Troubleshooting Next.js BasePath With NextAuth And Keycloak

Hey everyone! Today, we're diving into a common issue that Next.js developers encounter when trying to integrate Keycloak authentication using NextAuth.js while also utilizing the basePath configuration in their next.config.js. It's a bit of a tricky situation, but fear not! We'll break it down step-by-step and get your application working smoothly.

Understanding the Problem

So, you've got your shiny new Next.js project, and you're excited to use Keycloak for authentication. You've set up NextAuth.js, configured your Keycloak server, and everything seems to be humming along nicely. That is, until you decide to add a basePath to your next.config.js. This is often done when you want to deploy your application under a subpath on your domain (e.g., yourdomain.com/app).

Suddenly, things start to break. You might see errors like redirects failing, authentication loops, or your application simply not being able to communicate with your Keycloak server correctly. What gives?

The root of the problem lies in how Next.js, NextAuth.js, and Keycloak handle URLs and redirects, especially when a basePath is involved. NextAuth.js relies on callbacks and redirects to manage the authentication flow. When you introduce a basePath, you need to ensure that all these URLs are correctly prefixed with your basePath. If not, NextAuth.js might redirect to the wrong location, or Keycloak might not be able to find the correct callback URL.

This can be a particularly frustrating issue because the error messages aren't always clear, and it can feel like you're chasing a ghost. But don't worry, we're here to help you exorcise that ghost!

Why basePath Matters

Before we dive into solutions, let's quickly recap why you might be using basePath in the first place. The basePath configuration in next.config.js allows you to serve your Next.js application from a sub-directory on your domain. This is incredibly useful in a variety of scenarios:

• Reverse Proxies: If you're using a reverse proxy like Nginx or Apache, you might want to serve your Next.js application under a specific path.
• Multiple Applications: You might have multiple applications running on the same domain, each under its own subpath.
• Organization: Using a basePath can help you organize your application structure and make it more maintainable.

However, as we've seen, it also adds a layer of complexity when dealing with authentication.

Configuration Issues with Next.js basePath, NextAuth, and Keycloak

When integrating Keycloak with NextAuth.js in a Next.js application that uses a basePath, several configuration issues can arise. These issues stem from the way URLs are handled during the authentication flow, especially redirects and callbacks. Let's explore these challenges in detail.

1. Incorrect Callback URLs

One of the most common problems is the incorrect configuration of callback URLs. Keycloak needs to know where to redirect the user after successful authentication. This is specified through the redirect_uri parameter in the authentication request. When a basePath is introduced, you must ensure that this redirect_uri includes the basePath. If it doesn't, Keycloak might redirect the user to the wrong location, leading to errors or an authentication loop.

For example, if your basePath is /app and your NextAuth.js callback URL is /api/auth/callback/keycloak, the correct redirect_uri should be /app/api/auth/callback/keycloak. Failing to include the /app prefix will cause issues.

2. Misconfigured NextAuth.js Options

NextAuth.js relies on several options, such as NEXTAUTH_URL, to function correctly. When using a basePath, you need to ensure that these options are configured to include the basePath. For instance, if your application is served at yourdomain.com/app, the NEXTAUTH_URL environment variable should be set to yourdomain.com/app. If this is not set correctly, NextAuth.js might generate incorrect URLs, leading to authentication failures.

3. Keycloak Client Configuration

On the Keycloak side, you need to configure the client settings appropriately. This includes setting the valid redirect URIs and the Web Origins. These settings must match the URLs that NextAuth.js is using, including the basePath. If there's a mismatch, Keycloak will reject the authentication request.

For example, if your basePath is /app, you need to add yourdomain.com/app/* to the valid redirect URIs in your Keycloak client configuration. Similarly, if your application is making requests from yourdomain.com/app, you need to add this to the Web Origins.

4. Reverse Proxy Issues

If you're using a reverse proxy like Nginx or Apache, you need to configure it to correctly forward requests to your Next.js application, including the basePath. If the reverse proxy is not configured correctly, it might strip the basePath from the request, leading to URL mismatches and authentication failures.

For example, you need to ensure that requests to yourdomain.com/app/* are correctly proxied to your Next.js application, and that the basePath is preserved.

5. Cookie Path Problems

NextAuth.js uses cookies to manage sessions. When a basePath is involved, the cookie path needs to be set correctly. If the cookie path is not set to include the basePath, the browser might not send the cookie in subsequent requests, leading to session loss and authentication issues.

You can configure the cookie options in NextAuth.js to include the basePath. This ensures that the cookies are correctly associated with your application's path.

6. Environment Variable Configuration

Incorrectly set environment variables can be a major source of issues. Make sure that environment variables like NEXTAUTH_URL, KEYCLOAK_ID, KEYCLOAK_SECRET, and KEYCLOAK_ISSUER are correctly set and include the basePath where necessary.

For example, if your Keycloak issuer URL is yourkeycloakdomain.com/auth/realms/yourrealm, and your basePath is /app, you might need to adjust the issuer URL to yourkeycloakdomain.com/auth/realms/yourrealm/app if Keycloak is also served under the /app path.

Solutions and Code Examples

Okay, enough about the problems! Let's get into some solutions. Here's a breakdown of the steps you need to take to correctly configure Next.js, NextAuth.js, and Keycloak with a basePath.

1. Configure next.config.js

First, make sure your next.config.js is correctly configured with your basePath:

const basePath = '/app';

module.exports = {
  basePath: basePath,
  assetPrefix: basePath,
  // Other configurations...
};

Important: The assetPrefix is also set to basePath. This ensures that your static assets are served correctly under the basePath.

2. Set NEXTAUTH_URL Environment Variable

The NEXTAUTH_URL environment variable tells NextAuth.js the base URL of your application. Make sure this includes your basePath:

NEXTAUTH_URL=http://localhost:3000/app

If you're deploying to a production environment, this should be your production URL with the basePath:

NEXTAUTH_URL=https://yourdomain.com/app

3. Configure Keycloak Client

In your Keycloak admin console, you need to configure your client settings. This is where you tell Keycloak about your application's URLs.

• Valid Redirect URIs: Add your callback URL, including the basePath. For example:

  http://localhost:3000/app/api/auth/callback/keycloak
  https://yourdomain.com/app/api/auth/callback/keycloak
  

  You can use wildcards here, such as yourdomain.com/app/*, but be mindful of security implications.
• Web Origins: Add your application's origin, including the basePath. For example:

  http://localhost:3000/app
  https://yourdomain.com/app
  

4. Update NextAuth.js Configuration

In your NextAuth.js configuration (usually in pages/api/auth/[...nextauth].js), you need to configure your Keycloak provider. Make sure your URLs include the basePath where necessary.

import NextAuth from 'next-auth';
import KeycloakProvider from 'next-auth/providers/keycloak';

const basePath = '/app';

export default NextAuth({
  providers: [
    KeycloakProvider({
      clientId: process.env.KEYCLOAK_ID,
      clientSecret: process.env.KEYCLOAK_SECRET,
      issuer: process.env.KEYCLOAK_ISSUER, // e.g., 'http://localhost:8080/realms/myrealm'
    }),
  ],
  callbacks: {
    async redirect({ url, baseUrl }) {
      // Allows relative callback URLs
      if (url.startsWith("/")) return `${baseUrl}${url}`
      // Allows callback URLs on the same domain
      else if (new URL(url).origin === baseUrl) return url
      return baseUrl
    }
  },
  pages: {
    signIn: '/signin',
  },
  session: {
    strategy: "jwt"
  },
  debug: true,
});

Key Points:

• Ensure that clientId, clientSecret, and issuer are correctly set from your environment variables.

5. Handle Redirects in Callbacks

NextAuth.js provides callbacks that allow you to customize the authentication flow. The redirect callback is particularly important when using a basePath.

The above configuration will redirect correctly after login.

6. Cookie Configuration (Optional)

If you're still experiencing issues with session management, you might need to configure the cookie options in NextAuth.js. This is usually not necessary, but it can be helpful in certain scenarios.

// Inside your NextAuth configuration
session: {
  strategy: "jwt",
  // maxAge: 30 * 24 * 60 * 60, // 30 days
  updateAge: 24 * 60 * 60, // 24 hours
},
cookie: {
  options: {
    httpOnly: true,
    sameSite: 'lax',
    path: '/app', // Set the cookie path to your basePath
    secure: process.env.NODE_ENV === 'production',
  }
}

Note: The cookie.options.path is set to /app, which ensures that the cookies are associated with your application's basePath.

7. Reverse Proxy Configuration (If Applicable)

If you're using a reverse proxy, you need to configure it to correctly forward requests to your Next.js application. Here's an example Nginx configuration:

server {
    listen 80;
    server_name yourdomain.com;

    location /app/ {
        proxy_pass http://localhost:3000/app/;
        proxy_http_version 1.1;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection 'upgrade';
        proxy_set_header Host $host;
        proxy_cache_bypass $http_upgrade;
    }

    location / {
        return 301 https://$host$request_uri;
    }
}

Key Points:

• The location /app/ block proxies requests to your Next.js application, ensuring that the /app basePath is preserved.
• The proxy_set_header directives are important for WebSocket connections, which might be used by NextAuth.js.

Common Pitfalls and How to Avoid Them

Even with the correct configurations, you might still run into some common pitfalls. Here are a few things to watch out for:

• Forgetting the Trailing Slash: Make sure your URLs in Keycloak and your NextAuth.js configuration have the correct trailing slashes. A missing slash can cause redirects to fail.
• Cache Issues: Sometimes, your browser or Keycloak might cache incorrect URLs. Try clearing your browser cache or restarting your Keycloak server.
• Environment Variable Typos: Double-check your environment variables for typos. A small mistake can cause big problems.
• Inconsistent basePath: Ensure that your basePath is consistent across all your configurations (Next.js, NextAuth.js, Keycloak, and your reverse proxy).

Debugging Tips

If you're still stuck, here are some debugging tips:

• NextAuth.js Debug Mode: Set the debug option to true in your NextAuth.js configuration. This will print more detailed logs to the console.
• Browser Developer Tools: Use your browser's developer tools to inspect network requests and responses. This can help you identify redirect issues and URL mismatches.
• Keycloak Logs: Check your Keycloak server logs for errors. This can give you insights into authentication failures.
• Simplify Your Configuration: Try simplifying your configuration by removing the basePath temporarily. If everything works without the basePath, you know the issue is related to the basePath configuration.

Conclusion

Integrating Keycloak with NextAuth.js and a basePath can be challenging, but it's definitely achievable. By understanding the underlying issues and following the steps outlined in this article, you can get your application up and running smoothly. Remember to double-check your configurations, pay attention to detail, and don't be afraid to use debugging tools to identify and fix problems.

Good luck, and happy coding!