How to Debug Blocked Requests: Local-Network-Access in ChromeA developer's guide to tackling LNA preflight errors and CORS workarounds

Chrome has recently tightened its security policies with the introduction of Local-Network-Access (LNA) restrictions, introduced in Chrome v142+. While these policies aim to enhance security for local network requests, they often present developers with mysterious blocked requests, preflight errors, or unexpected CORS warnings. Debugging these issues can seem daunting, but with the right approach, they can be resolved with confidence.

In this post, we'll dive deep into why LNA restrictions exist, how they impact modern web applications, and how you can debug and fix blocked requests. We'll also provide code examples, step-by-step techniques, and actionable insights.

Why Chrome Enforces Local-Network-Access Restrictions

Local-Network-Access restrictions protect users from unauthorized access to their local devices (like routers, IoT devices, and local servers). With Local Network Access (LNA), Chrome ensures that websites can no longer silently make requests to devices on your local network without explicit user permission.

For example, prior to Chrome v142, any website could potentially make requests to http://127.0.0.1, http://192.168.x.x, or .local domains, potentially exposing sensitive resources. By restricting such access and introducing explicit permission prompts, Chrome makes local network interactions safer.

Key motivations behind LNA:

• Prevent potential Cross-Site Request Forgery (CSRF) attacks targeting local devices.
• Protect LAN-only resources from unauthorized access or malicious use.
• Standardize secure practices for local and private network requests.

If your application interacts with localhost-based microservices, IoT devices, or internal enterprise resources, understanding LNA is critical.

Identifying a Blocked Local-Network-Access Request

The first step in debugging is recognizing when Chrome's LNA policies block a request. Common symptoms include:

Example Error Messages in Chrome DevTools

1. Blocked Request in DevTools Console:

   Access to fetch at 'http://127.0.0.1:8080/data' from origin 'https://example.com' 
   has been blocked by CORS policy: Permission was denied for this request to access the `unknown` address space.
   

2. Preflight Request Denial:

   Error: OPTIONS http://192.168.1.1/api net::ERR_FAILED
   

3. Address Space Mismatch Warnings:

   Request had a target IP address space of `unknown`, but the resource is in address space `loopback`.
   

To confirm the issue is LNA-related, open DevTools → Network Tab → Affected Request. Look for:

• CORS errors
• Labeled issues such as Access-Control-Allow-Private-Network
• Blocked requests with mentions of unknown, loopback, or local-network-access.

Understanding Key Header Requirements for LNA

To make local requests work, Chrome imposes several requirements:

1. Access-Control-Allow-Origin: This specifies which websites are authorized to access local resources (e.g., * or the specific origin).
2. Access-Control-Allow-Credentials: If credentials are being passed, this must be explicitly true.
3. Access-Control-Allow-Private-Network: Introduced for LNA, this must be set to true for preflight requests originating from private networks or loopback addresses.

Header Example for a Successful LNA Request

Below is an example of valid headers required for local network requests:

Access-Control-Allow-Origin: https://example.com
Access-Control-Allow-Methods: GET, POST, OPTIONS
Access-Control-Allow-Credentials: true
Access-Control-Allow-Private-Network: true

Failure to include these headers can result in blocked requests, particularly when dealing with 127.0.0.1, .local domains, or private IPs.

Debugging with DevTools and Console Scripts

Debugging Preflight Requests in Chrome

1. Inspect Preflight Requests:

   • Open Chrome DevTools.
   • Select the Network tab.
   • Filter requests by the OPTIONS method.
   • Identify failures with headers like Access-Control-Allow-Private-Network.

2. Test LNA Permission with navigator.permissions.query:

navigator.permissions.query({ name: "local-network-access" })
  .then(result => {
    console.log('LNA Permission State:', result.state); // granted, denied, prompt
  })
  .catch(error => {
    console.error('Error checking LNA permissions:', error);
  });

This script helps verify whether the browser has granted, denied, or still prompts for local-network-access permissions.

Applying Fixes and Testing Allow Policies

Add the allow="local-network-access" Attribute for Iframes

One common cause of LNA failures is nested iframes. Each iframe must explicitly allow local network requests by including this attribute:

<iframe 
    src="https://app.example.com" 
    allow="local-network-access">
</iframe>

This allows the embedded iframe to inherit permissions to interact with local network endpoints.

Workarounds for CORS-Related Issues

Use a Proxy for Development

Set up a local proxy to handle headers and redirect requests. Here's an Express.js example:

const express = require('express');
const { createProxyMiddleware } = require('http-proxy-middleware');

const app = express();

app.use('/api', createProxyMiddleware({
    target: 'http://127.0.0.1:8080',
    changeOrigin: true,
    onProxyRes: (proxyRes) => {
        proxyRes.headers['Access-Control-Allow-Origin'] = '*';
        proxyRes.headers['Access-Control-Allow-Private-Network'] = 'true';
    }
}));

app.listen(3000, () => {
    console.log('Proxy running on http://localhost:3000/api');
});

Concluding Thoughts

The introduction of Local-Network-Access in Chrome is a powerful step forward in improving web security, but it can create frustration for developers working with local services. Debugging blocked requests requires a detailed understanding of LNA policies, headers, and Chrome's expectations.

By following the steps outlined in this guide, you can confidently resolve LNA-related issues, ensuring your web applications remain functional without compromising security.