DNS Record Diff

Compare two DNS zone snapshots and report what the change does. Losing the last record of a type at a name stops something resolving once caches expire, and losing one value out of several does not, so the two are separated. A value change at one name is reported as a change rather than as an unrelated removal and addition. A TTL change is reported for its own reason: the old TTL governs how long the old answer survives, so a new one does not take effect until the old has expired.