Azure Application Gateway Best Practices

This document outlines best practices for designing, configuring, and managing Azure Application Gateway to ensure optimal performance, security, and reliability for your web applications.

Introduction

Azure Application Gateway is a scalable, secure, and fully managed web traffic load balancer that enables you to manage traffic to your web applications. It offers features like Web Application Firewall (WAF), SSL termination, cookie-based session affinity, and URL-based routing.

Design Considerations

Before deploying an Application Gateway, consider the following:

• Sizing: Choose the appropriate SKU (Standard_v2, WAF_v2) and instance count based on your expected traffic load, latency requirements, and throughput needs. v2 SKUs offer autoscaling and zone redundancy.
• Availability Zones: Deploy Application Gateway across multiple availability zones for high availability and resilience.
• Virtual Network Integration: Ensure Application Gateway is deployed in a dedicated subnet, large enough to accommodate future scaling and private IP address requirements.
• Backend Pool Design: Group similar backend resources (e.g., VMs, VM Scale Sets, App Services) into logical backend pools.

Configuration Best Practices

Listeners

• Use HTTPS listeners for all external traffic to enforce secure communication.
• Configure health probes correctly for each listener.
• Implement custom error pages for a better user experience during outages.

Rules (Routing Rules)

• Leverage path-based routing for microservices or different application modules.
• Use host-based routing for serving multiple domains from a single Application Gateway.
• Configure appropriate priorities for your rules.

Health Probes

• Configure health probes to accurately reflect the health of your backend applications.
• Use appropriate protocol (HTTP, HTTPS), port, and URI for probes.
• Set reasonable intervals, timeouts, and unhealthy threshold values.

Backend Settings

• Enable cookie-based session affinity if your application requires it.
• Configure appropriate connection draining timeouts.
• Use custom HTTP settings for specific backend requirements.

Security

Web Application Firewall (WAF)

• Enable WAF on your Application Gateway to protect against common web vulnerabilities (e.g., SQL injection, cross-site scripting).
• Choose between WAF_v2 SKU for enhanced features and performance.
• Keep WAF managed rulesets updated.
• Configure WAF in Prevention mode for blocking malicious requests.
• Regularly review WAF logs for suspicious activity.

SSL/TLS Management

• Use strong TLS policies (e.g., TLS 1.2) and disable older, less secure protocols.
• Manage SSL certificates efficiently: use Key Vault for certificate storage and auto-renewal.
• Terminate SSL at the Application Gateway to offload processing from backend servers.

Performance and Scalability

• Monitor backend health and latency to identify bottlenecks.
• Configure autoscaling for v2 SKUs to dynamically adjust capacity based on traffic.
• Right-size your Application Gateway instances based on performance metrics.
• Optimize backend application performance.

Monitoring and Logging

• Enable diagnostic logs for Application Gateway (Request Logs, Activity Logs, WAF Logs).
• Send logs to Azure Monitor Logs (Log Analytics) or Azure Storage for analysis and retention.
• Create alerts based on key metrics like request latency, error rates, and WAF incidents.
• Utilize Application Gateway metrics in Azure Monitor for performance insights.

Troubleshooting

Refer to the following for common troubleshooting scenarios:

• Connection Errors: Verify NSG rules, backend health, and firewall configurations.
• SSL/TLS Issues: Check certificate validity, chain, and TLS policy configuration.
• WAF Blocking Legitimate Traffic: Review WAF logs and consider tuning WAF rules or creating exceptions.