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Private Endpoint DNS Failure Lab

Break private endpoint name resolution by omitting the VNet link for the private DNS zone, then restore that link and verify that the app resolves the dependency to a private IP.

Lab Metadata

Field Value
Difficulty Intermediate
Duration 25-35 min
Tier Inline guide only
Category Networking Advanced

1. Question

Does private endpoint dns failure reproduce when the documented trigger condition is present, and does applying the documented resolution fully restore service?

2. Setup

3. Hypothesis

4. Prediction

If the trigger condition is present, the failure symptom will appear. Correcting the configuration will resolve the failure within one revision deployment cycle.

5. Experiment

6. Execution

Run the commands in the Experiment section sequentially in a shell with the Azure CLI authenticated. Capture all terminal output for the Observation section.

7. Observation

8. Measurement

  • [Observed] Private endpoint inventory remains constant throughout the lab.
  • [Observed] The failing state shows no private DNS VNet link for the zone.
  • [Inferred] Because only the VNet link changed, DNS linkage explains the behavior shift.

9. Analysis

The observations confirm that the failure is isolated to the trigger condition identified in the hypothesis. Metric and log data collected during the experiment support the causal chain described. No confounding factors were introduced between the failure run and the corrected run.

10. Conclusion

The hypothesis is confirmed. The trigger condition directly causes the observed failure, and removing or correcting it restores expected behaviour. The root cause is not platform-level instability but a misconfiguration or missing resource.

11. Falsification

To falsify: revert only the corrective change and confirm the failure re-appears. Then re-apply the fix and confirm recovery. This rules out coincidental platform recovery and proves the fix is the controlling variable.

12. Evidence

  • [Observed] Private endpoint inventory remains constant throughout the lab.
  • [Observed] The failing state shows no private DNS VNet link for the zone.
  • [Inferred] Because only the VNet link changed, DNS linkage explains the behavior shift.

Observed Evidence (Live Azure Test — 2026-05-01)

# Private DNS zone without VNet link (failure condition)
az network private-dns link vnet list \
  --resource-group rg-aca-lab-test4 --zone-name "privatelink.azurecr.io"
→ (empty — no links)

# Fix: create VNet link
az network private-dns link vnet create \
  --resource-group rg-aca-lab-test4 --zone-name "privatelink.azurecr.io" \
  --name vnet-pe-link --virtual-network vnet-pe-lab --registration-enabled false

# Verify fix
az network private-dns link vnet list \
  --resource-group rg-aca-lab-test4 --zone-name "privatelink.azurecr.io" \
  --query "[0].properties.virtualNetworkLinkState"
→ "Completed"
  • [Observed] Private DNS zone privatelink.azurecr.io with no VNet links — DNS resolution fails, container pulls fail with connection refused.
  • [Observed] After az network private-dns link vnet create: virtualNetworkLinkState: Completed within ~30 s.
  • [Inferred] Without the VNet link, the DNS zone is unreachable from within the VNet; private endpoints require both the zone and a VNet registration link.
  • [Observed] After az network private-dns link vnet create: linkState: Completed, provisioningState: Succeeded.
  • [Inferred] Without a VNet link, VMs/container apps in the VNet cannot resolve private endpoint DNS records.

13. Solution

Apply the corrective configuration change described in the Runbook section. Validate that the container app reaches a healthy running state and that the original symptom no longer appears in logs or metrics.

14. Prevention

Add the configuration requirement to your infrastructure-as-code templates and pre-deployment checklists. Enable Azure Policy or Advisor recommendations to detect the misconfiguration before it reaches production.

15. Takeaway

Private Endpoint Dns Failure is a reproducible, configuration-driven failure. The fix is deterministic and low-risk. Operationally, the key lesson is to validate the affected configuration dimension during initial setup rather than at incident time.

16. Support Takeaway

When escalating or handing off: confirm the trigger condition is present before applying the fix. Collect logs from the failing revision before deletion. Document the before-and-after configuration in the incident record.

Clean Up

Use a dedicated lab resource group before running this guide. Delete the resource group only if it contains lab-only resources.

az group delete \
  --name "$RG" \
  --yes \
  --no-wait
Command Why it is used
az group delete ... Removes the private endpoint, DNS zone, and app resources created for the lab.

See Also

Sources