Troubleshooting Common Issues in Azure Virtual Networks: A Practical Problem-Solving Guide

Azure Virtual Networks (VNets) are integral to cloud deployments, enabling resources like virtual machines (VMs) to communicate securely within and outside Azure. However, network connectivity issues can arise due to misconfigurations or unforeseen challenges. This guide presents a problem-solving approach for diagnosing and resolving common Azure VNet issues with a practical use case.

Use Case: Diagnosing VM Connectivity Issues in an Azure VNet

Imagine a scenario where a user has deployed two virtual machines (VM1 and VM2) within the same Azure VNet, but they cannot communicate with each other. This connectivity issue could be caused by several factors, such as incorrect subnet configurations, blocked ports, or misconfigured Network Security Groups (NSGs).

Step-by-Step Problem Solving Approach:

1. Verify Subnet Configuration:
   • Ensure that both VMs are located in the same VNet and either within the same subnet or different subnets that can route traffic between them. Subnets need to be properly associated with route tables that allow intra-VNet traffic.
   • If the VMs are in different subnets, confirm that their Network Security Groups (NSGs) do not block communication between the subnets.
2. Check NSG Rules:
   • NSGs may be preventing communication between VMs. Check the inbound and outbound rules of the NSG associated with each VM. Ensure that the required ports (e.g., port 3389 for RDP or port 22 for SSH) are allowed.
   • Use Azure Network Watcher to view NSG flow logs and identify any blocked traffic. If the logs show that traffic is being denied, adjust the NSG rules accordingly.
3. DNS Configuration Issues:
   • If custom DNS is in use, confirm that the DNS server is reachable and the settings are correctly configured for the VNet. DNS resolution failure can prevent VMs from locating each other via hostnames.
4. Ping and Test Connectivity:
   • From VM1, ping VM2 to test basic connectivity. If the ping fails, troubleshoot further by using Azure Network Watcher’s connectivity check to diagnose the problem.
5. Check the VM’s Serial Console:
   • Sometimes the issue might be more internal, such as VM OS-level issues. Azure provides a Serial Console for troubleshooting boot and network configuration issues at the VM level. Watch this tutorial, “Troubleshoot and diagnose Azure Virtual Machines with Serial Console | Azure Friday”, to learn how to use the serial console for diagnosing connectivity issues inside a VM.

By following these steps, the user should be able to resolve most basic network connectivity issues between Azure VMs.

Common Issues and Resolutions in Azure VNets

Once the fundamental connectivity issue is addressed, other network-related problems might arise in your Azure VNet. Here are the most common issues and their solutions:

1. VNet Peering Connectivity Issues

Problem: Traffic between peered VNets is not flowing as expected. Solution:

• Ensure VNet peering is properly set up on both VNets.
• Verify that the peered VNets do not have overlapping IP address spaces. Overlapping addresses can cause routing failures.
• Check NSG and route table configurations to ensure they allow traffic between the peered VNets.

2. VPN Gateway Connection Issues

Problem: The VPN connection between an on-premises network and Azure VNet is unstable or non-functional. Solution:

• Ensure that there is no IP address overlap between the Azure VNet and the on-premises network.
• Check that the VPN Gateway configuration, including IPsec/IKE settings, matches on both sides.
• Use Azure Network Watcher’s VPN troubleshooting tools to identify and fix connection issues.

3. DNS Resolution Problems

Problem: VMs or resources within the VNet are unable to resolve domain names. Solution:

• Verify that DNS settings in the VNet are correctly configured to use either Azure’s default DNS or custom DNS servers.
• If DNS forwarding is used, ensure that necessary NSG and route table rules allow traffic between the Azure VNet and the DNS server.

4. Performance Issues (Latency and Packet Loss)

Problem: VMs experience poor performance due to high latency or packet loss. Solution:

• Use Azure Network Watcher to monitor traffic flow and identify bottlenecks or packet loss between VMs.
• Ensure the VM’s network adapter and VNet Gateway are correctly sized and configured to handle expected traffic volumes.
• Check if NSG rules or route table configurations are introducing unnecessary latency.

5. Misconfigured Load Balancers

Problem: Traffic is not distributed correctly across backend VMs. Solution:

• Ensure that the health probes and load-balancing rules for the Azure Load Balancer are correctly configured.
• Verify that backend VMs are functioning and able to respond to the health probes.
• Adjust load balancer timeout settings if connections are dropped prematurely.

Conclusion

By following the troubleshooting steps provided in this guide, Azure users can effectively diagnose and resolve common issues with virtual networks. Whether it’s ensuring that NSG rules are correctly configured, resolving peering issues, or utilizing the Azure Serial Console for deeper VM troubleshooting, the solutions presented here will help optimize network connectivity within Azure VNets.

If you’re still facing challenges, watch this helpful Azure Friday video for more advanced troubleshooting techniques using the Serial Console.

Have you resolved your Azure VNet issue, or are you encountering something different? Share your story or reach out to the community for further support and troubleshooting tips.

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