Validation and Resolution Steps
To diagnose and resolve issues, sign in to the backend VM and check if the application is listening on the data port using netstat -an. Then, list the network security groups configured on the backend VM and ensure they allow incoming and outgoing traffic on the data port. Make sure the load balancer IP (168.63.129.16) is allowed through any NSG rules, because blocking this IP will cause all health probes to fail and stop traffic from reaching the backend.
Investigate Traffic Flow and Diagnostic Insights
Azure Monitor provides multi-dimensional metrics that give real-time visibility into load balancer operations. Critical metrics include Data Path Availability, which measures the health of load-balancing rules' frontend IP and port combinations, and Health Probe Status, which indicates backend instance health. SYN Count metrics help detect potential security threats by tracking TCP connection attempts, while SNAT Connection Count and Used SNAT Ports metrics reveal outbound connectivity issues such as port exhaustion that leads to failed connections.
Analyzing Traffic Patterns and Anomalies
VNet flow logs are essential for analyzing traffic patterns flowing through the load balancer, helping identify anomalous behavior or security threats. By enabling these logs, you can trace packet paths and detect issues like uneven traffic distribution, often caused by misconfigured session persistence such as source IP affinity, or by rule configurations. Additionally, configure diagnostic settings to send logs to Azure Monitor, Storage, or Event Hubs for comprehensive analysis and alerting.
Troubleshooting Common Issues
When investigating dropped traffic or health probe failures, start by verifying backend instance health via health probes. Ensure network security groups (NSGs) allow traffic from the Azure infrastructure IP (168.63.129.16) for probes. For SNAT exhaustion, monitor the Used SNAT Ports metrics and consider using Azure NAT Gateway for more predictable outbound connectivity. If uneven distribution is suspected, review the distribution mode—5-tuple hash versus session persistence—and adjust rules to balance load effectively.
Advanced Insights and Best Practices
Leverage Load Balancer Insights for preconfigured dashboards that visualize flow distribution, dependency views, and connection monitors. Set up Azure Monitor alerts for metrics like Data Path Availability dropping below thresholds or Health Probe Status indicating unhealthy instances. For robust security, integrate with Azure Firewall for traffic inspection and use TCP reset on idle timeouts to ensure clear connection state information. Regularly review Azure Policy controls and resource tagging to maintain compliance and governance across deployments.
Assess Health Probe and Backend Pool Functionality
Health Probe Configuration and Behavior
Health probes are essential mechanisms used by Azure Load Balancer to determine the health status of instances within a backend pool. These probes periodically send requests to backend instances using configured protocols (TCP, HTTP, or HTTPS) and ports. An instance is marked as healthy only if it responds correctly, ensuring that new traffic is directed solely to operational instances. Properly configured health probes prevent connection attempts to failed or unresponsive services, maintaining application availability. The probe protocol must align with the application's listener; for example, an HTTP probe requires the backend to return an HTTP 200 status, whereas a TCP probe only requires a successful connection. The probe port must match the port where the application is actively listening, which can be confirmed using commands like netstat -an on the backend VM.
Troubleshooting Backend Pool Connectivity
When backend instances are not receiving traffic, the issue often lies with misconfigured network security rules or application-level failures. NSGs must permit traffic from the load balancer's IP and the health probe port. A common mistake is having a deny rule with higher priority than the default allow rule for Azure Load Balancer. Additionally, the application itself must be running and listening on the expected port. High CPU utilization on a backend VM can also prevent it from responding to probes in a timely manner, causing it to be marked unhealthy.
Analyzing Advanced Failure Scenarios
For persistent issues, more advanced diagnostics are required. Tools like Psping or TCPing can be used from another VM within the same virtual network to test connectivity to the probe port, helping to isolate whether the problem is network-related or specific to the target VM. Simultaneous network traces (using netsh on Windows) on both the backend VM and a test VM can capture probe packets. If incoming packets are absent on the backend, the cause is likely an NSG or user-defined route (UDR) misconfiguration. If outgoing packets are missing, the issue is likely on the VM itself, such as an application or OS firewall blocking the response.
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