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Network L6 NETWORK-036 · 24 min

Path MTU black hole appears because fragmentation-needed ICMP is filtered

Small requests succeed while large responses hang, because the firewall blocks the control message needed for PMTUD to adjust the path.

NetworkFoundation OpsLevel 6Free24 min
Scenario

Small requests succeed while large responses hang, because the firewall blocks the control message needed for PMTUD to adjust the path.

What to check first
  • Identify the primary failure signal in the Transport Path Analysis scenario.
  • Separate visible symptoms from the underlying technical dependency.
  • Describe the safest recovery path and the follow-up prevention work.
Checking checklist
  1. Summarize the current impact and the last known change.
  2. Collect direct evidence from logs, runtime state, and configuration before changing anything.
  3. Separate immediate recovery from permanent prevention work.
Recovery and prevention

Choose the smallest safe recovery action first, then record the prevention work that reduces repeat incidents.

Questions worth viewing together
What makes PMTUD failures hard to spot quickly?

Only larger payloads fail, so the path can look healthy until the missing fragmentation-needed signal matters.