GCP Load Balancer and IAP: The Unholy Union?

Are you struggling to get your GCP Load Balancer to work with IAP (Identity-Aware Proxy) settings? You’re not alone! Many developers have reported issues with this seemingly straightforward combination. In this article, we’ll dive into the depths of this problem and provide you with a step-by-step guide to resolve the issue once and for all.

The Problem: GCP Load Balancer vs IAP

At first glance, it seems like a match made in heaven: GCP Load Balancer, a powerful tool for distributing traffic, and IAP, a robust security solution for controlling access. However, the reality is far from it. When you try to combine these two services, things start to fall apart. Your load balancer refuses to play nice with IAP, leaving you with a bunch of 502 errors and frustrated users.

So, what’s going on? To understand the issue, let’s take a closer look at how both services work:

GCP Load Balancer 101

A GCP Load Balancer is a managed service that helps distribute incoming traffic across multiple backend instances. It provides scalability, high availability, and improved responsiveness. Here’s a simplified overview of the load balancing process:

• Client sends a request to the load balancer’s IP address
• Load balancer receives the request and selects a backend instance based on the configured routing rules
• Request is forwarded to the chosen backend instance
• Backend instance processes the request and sends a response back to the load balancer
• Load balancer forwards the response to the client

IAP 101

IAP is a security service that controls access to your resources based on user identity. It uses authentication and authorization mechanisms to ensure that only authorized users can access your resources. Here’s a simplified overview of how IAP works:

• User sends a request to the IAP-protected resource
• IAP intercepts the request and checks the user’s identity
• If the user is authorized, IAP forwards the request to the backend instance
• Backend instance processes the request and sends a response back to IAP
• IAP forwards the response to the user

The Conflict: Load Balancer and IAP

Now that we’ve covered the basics of both services, let’s see what happens when we try to combine them:

  +---------------+
  |  Client   |
  +---------------+
           |
           | (Request)
           v
  +---------------+
  |  Load Balancer  |
  +---------------+
           |
           | (Request)
           v
  +---------------+
  |  IAP     |
  +---------------+
           |
           | (Request)
           v
  +---------------+
  |  Backend Instance  |
  +---------------+

In this setup, the load balancer receives the client’s request and forwards it to IAP. IAP then checks the user’s identity and, if authorized, forwards the request to the backend instance. However, this is where things start to go wrong.

The load balancer is designed to distribute traffic, not handle authentication. When IAP intercepts the request, it expects the load balancer to forward the authenticated request to the backend instance. But the load balancer doesn’t understand IAP’s authentication mechanism, and it can’t forward the request properly.

This mismatch leads to a series of 502 errors, failed requests, and frustrated users. So, what can you do to resolve this issue?

The Solution: Configure IAP to Trust the Load Balancer

To make IAP and the load balancer work together harmoniously, you need to configure IAP to trust the load balancer. This involves creating an IAP-enabled backend service and specifying the load balancer’s IP address as a trusted proxy.

Here’s a step-by-step guide to get you started:

Step 1: Create an IAP-enabled Backend Service

In the Google Cloud Console, navigate to the Navigation menu ( three horizontal lines in the top left corner) and select **APIs & Services** > **Dashboard**. Click on **Enable APIs and Services** and search for **Identity-Aware Proxy API**. Click on the result and enable the API.

Once the API is enabled, create a new backend service:

gcloud compute backend-services create [BACKEND_SERVICE_NAME] \
  --global \
  --enable-iap

Step 2: Configure IAP Settings

Create an IAP settings file with the following content:

{
  "identityAwareProxy": {
    "enabled": true,
    "oauth2ClientId": "[OAUTH2_CLIENT_ID]",
    "oauth2ClientSecret": "[OAUTH2_CLIENT_SECRET]"
  }
}

Replace `[OAUTH2_CLIENT_ID]` and `[OAUTH2_CLIENT_SECRET]` with the values from your OAuth 2.0 client ID and secret, respectively.

Step 3: Update the Load Balancer Configuration

Update the load balancer configuration to use the IAP-enabled backend service:

gcloud compute backend-services update [BACKEND_SERVICE_NAME] \
  --global \
  --iap-settings-file [IAP_SETTINGS_FILE]

Step 4: Add the Load Balancer’s IP Address as a Trusted Proxy

In the IAP settings file, add the load balancer’s IP address as a trusted proxy:

{
  "identityAwareProxy": {
    "enabled": true,
    "oauth2ClientId": "[OAUTH2_CLIENT_ID]",
    "oauth2ClientSecret": "[OAUTH2_CLIENT_SECRET]",
    "trusted_proxies": [
      "[LOAD_BALANCER_IP_ADDRESS]"
    ]
  }
}

Replace `[LOAD_BALANCER_IP_ADDRESS]` with the load balancer’s IP address.

Step 5: Verify the Configuration

Verify that the load balancer and IAP are working together correctly:

gcloud compute backend-services describe [BACKEND_SERVICE_NAME] \
  --global

Check the output to ensure that the IAP settings are correctly configured and the load balancer’s IP address is listed as a trusted proxy.

Before	After
Client sends a request to the load balancer Load balancer forwards the request to IAP IAP intercepts the request and checks the user’s identity IAP forwards the request to the backend instance Backend instance processes the request and sends a response back to IAP IAP forwards the response to the client	Client sends a request to the load balancer Load balancer forwards the request to IAP (trusted proxy) IAP checks the user’s identity and forwards the request to the backend instance Backend instance processes the request and sends a response back to IAP IAP forwards the response to the load balancer Load balancer forwards the response to the client

With these steps, you’ve successfully configured IAP to trust the load balancer. Your users should now be able to access your resources without any issues.

Conclusion

In conclusion, the combination of GCP Load Balancer and IAP can be a powerful security solution for your resources. By configuring IAP to trust the load balancer, you can ensure seamless authentication and authorization for your users. Remember to follow the steps outlined in this article to avoid any potential pitfalls and ensure a smooth deployment.

If you’re still experiencing issues, don’t hesitate to reach out to the GCP support team or explore the official documentation for further guidance.

Happy deploying!

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