Timeouts After Migration: DNS, MTU, and Firewall Checks Most Teams Miss
A migration may finish successfully yet users still see timeouts. This checklist covers the hidden post-cutover traps.
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Timeouts After Migration: DNS, MTU, and Firewall Checks Most Teams Miss
Many migrations fail after the “success” announcement. Services appear online, but users report intermittent timeouts or partial failures.
The cause is often not application logic. It is path inconsistency between DNS, routing, MTU, and firewall policy.
DNS checks
After cutover:
- verify authoritative records, not just local resolver output
- confirm TTL behavior during transition window
- check for stale AAAA/A records across major public resolvers
A subset of users hitting old endpoints creates confusing “works for me” incidents.
MTU checks
Tunnels, overlays, and provider differences can introduce MTU mismatch.
Symptoms:
- random timeouts on larger payloads
- protocol-specific failures
- retransmit spikes without obvious host saturation
Validate path MTU end-to-end if network topology changed.
Firewall checks
Post-migration firewalls often drift:
- old allowlists not carried over
- edge source ranges updated but origin rules stale
- ephemeral ports unexpectedly blocked for upstream dependencies
Run explicit rule verification against real source paths.
Application adjacency checks
Also verify:
- callback/webhook source IP assumptions
- outbound egress ACLs for third-party APIs
- health checks from actual edge locations
An app that starts successfully can still fail in cross-service traffic.
Communication discipline
In the first 24 hours after migration:
- publish increased monitoring window
- define rollback criteria
- keep a single incident owner for post-cutover issues
Most migration pain comes from fragmented responsibility.
Final takeaway
Migration timeouts are typically integration-path failures, not random bad luck. DNS consistency, MTU sanity, and firewall truth checks should be mandatory post-cutover steps in every VPS migration runbook.