Open Resolvers pose a significant threat to the global network infrastructure by answering recursive queries for hosts outside of its domain. They are utilized in DNS Amplification attacks and pose a similar threat as those from Smurf attacks commonly seen in the late 1990s.
We have collected a list of 32 million resolvers that respond to queries in some fashion. 28 million of these pose a significant threat (as of 27-OCT-2013). Detailed History and Breakdown
What can I do?
If you operate a DNS server, please check the settings.
Recursive servers should be restricted to your enterprise or customer IP ranges to prevent abuse. Directions on securing BIND and Microsoft nameservers can be found on the Team CYMRU Website - If you operate BIND, you can deploy the TCP-ANY patch
Authoritative servers should not offer recursion, but can still be used in an attack. Configure your Authoritative DNS servers to use DNS RRL [Response Rate Limiting] Knot DNS and NLNetLabs NSD include this as a standard option now. BIND requires a patch.
CPE DEVICES SHOULD NOT listen for DNS packets on the WAN interface, including NETWORK and BROADCAST addresses.
Prevent spoofing on your network!
Configure Source Address Validation/uRPF/BCP-38 on all CPE and Datacenter equipment edges that have fixed IP ranges. This could be as simple as setting ip verify unicast source reachable-via rx on a router interface. Any staticly routed customer should receive this setting by default.
If you are in the security community:
Please contact dns-scan /at/ puck.nether.net for access to raw data.
We can provide you a List of Open Resolvers by ASN if you e-mail dns-scan /at/ puck.nether.net