How can I configure an Azure Load Balancer to route traffic based on URL path?"

To configure Azure Application Gateway to route traffic based on the URL path correctly, here are some steps and troubleshooting tips:

1. Verify the Configuration

Ensure that the URL path-based routing rules are set up correctly.

• Path-based Routing Rules:
  • Go to the Azure Portal > Application Gateway > HTTP Settings.
  • Under Routing Rules, ensure that:
    • Path-based rules are enabled.
    • The correct backend pools are mapped to specific paths.
    • Example:
      • /api/* → Backend Pool A
      • /web/* → Backend Pool B
• HTTP Settings:
  • Verify that each backend pool has the correct HTTP settings configured, including protocols, timeouts, and custom headers if required.
  • Ensure that hostnames and paths match what your backend services expect.

2. Check Backend Health

• Go to Backend Health in the Application Gateway settings.
• Verify that all backend targets in your backend pools are healthy.
• If a backend is unhealthy, requests will default to another healthy backend or fail.

3. Set Up Listener and Rules Correctly

• Ensure that the HTTP or HTTPS Listener is properly configured:
  • The frontend IP configuration should match the expected incoming requests.
  • The Host Header should not be causing mismatches.
• Double-check the rule priority:
  • Path-based routing rules are processed sequentially. Ensure there is no conflict or incorrect priority among your rules.
  • The default rule should not override path-specific rules.

4. Common Issues and Fixes

Issue: Traffic Always Routes to Default Pool

• Cause: Incorrect or missing path-based routing rules.
• Fix:
  • Ensure the /* path is not overlapping with specific paths like /api/*.
  • Add a fallback path only for traffic that doesn’t match other paths explicitly.

Issue: Misconfigured Backend Settings

• Cause: Incorrect backend pool configurations.
• Fix:
  • Ensure the backend web apps have correct FQDNs or IP addresses.
  • If using FQDN, ensure proper DNS resolution and SSL certificates (if HTTPS).

Issue: Backend Authentication Issues

• Cause: Web apps in different resource groups may have different authentication requirements.
• Fix:
  • Use Managed Identity or a Service Principal for authentication.
  • If the apps require client certificates, ensure they are configured in HTTP settings.

Example Configuration

Path Rules:

1. /api/* → Backend Pool: API-WebApp
2. /web/* → Backend Pool: Frontend-WebApp
3. /* → Backend Pool: Default-App

Ensure the /* fallback rule is last in priority.

If these steps do not resolve the issue, provide more details such as:

• Your specific routing rules.
• Access log insights.
• Backend health status.