Application Gateways

What is an Application Gateway?

An Application Gateway is a load balancer that enables you to manage traffic to your web applications. It operates at Layer 7 (HTTP/HTTPS) and can route requests based on various criteria, including path-based routing, host-based routing, and header-based routing. It's a crucial component for building scalable, highly available, and secure web architectures.

Key Features

• Load Balancing: Distributes incoming application traffic across multiple backend servers.
• SSL Termination: Handles the decryption of SSL/TLS traffic at the gateway, offloading this CPU-intensive task from your backend servers.
• Web Application Firewall (WAF): Protects your web applications from common web exploits and vulnerabilities such as SQL injection, cross-site scripting, and more.
• Path-Based Routing: Routes traffic to different backend pools based on the URL path of the request. For example, /images/* could be routed to one set of servers, while /api/* is routed to another.
• Host-Based Routing: Routes traffic to different backend pools based on the host name in the request header. This is useful for hosting multiple websites on the same IP address.
• Cookie-Based Session Affinity: Ensures that requests from a particular client are consistently sent to the same backend server.
• Health Probes: Continuously monitors the health of backend instances and removes unhealthy instances from rotation.
• Autoscaling: Automatically adjusts the capacity of the gateway based on traffic load.

Use Cases

• Securing web applications with a WAF.
• Implementing SSL offloading for improved performance.
• Deploying microservices with path-based routing.
• Hosting multiple domains on a single IP address.
• Ensuring high availability and scalability of web applications.

Getting Started

To get started with Application Gateways, you typically need to define the following:

• A frontend IP configuration (public or private).
• One or more listeners to accept incoming traffic (e.g., HTTP on port 80, HTTPS on port 443).
• Backend HTTP settings, which define how requests are forwarded to backend servers.
• Backend pools, which contain the IP addresses or FQDNs of your backend servers.
• Health probes to monitor backend server health.
• Rules to define the routing logic.

Example Configuration Snippet (Conceptual)

{
  "name": "myAppGateway",
  "location": "East US",
  "properties": {
    "sku": {
      "name": "Standard_WAF",
      "tier": "Standard_v2"
    },
    "enableHttp2": true,
    "frontendIPConfigurations": [
      {
        "name": "frontendPublicIP",
        "properties": {
          "publicIPAddress": {
            "id": "/subscriptions/.../providers/Microsoft.Network/publicIPAddresses/myPublicIP"
          }
        }
      }
    ],
    "backendAddressPools": [
      {
        "name": "myBackendPool",
        "properties": {
          "backendIPConfigurations": [],
          "backendAddresses": [
            {
              "ipAddress": "10.0.0.4"
            },
            {
              "ipAddress": "10.0.0.5"
            }
          ]
        }
      }
    ],
    "httpListeners": [
      {
        "name": "httpListener",
        "properties": {
          "frontendIPConfiguration": {
            "id": "/subscriptions/.../providers/Microsoft.Network/applicationGateways/myAppGateway/frontendIPConfigurations/frontendPublicIP"
          },
          "frontendPort": {
            "id": "/subscriptions/.../providers/Microsoft.Network/applicationGateways/myAppGateway/frontendPorts/port80"
          },
          "protocol": "Http"
        }
      }
    ],
    "requestRoutingRules": [
      {
        "name": "basicRule",
        "properties": {
          "ruleType": "Basic",
          "httpListener": {
            "id": "/subscriptions/.../providers/Microsoft.Network/applicationGateways/myAppGateway/httpListeners/httpListener"
          },
          "backendAddressPool": {
            "id": "/subscriptions/.../providers/Microsoft.Network/applicationGateways/myAppGateway/backendAddressPools/myBackendPool"
          },
          "backendHttpSettings": {
            "id": "/subscriptions/.../providers/Microsoft.Network/applicationGateways/myAppGateway/backendHttpSettingsCollection/appGatewayBackendHttpSettings"
          }
        }
      }
    ],
    "backendHttpSettingsCollection": [
      {
        "name": "appGatewayBackendHttpSettings",
        "properties": {
          "port": 80,
          "protocol": "Http",
          "cookieBasedAffinity": "Disabled",
          "requestTimeout": 20
        }
      }
    ]
  }
}
                

Resources

For more detailed information and tutorials, please refer to the official documentation:

• Azure Application Gateway Documentation
• Azure WAF Documentation