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¶

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¶

Prepare a dedicated lab resource group, set $RG, $LOCATION, $ACA_ENV_NAME, and $APP_NAME, and confirm Azure CLI authentication before running the scenario.

3. Hypothesis¶

The documented trigger condition is sufficient to reproduce the symptom, and removing only that condition should restore normal Azure Container Apps behavior.

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¶

Run the trigger steps from the runbook, capture system logs and relevant az containerapp output, then apply only the stated remediation before taking a second measurement.

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¶

Record before-and-after CLI output, ContainerAppSystemLogs or ConsoleLogs evidence, and any metrics that show the failure changing after the fix.

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 — CLI-only reproduction; no Portal captures yet)¶

# 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 remediation in the Runbook section for this lab, then verify the corrected Container Apps resource reaches a healthy state and 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

Related Playbook¶

• Private Endpoint DNS Failure

Page Flow¶

flowchart TD
    A["Private Endpoint DNS Failure Lab"]
    B["Lab Metadata"]
    C["1. Question"]
    D["2. Setup"]
    E["3. Hypothesis"]
    A --> B
    B --> C
    C --> D
    D --> E

See Also¶

• Private Endpoints
• Internal DNS and Private Endpoint Failure
• Deployment Networking Operations

Sources¶

• Azure private endpoint DNS configuration
• Networking in Azure Container Apps environment
• What is Azure Private DNS?