Some Country for Old Men

By Yorgos Thessalonikefs

In March 2020, RFC 8767 was published that specifies when and how a resolver can serve stale data from its cache. Normally, that would break the contract between an authoritative name server and a caching resolver on the amount of time a record is permitted to be cached. The now updated TTL definition in section 4 of the RFC states that:

If the data is unable to be authoritatively refreshed when the TTL expires, the record MAY be used as though it is unexpired.

Serving expired records is not a novel idea and it was already present in various forms (e.g., increased cache-hit ratio, fallback when upstream is not reachable) in various resolvers. Unbound’s own form was called serve-expired and its main purpose was to increase the cache-hit ratio.

As the RFC landed in the standards track, Unbound gained support for it but still kept the original serve-expired logic. Certain aspects of the RFC, such as timers, were already present in Unbound and their functionality is shared by both modes of operation.

The following sections try to clarify the differences between serve-expired and RFC 8767 (serve-stale) and give some insight into when one may be preferable over the other. I will refrain from using the RFC term serve-stale in order to avoid any confusion between the terms and the configuration options later on.

serve-expired

Since version 1.6.0, Unbound has the ability to answer with expired records. Before trying to resolve, Unbound will also consider expired cached records as possible answers. If such a record is found it is immediately returned to the client (cache response speed!). But contrary to normal cache replies, Unbound continues resolving and hopefully updating the cached record.

The immediate downside is obvious: the expired answers rely heavily on the cache state. Unbound already has the tools to try and tip the scales in its favor with the prefetch and serve-expired-ttl options.

With prefetch, Unbound tries to update a cached record (after first replying to the client) when the current TTL is within 10% of the original TTL value. The logic is similar to serve-expired: if a cached record is found and the record is within 10% of the TTL, it is returned to the client but Unbound continues resolving in order to update the record. Although prefetching comes with a small penalty of ~10% in traffic and load from the extra upstream queries, the cache is kept up-to-date, at least for popular queries.

Rare queries have the inescapable fate of having their records expired past any meaningful time. The option serve-expired-ttl limits the amount of time an expired record is supposed to be served. A suggested value per RFC is between one and three days.

A note on the expired reply’s TTL value: prior to the RFC, Unbound was using TTL 0 in order to signal that the expired record is only meant to be used for this DNS transaction and not to be cached by the client. The RFC now RECOMMENDS a value of 30 to be returned to the client.

A simple configuration for the primal serve-expired behavior could then be:

server:
  prefetch: yes
  serve-expired: yes
  serve-expired-ttl: 86400  # one day, in seconds

This will allow Unbound to:

• prioritize (expired) cached replies,
• keep the cache fairly up-to-date,
• in the likelihood that an expired record needs to be served (e.g., rare query, issue with upstream resolving), make sure that the record is not older than the specified limit.

RFC 8767

Starting with version 1.10.0 (1.11.0 really, as 1.10.0 contains a bug when the RFC behavior is activated), Unbound also supports serving expired records following the RFC guidelines.

The RFC behavior is mainly focused on returning expired answers as fallback for normal resolution. The new option to control that is serve-expired-client-timeout and setting it to a value greater than 0 enables the RFC behavior.

With the value set, Unbound has a limit on how much time it can spend resolving a client query. When that limit is passed, Unbound pauses resolution and checks if there are any expired records in the cache that can answer the initial query. If that is the case, Unbound answers with the expired record before resuming resolution. The result of the resolution will be used to update the cache if possible.

Similar to the client timeout, Unbound will also try and use expired answers instead of returning SERVFAIL to the client where possible.

A simple configuration for the RFC behavior could then be:

server:
  serve-expired: yes
  serve-expired-ttl: 86400            # one day, in seconds
  serve-expired-client-timeout: 1800  # 1.8 seconds, in milliseconds

This will allow Unbound to use expired answers only as fallback from normal resolving:

• when 1.8 seconds have passed since the client made the query,
• instead of returning SERVFAIL;
• in the likelihood that an expired record needs to be served (e.g., issue with upstream resolving), make sure that the record is not older than the specified limit.

Conclusion

Unbound offers two distinct modes for serving expired records.

The safest approach is to use the RFC behavior where expired records are used as a fallback to availability, network or configuration errors. This will serve expired records as a last resort instead of returning SERVFAIL or the client giving up.

If more client-side performance is required, the default original serve-expired behavior can keep the cache-hit ratio higher. Using it together with the prefetch option is highly recommended in order to try and keep an updated cache.

In all cases make sure to consult the man page of your installed Unbound for defaults and suggested values. And always remember that serving expired records should be approached with caution; you may be directing your clients to places long gone.