IPv6 Network - IPV6 AND THE ROOT SERVERS

For nameservers to resolve information over IPv6, it’s necessary that the root nameservers gain IPv6 support.Nameservers find the root DNS server addresses in a local “hints” file, so it would appear that providing the roots with IPv6 connectivity and updating the hints file would do the trick. Unfortunately, there are some complications.

Because hints files tend to get out of date, the first thing a nameserver does upon startup is ask one of the nameservers listed in the hints file for the current list of root nameservers. So, for nameservers to be able to communicate with the root nameservers over IPv6, the answer to this initial query must contain IPv6 addresses for at least a subset of all root servers.

In theory, it’s fairly trivial to add IPv6 addresses for the root nameservers as “glue” records in the root zone. The problem is that the original DNS specifications only allow for 512 byte DNS messages over UDP.

For the current list of 13 root servers, the initial response message is 436 bytes (and that’s after “label compression” to avoid repeating the same domain name for different servers), so there is no room to add IPv6 addresses for all root servers without potentially going over the limit. When that happens, some addresses must be dropped from the response, which often means the request must be repeated over TCP. Because many people are unaware that regular DNS queries and not just zone transfers can happen over TCP, this is often filtered in firewalls. RFC 2671 adds support for larger DNS messages through the “EDNS0” mechanism, but a sizeable minority of all nameservers on the Internet don’t support EDNS0.

Since mid-2004, TLD registries may have IPv6 addresses included in the root zone as glue records, and some TLDs allow end users to register IPv6 nameserver addresses for their domains. Many of the root nameservers are already reachable over IPv6 (see http://www.root-servers.org/). ICANN and the root server operators are proceeding very cautiously, but addition of IPv6 glue records to the root zone is expected in the not too distant future.