obvious way to provide an open resolver that does not have this flaw: For hosts not on your own network, provide DNS over TCP only.
I hope that someone will hack this feature into unbound , so people can easily deploy open resolvers in a reasonably safe way, without disrupting the Internet. Currently, unbound's do-udp setting is only a combined setting for incoming and outgoing queries, causing upstream name servers excessive load.
Thank you for reading!
|||See eg. http://openresolverproject.org/|