Azure VM Connectivity Troubleshooting: NSG, Routes and TCP Tests

Define the failed path

Write down source, destination, protocol and port. A failed ping and a failed TCP connection are different symptoms.

Check DNS

Confirm that the source resolves the destination name to the expected private or public address. Test the resolved address directly when possible.

Check NSGs on the network interfaces

Review inbound and outbound NSG rules attached to the relevant network interface and subnet. Look for a lower-priority deny that blocks the intended flow.

Check routes

Use Network Watcher connectivity checks and review user-defined routes when traffic is sent through a network virtual appliance or another next hop.

Test the actual port

From Windows, use Test-NetConnection destination -Port 443 or the application port. This gives a more useful signal than ICMP alone.

Validate the application

If TCP connectivity succeeds but the application still fails, inspect the listener, certificate, application configuration and backend dependencies.

Useful commands

Test-NetConnection <destination> -Port 443
Resolve-DnsName <destination>
ipconfig /all

Production checklist: capture the original state, test one dependency at a time, make the smallest controlled change and validate the original symptom before closing the incident.

What good troubleshooting looks like

Use evidence before configuration changes. Record the symptom, test result, change made and validation result so another engineer can repeat the procedure.

Workflow
Symptom → hypothesis → direct test → controlled change → validation → documentation

Frequently asked questions

What should I check first?

Start with the exact failed path or dependency and test it directly before changing configuration.

Should I change production configuration immediately?

No. Capture the current state first and make one controlled change at a time.

How should I document the fix?

Record the symptom, commands, result, configuration change and validation result.

Related TechRunbook guides

Primary reference

This guide was independently written for TechRunbook. Technical details were checked against current Microsoft documentation. Microsoft Learn reference →

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