1 / 4 DNS, Anycast & Global Routing

The Old Number Lingers

An A record has been published with a TTL of 86400 for years. Two hours before a planned IP migration you lower the TTL to 60, then cut over. For most of the following day a slice of users still lands on the old address. Why?

A

TTL applies only to negative answers; positive A record answers are cached for the interval in the zone's SOA minimum field, which was still set to one day

B

Resolvers that cached the record before the change keep it for up to the original 86400 seconds, so the short TTL governs only later lookups

C

TTL values are advisory, so recursive resolvers substitute their own fixed caching interval and the published number has no bearing on how long an answer is held

D

A TTL change does not reach resolvers until the SOA serial is incremented and every secondary nameserver completes a zone transfer, which is what actually took the extra day