Encountering a 502 Bad Gateway error while using Azure Application Gateway can be frustrating, particularly because it often points to a failure in the communication between Application Gateway and backend servers. It doesn’t necessarily mean that your backend is completely down, but it does suggest that something in the connection is misbehaving. The good news is that with a systematic troubleshooting approach, you can identify and resolve the problem quicker than you might think.
In this article, we will explore how to diagnose and resolve a 502 error in Azure Application Gateway using practical steps, Azure tools, and expert tips. Whether you’re a seasoned Azure user or just getting started, this guide will deepen your understanding and help keep your applications running smoothly.
Understanding the 502 Error in Azure Application Gateway
The 502 Bad Gateway error typically indicates that the Application Gateway was unable to receive a valid response from the backend server within the expected timeframe. Some common causes include:
- Health probe failures
- Incorrect backend configurations
- Firewall or network security group (NSG) blocks
- SSL/TLS protocol mismatches
- Timeouts or crashes on the backend server
Since multiple layers can influence the cause of the error, it’s critical to approach your troubleshooting in a structured manner. Let’s take a deep dive into the steps you should follow to resolve the issue.
Step-by-Step Troubleshooting Guide
1. Check Backend Pool Health
Start by checking the health of your backend pool. This is crucial because the Application Gateway depends on responses from backend servers to work correctly. To do this:
- Go to the Azure Portal.
- Navigate to your Application Gateway instance.
- Select “Backend health” under the Monitoring section.
In this view, you’ll see the health status of each backend server associated with your Application Gateway. Healthy servers will appear in green, while misconfigured or unreachable ones will appear in red or yellow.
If servers are marked unhealthy:
- Check if the server is running and accessible over the right port.
- Ensure that the backend server is responding to the Application Gateway’s health probe.
2. Verify Health Probe Configuration
Incorrect health probe configuration is a major contributor to 502 errors. You need to ensure that the probes are using the correct protocol, port, and path. Here are a few items to verify:
- Protocol: Does it match what your backend supports (HTTP/HTTPS)?
- Port: Are you probing the correct port that the backend is listening on?
- Path: Ensure the path you are probing returns a 200 OK response.
Use tools like curl or Postman to manually test the probe URL to verify its response. If it doesn’t return HTTP 200, your Application Gateway will continue to mark the backend as unhealthy.
3. Examine Backend HTTP Settings
Incorrect backend HTTP settings can also result in communication failures. From within Azure, navigate to the “HTTP settings” section of your Application Gateway configuration and confirm the following:
- Use the right port (80 for HTTP, 443 for HTTPS).
- Ensure “Pick host name from backend address” is appropriately selected based on your backend.
- If you’re using HTTPS, make sure the backend is configured with a valid SSL certificate.
You may also toggle the Use custom probe option if you need more control over how the Application Gateway verifies backend health.
4. Analyze Network Security Groups and Firewalls
Azure uses Network Security Groups (NSGs) to control traffic at the subnet and VM level. If the port you’re using is blocked by an NSG or firewall, the Application Gateway won’t be able to communicate with the backend.
Make sure that:
- The relevant ports are open for inbound communication on the backend VMs (e.g., ports 80 or 443).
- You have allowed traffic from the Application Gateway subnet to reach the backend IP addresses.
Use the “Effective security rules” in Azure Network Watcher to confirm the applied rules and identify any conflicting policies that might be blocking traffic.
5. Check SSL Certificate Trust
If your backend server uses HTTPS and SSL termination is enabled at the Application Gateway level, it needs to trust the backend’s certificate. Self-signed certificates, or those from untrusted authorities, are likely to trigger 502 errors.
To fix this:
- Ensure your backend uses a publicly trusted SSL certificate.
- Or, upload the root certificate to your Application Gateway’s HTTP settings section to establish trust.
Also, check if your backend enforces TLS versions that are not compatible with Azure Application Gateway (e.g., TLS 1.0 is often disabled for security reasons).
6. Enable Diagnostic Logs
Azure offers comprehensive diagnostic logs for Application Gateway. You can enable logs via:
- Azure Portal → Application Gateway → Diagnostics settings
- Select the log categories: ApplicationGatewayAccessLog, ApplicationGatewayPerformanceLog, and ApplicationGatewayFirewallLog (if WAF is enabled).
Export these logs to Azure Monitor, storage accounts, or Event Hub, depending on your needs. With this data, you can use Azure Log Analytics to query for 502 errors and analyze them in context.
Common Scenarios and Fixes
Understanding common real-world scenarios can help you act faster. Here are a few:
- Scenario: Probes return 403 errors.
Fix: Some web servers block probes due to lack of authentication. Set up dedicated probe pages or whitelist the Application Gateway’s IP address. - Scenario: Backend times out under load.
Fix: Check for performance bottlenecks on the backend or adjust timeout settings under HTTP configuration. - Scenario: Backend SSL certificate expired.
Fix: Renew the certificate and ensure it’s trusted by the Application Gateway.
Conclusion
Troubleshooting a 502 Bad Gateway error on Azure Application Gateway may seem daunting, but with a structured approach, it becomes manageable. Always start by checking backend health and probe configurations, then move on to more advanced aspects like SSL trust and diagnostic logs.
The key is not to assume the problem sits with only one component. Instead, verify each layer—from DNS resolution to NSGs and backend server performance. By doing so, you’ll not only resolve the 502 error but improve the overall resilience and reliability of your application environment.
With consistent monitoring and best practices, you can reduce the chances of future outages and maintain seamless service for your users.
