Exposing Azure Functions Through Azure Application Gateway Using Path-Based Routing

Introduction:

When building enterprise-grade serverless applications with Microsoft Azure Functions, it is common to secure APIs with Microsoft Azure Application Gateway rather than exposing the Function App endpoint directly to the internet.

Using Application Gateway provides several advantages:

• Centralized API exposure
• Web Application Firewall (WAF) protection
• SSL termination
• Path-based routing
• URL rewrite capabilities
• Better traffic management
• Integration with private networking

One common scenario is exposing Azure Functions behind a custom path such as:

https://mydomain.com/[your path]/Ping

Instead of directly using the Azure Function URL:

https://myfunction.azurewebsites.net/api/Ping

In this article, we will learn how to:

• Configure Azure Functions for custom routing
• Understand the default /api route behavior
• Modify host.json
• Configure Application Gateway path-based routing
• Configure URL rewrite rules
• Test the final setup

If you are new to path-based routing with Azure Application Gateway, please go through my previous article on Azure Application Gateway path-based routing with App Services

Understanding the Default Azure Function Route

By default, HTTP-triggered Azure Functions automatically use the /api route prefix.

For example:

[Function("Ping")]
public async Task<HttpResponseData> Run(
    [HttpTrigger(AuthorizationLevel.Function, "get")]
    HttpRequestData req)

is exposed as:

https://<functionapp>.azurewebsites.net/api/Ping

The /api prefix is automatically added by the Azure Functions runtime.

This behavior is controlled through the host.json configuration file.

Default host.json Configuration

A typical host.json file looks like this:

{
  "version": "2.0",
  "logging": {
    "applicationInsights": {
      "samplingSettings": {
        "isEnabled": true,
        "excludedTypes": "Request"
      },
      "enableLiveMetricsFilters": true
    }
  }
}

Since there is no HTTP route configuration present, Azure Functions automatically applies the default /api route prefix.

Customizing or Removing the /api Prefix

You can customize or completely remove the /api prefix by adding the extensions.http.routePrefix configuration inside host.json.

Remove the /api Prefix

{
  "version": "2.0",
  "logging": {
    "applicationInsights": {
      "samplingSettings": {
        "isEnabled": true,
        "excludedTypes": "Request"
      },
      "enableLiveMetricsFilters": true
    }
  },
  "extensions": {
    "http": {
      "routePrefix": ""
    }
  }
}

After deployment, the function endpoint changes from:

https://.azurewebsites.net/api/Ping

to:

https://.azurewebsites.net/Ping

Using a Custom Route Prefix

You can also replace /api with a custom path.

{
  "version": "2.0",
  "extensions": {
    "http": {
      "routePrefix": "b2crole"
    }
  }
}

Now the endpoint becomes:

https://<functionapp>.azurewebsites.net/b2crole/Ping

This is useful when organizing APIs under specific namespaces.

Important Clarification About Path Mappings

Many developers confuse Azure Function App Path Mappings with HTTP routing.

Inside the Function App configuration, there is a feature called:

Path mappings

This feature is only used for:

• Mounting Azure Files
• Mounting Blob Storage
• Accessing storage as local filesystem paths

This does NOT affect:

• HTTP URLs
• API endpoints
• Routing
• Application Gateway behavior

HTTP routing is controlled only through:

• host.json
• Function Route attributes
• Application Gateway rewrite/path rules

Configuring Azure Application Gateway

Now let’s expose the Function App through Microsoft Azure Application Gateway.

Scenario

Azure Function Endpoint

https://b2crole.azurewebsites.net/Ping

Desired Public URL

https://mydomain.com/b2crole/Ping

Configure Backend Pool

In Application Gateway:

Backend Pool

Add the Azure Function hostname:

b2crole.azurewebsites.net

Configure HTTP Settings

In HTTP Settings:

• Backend protocol → HTTPS
• Pick hostname from backend target → Enabled

This ensures Application Gateway correctly forwards requests to the Function App.

Configure Path-Based Routing

Create a Path-Based Rule.

Listener

Example:

https://mydomain.com

Path Rule

/b2crole/*

Forward traffic to the Azure Function backend pool.

Testing the Configuration

Once deployed, test the endpoint:

https://mydomain.com/b2crole/Ping?code=<function-key>

Expected response:

200 OK

Summary

In this article, we explored how to expose Azure Functions through Microsoft Azure Application Gateway using path-based routing.

We covered:

The default /api route behavior in Azure Functions
How host.json controls the route prefix
How to remove or customize the route prefix
The difference between Path Mappings and HTTP routing
Configuring Application Gateway backend pools
Configuring path-based routing
Using rewrite rules to correctly forward requests

Using Application Gateway in front of Azure Functions is a powerful approach for building secure, scalable, and enterprise-ready serverless APIs.