DNS cold start exposes resolver dependency risks

DNS resolvers face a fundamental challenge when their caches are empty, a scenario known as a cold start. This issue, recently examined in an APNIC podcast, mirrors the complexities of restarting power grids after a blackout, where initial dependencies create circular problems. Without cached data, resolvers must perform multiple lookups to locate authoritative nameservers, leading to increased latency and query volume across the internet infrastructure.

How cold starts disrupt DNS resolution

A cold start occurs when a resolver lacks prior query data, forcing it to build responses from scratch. Typically, resolvers rely on cached information to avoid repeated network requests. For example, resolving a domain like www.apnic.net may require querying multiple hierarchical nameservers, each responsible for a segment of the domain. When no cached data exists, the resolver must first locate the nameservers for .net, then for apnic.net, and finally for www.apnic.net.

The circular dependency emerges when the resolver attempts to find the nameservers for .net but discovers they are hosted under .net itself (e.g., a.net and b.net). Without prior knowledge of these nameservers' IP addresses, the resolver cannot proceed, creating a deadlock. This problem is not theoretical—it affects real-world deployments, particularly during system restarts or when new resolvers are deployed without preloaded cache data.

Infrastructure and operator choices

Geoff Huston, APNIC’s Chief Scientist, discussed how operator decisions influence cold-start behavior. The design of DNS infrastructure, including the placement of nameservers and the use of intermediary services, can exacerbate or mitigate the issue. For instance, some operators rely on third-party DNS providers or load balancers, which may introduce additional dependencies. These intermediaries can increase the number of queries required to resolve a domain, particularly during cold starts.

Huston’s analysis, prompted by a presentation from Ondřej Surý at the RIPE 92 DNS Working Group, highlights how cold starts reveal inefficiencies in DNS design. Operators who distribute nameservers across multiple domains or use complex referral chains may inadvertently increase the query load on resolvers. Conversely, those who preload cache data or use simpler delegation models can reduce the impact of cold starts.

Global implications and mitigation strategies

The cold-start problem extends beyond individual resolvers. When multiple resolvers experience cold starts simultaneously—such as after a widespread outage—the cumulative query load can strain authoritative nameservers globally. This effect is amplified by the growing use of DNS-based security mechanisms, such as DNSSEC, which require additional lookups to verify responses.

Mitigation strategies include preloading cache data during resolver initialization, using anycast routing to distribute query loads, and minimizing circular dependencies in nameserver delegation. Operators can also monitor cold-start behavior to identify inefficiencies in their DNS infrastructure. However, these solutions require coordination across multiple stakeholders, including registries, registrars, and DNS service providers.

What to watch

As DNS continues to evolve, the cold-start problem may become more pronounced with the adoption of new protocols and security mechanisms. Operators should prioritize designs that minimize dependencies and reduce the risk of circular lookups. Research into automated cache preloading and dynamic delegation models could also provide long-term solutions.