Troubleshooting Next.js BasePath With Next-auth And Keycloak

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So, you're diving into the world of Next.js, trying to wrangle next-auth and Keycloak, and you've thrown in a basePath for good measure? Awesome! But, as you might have discovered, things can get a little hairy when these technologies meet. Let's break down a common issue: getting your Next.js app, configured with a basePath, to play nicely with next-auth and Keycloak.

The Challenge: basePath and Authentication

The core problem often stems from how Next.js handles routing when a basePath is involved. Imagine you've set your basePath to /app. This means your Next.js application will be served under yourdomain.com/app. Now, when next-auth and Keycloak come into the picture, they need to correctly redirect users for authentication. If the URLs aren't configured properly, you might find yourself in a redirect loop or facing errors.

Understanding the Root Cause

To really grasp what's happening, let's zoom in on the moving parts:

  • Next.js Routing: Next.js uses its file system-based router. When you add a basePath, it essentially prefixes all your routes. So, /api/auth/[...nextauth] becomes /app/api/auth/[...nextauth]. This is a crucial detail.
  • next-auth: This library simplifies authentication in Next.js apps. It provides built-in support for various providers, including Keycloak. However, it relies on correct URLs for callbacks and redirects.
  • Keycloak: Your identity provider. It needs to know the correct URLs to redirect users back to your application after they've authenticated.

Keycloak Configuration

First off, Keycloak needs to be in the loop about your basePath. This means you'll need to configure the Valid Redirect URIs and Web Origins in your Keycloak client settings. Think of these as Keycloak's whitelist – it's gotta know where it's allowed to send users back.

  • Valid Redirect URIs: This is where you tell Keycloak the exact URLs next-auth will use for the authentication dance. It's crucial to include your basePath here. For instance, if your basePath is /app, a valid redirect URI would look something like yourdomain.com/app/api/auth/callback/keycloak. You'll also likely need to add yourdomain.com/app/* to cover various scenarios.
  • Web Origins: This setting is about CORS (Cross-Origin Resource Sharing). It specifies the origins that are allowed to make requests to Keycloak. Again, your basePath plays a role here. Include yourdomain.com/app in your Web Origins.

Next.js Configuration (next.config.js)

Now, let's peek into your next.config.js file. This is where you define your basePath. It's pretty straightforward:

const nextConfig = {
  basePath: '/app',
  // ... other configurations
};

module.exports = nextConfig;

This tells Next.js to serve your application under the /app path. But remember, this also means you need to adjust your environment variables, especially the ones related to authentication.

Environment Variables

Your environment variables are the unsung heroes here. They bridge the gap between your Next.js app and next-auth, ensuring everything knows about your basePath. You'll likely need to tweak these:

  • NEXTAUTH_URL: This is the base URL for your application, and it must include your basePath. If your app is hosted at yourdomain.com/app, then NEXTAUTH_URL should be set to yourdomain.com/app. This is super important!
  • NEXTAUTH_URL_INTERNAL: In some setups, you might need to explicitly set the internal URL. This is often the same as NEXTAUTH_URL but can be different in complex deployments.
  • Keycloak Client ID and Secret: These are specific to your Keycloak setup. Make sure they're correctly configured and accessible in your environment.

The Tricky Part: Callbacks and Redirects

Here's where things can get a little tangled. next-auth uses callbacks to handle the authentication flow. When you introduce a basePath, you need to ensure these callbacks are correctly configured.

  • The Default Callback URL: By default, next-auth uses /api/auth/callback/:provider. With a basePath, this becomes /app/api/auth/callback/:provider. This is the URL you configured in your Keycloak's Valid Redirect URIs.
  • Custom Callbacks (If Needed): In some scenarios, you might need more control over the callback process. You can define custom callbacks in your next-auth configuration. This gives you the flexibility to handle redirects and session management precisely as you need.

Troubleshooting Tips

Okay, so you've configured everything, but things still aren't working? Don't panic! Here are some battle-tested tips for debugging:

  1. Console Logging: Sprinkle console.log statements throughout your code, especially in your callbacks and API routes. This helps you trace the flow of execution and identify where things are going wrong.
  2. Network Tab: Your browser's developer tools are your friend. The Network tab shows you the HTTP requests and responses, including redirects. This can reveal URL mismatches or other issues.
  3. next-auth Debug Mode: Enable debug mode in your next-auth configuration. It provides verbose logging that can help pinpoint problems.
  4. Keycloak Logs: Check your Keycloak server logs. They often contain valuable clues about authentication failures or misconfigurations.
  5. Double-Check Everything: It sounds obvious, but go through your configurations again. Typos or small errors can be sneaky! Pay close attention to URLs, environment variables, and Keycloak settings.

Example Scenario: A Step-by-Step Debugging Session

Let's walk through a hypothetical scenario. Imagine you're getting a redirect loop after logging in. Here's how you might approach debugging:

  1. Check Keycloak: Verify that your Valid Redirect URIs and Web Origins are correctly configured with your basePath. A common mistake is forgetting the /* wildcard in the redirect URIs.
  2. Inspect NEXTAUTH_URL: Ensure your NEXTAUTH_URL environment variable includes the basePath. If it's missing, next-auth will generate incorrect URLs.
  3. Examine Network Requests: Open your browser's developer tools and watch the network requests during the login process. Look for redirects and check if the URLs are what you expect.
  4. Debug Callbacks: If you're using custom callbacks, add console.log statements to see the values of the url, baseUrl, and other relevant parameters. This can help you understand how redirects are being handled.
  5. Keycloak Logs: Dive into your Keycloak server logs. They might reveal errors related to client authentication or redirect URI mismatches.

Real-World Considerations

Beyond the technical configurations, let's think about some real-world scenarios.

  • Deployment Environments: You might have different basePath settings for development, staging, and production. Use environment variables to manage these variations.
  • Reverse Proxies: If you're using a reverse proxy (like Nginx or Apache), make sure it's configured to correctly forward requests to your Next.js application, including the basePath.
  • Subdomains: If your application is served on a subdomain (e.g., app.yourdomain.com), your basePath might be just /. The key is to ensure all your configurations align with your deployment setup.

The Importance of Clear Documentation

Finally, let's talk about documentation. When you're working with complex setups like this, clear documentation is your best friend. Document your basePath configuration, Keycloak settings, and any custom callbacks you've implemented. This will save you (and your team) headaches down the road.

Conclusion: basePath, next-auth, and Keycloak – A Powerful Trio

Integrating next-auth and Keycloak with a Next.js basePath can be a bit of a puzzle, but it's totally achievable. By understanding how these technologies interact and carefully configuring your URLs and environment variables, you can create a secure and robust application. Remember to double-check your settings, use debugging tools, and document your setup. With a little patience and a systematic approach, you'll conquer the basePath challenge and unlock the full potential of this powerful trio.

So, there you have it, folks! Go forth and build amazing, secure applications with Next.js, next-auth, and Keycloak. And remember, when things get tricky, don't hesitate to reach out to the community for help. We're all in this together!