In this episode of PING, APNIC’s Chief Scientist, Geoff Huston, discusses the surprisingly vexed question of how to say ‘no’ in the DNS. This conversation follows a presentation by Shumon Huque at the recent DNS OARC meeting, who will be on PING in a future episode talking about another aspect of the DNS protocol.
You would hope this is a simple, straightforward answer to a question, but as usual with the DNS, there are more complexities under the surface. The DNS must indicate whether the labels in the requested name do not exist, whether the specific record type is missing, or both. Sometimes, it needs to state both pieces of information, while other times, it only needs to state one.
The problem is made worse by the constraints of signing answers with DNSSEC. There needs to be a way to say ‘no’ authoritatively, and minimize the risk of leaking any other information.
NSEC3 records are designed to limit this exposure by making it harder to enumerate an entire zone. Instead of explicitly listing ‘before’ and ‘after’ labels in a signed response denying a label’s existence, NSEC3 uses hashed values to obscure them. In contrast, the simpler NSEC model reveals adjacent labels, allowing an attacker to systematically map out all existing names — a serious risk for domain registries that depend on name confidentiality. This is documented in RFC 7129.
Saying ‘no’ with authority also raises the question of where signing occurs — at the zone’s centre (by the zone holder) or at the edge (by the zone server). These approaches lead to different solutions, each with its own costs and consequences.
In this episode of PING, Geoff explores the differences between a non-standard, vendor-explored solution, and the emergence of a draft standard in how to say ‘no’ properly.