Question 420
A Standard Azure Load Balancer probes TCP port 8080 on backend VMs. The service listens on 8080 and responds locally, and the probe protocol and port are correct. Effective NSG rules show a priority-100 Deny matching probe source 168.63.129.16 to TCP 8080, ahead of the default AzureLoadBalancer allow. What must be corrected?
Explanation
Correct Answer
Azure Load Balancer probes originate from 168.63.129.16, represented in NSGs by the AzureLoadBalancer service tag. The default NSG allow for probe traffic can be overridden by a custom rule evaluated first, such as the scenario's priority-100 matching Deny. A narrow corrective allow or removal/adjustment of the incorrect deny must permit probe traffic to the known TCP 8080 listener without broadly opening application access. Then verify the backend's observed health-probe status and the application's own listener/firewall behavior. This explanation identifies an actual verified blocked five-tuple rather than guessing among several possible probe failures.
Incorrect Answers
An HTTP path is meaningful for HTTP or HTTPS application-layer probes, but this health check uses raw TCP on port 8080. There is no URL path to rewrite. The service listener and the configured TCP probe protocol and port have already been confirmed, so a path mismatch cannot explain the failure. The evidence identifies a priority-100 network security group deny that matches the Azure Load Balancer probe source address. Fix that specific rule under change control rather than altering an unrelated HTTP endpoint or continuing to probe through a blocked network path.
A load-balancing rule maps client frontend traffic to backend destinations; it is different from the health-probe connection that marks a backend eligible. This case states the health probe itself uses the correct TCP 8080 port but is blocked by an effective NSG deny for its source IP. Changing only the frontend-to-backend port rule cannot bypass the NSG rule that denies the probe. A port mapping adjustment might fix client traffic in another incident, but the observed failure here requires allowing the probe's specific source and destination tuple.
Moving the probe to a known closed port makes reachability worse, and increasing a failure threshold does not turn denied packets into successful health checks. The backend listens on TCP 8080; the existing probe uses that port correctly. Its failure comes from a high-priority matching NSG rule, not transient application restarts or a noisy probe interval. Correct the effective network security rule while retaining a supported probe that tests the actual listener, then validate the health status after NSG propagation and application availability checks.






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