Clients Using Stale DNS Cache

Question

Canary rollout passed, but a portion of clients hit decommissioned IPs due to stale DNS caches. What do you do?

Accepted Answer

This is a critical issue that points to a gap in our release process, specifically around DNS propagation and client behavior. My immediate priority would be to diagnose the root cause while minimizing further impact and then implement preventative measures. **Confirm Stale Cache:** My first step would be to confirm the DNS cache issue. I'd check the Time-To-Live (TTL) values for the DNS records involved in the migration. I would also attempt to reproduce the issue by querying different client resolvers (e.g., using `dig` or `nslookup` with specific upstream DNS servers) to see if they are indeed returning old IP addresses. I'd also examine client-side logs if possible to see if they are reporting connection errors to decommissioned IPs. **Mitigate Impact:** To stop further clients from hitting decommissioned IPs, I would immediately revert the DNS changes back to the old IPs, understanding this will also take time to propagate. Simultaneously, I'd work with the infrastructure team to ensure the old IPs remain accessible for a grace period, even if they are no longer serving active traffic, to allow stale caches to resolve correctly. **Graceful Drain and Redirects:** If the old IPs are still reachable, the ideal solution would be to enable a graceful drain on them. This means configuring the old IP addresses to respond with HTTP 301 or 302 redirects to the new, active IPs for a defined period. This would automatically guide clients with stale caches to the correct endpoints without manual intervention or requiring them to clear their caches. **Lower TTLs & Overlapping Windows:** Moving forward, to prevent this, I would implement a policy of significantly lowering TTLs on DNS records for a period leading up to any IP migration or service change. This ensures that clients and intermediary DNS servers will refresh their records more frequently, reducing the window of time during which stale data can be served. Furthermore, I would aim to use overlapping IP windows where both old and new IPs are active and resolving for a period, facilitating a smoother transition. **Enhanced Monitoring & Alerting:** Post-incident, I would ensure we have robust monitoring in place. This would include synthetic monitoring that periodically queries our services using diverse DNS resolvers to detect inconsistencies. We should also set up alerts for increased error rates on decommissioned IP ranges or unusual DNS resolution patterns that could indicate a similar issue in the future. Resolving this requires a multi-pronged approach focusing on immediate containment, graceful handling of existing issues, and robust process improvements. By integrating DNS TTL management and overlapping IP strategies into our release engineering, we can build greater resilience against such propagation delays.

Clients Using Stale DNS Cache

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