Service providers everywhere are executing on IPv6 transition strategies, some with more urgency than others. Numerous approaches to enable the transition are being implemented, with a goal of maximizing the utility of IPv4 addresses while ensuring 100% connectivity to the small but rapidly growing base of IPv6 addressed hosts. Regardless of technologies being deployed it’s important not to overlook the DNS since new stresses will be placed on it during the transition. Since every service provider has allocated budget for IPv6 readiness, now’s a great time to ensure the DNS is really “ready”. A couple of simple steps will ensure customers continue to enjoy fast response times and high service levels.
One of the stresses that will be imposed on the DNS during the transition to IPv6 is an increase in query volume. In fact it’s already happening. The default behavior of MacOS X is to request both A and AAAA records even when the clients making the queries aren’t provisioned on an IPv6 network! In many cases it is believed to be a major source of IPv6 DNS traffic, measurements show the query volume approximates MacOS X adoption.
Windows 7 and Vista will also query for both AAAA and A records if the OS sees a publicly routable (non-local link) IPv6 address configured. As more and more clients are transitioned to IPv6, operating system behaviors like this will cause query volumes to grow rapidly since from the standpoint of the DNS, adding a new IPv6 address will be almost equivalent to adding a new host. Because these operating systems dominate the market the aggregate effect will be a noticeable bump in query volume.
Since the shelves at IANA are now bare, Service Providers have to accept the fact that IPv4 addresses are officially scarce and thus have tangible value; they’re no longer “free”. There may come a day when dual stack deployments for new subscribers will not be economical, especially for consumer services, due to the “cost” of an IPv4 address. This is causing network operators to consider technologies such as DNS64/NAT64 to preserve precious IPv4 addresses. However this also results in a corresponding increase in DNS queries, since IPv6 hosts will query a caching server for a AAAA record and when it does not exist (often for now) the caching server will re-query for an A record. Bottom line: regardless of which transition technologies predominate DNS query volumes will increase.
DNS queries for both A and AAAA records could continue long into the future, essentially until the last IPv4 addresses are retired – which could be a long time. It’s possible when a significant majority of web content is migrated to IPv6 operating systems could be modified again to be biased toward IPv6 (only issuing AAAA queries unless an IPv6 record is not found) but it appears as though we are a ways away from that!
The good news is ensuring critical systems like the DNS are truly ready for the transition to IPv6 is straightforward. The most important task is ensuring the DNS infrastructure is dimensioned to account for the additional load. The primary variable that needs to be considered is processor utilization, with Best Practices calling for average utilization of around 20%. This provides headroom in the event of a DoS attack, or other network event, that spikes query volume (DoS attacks have been measured that increase query volume as much as 800%).