NextAuth with Nginx proxy: Invalid grant code error when using proxied URL

[Guruprasad16]

Environment

Next.js running as a container
NextAuth.js
Nginx as reverse proxy running as a container
Keycloak as OAuth provider running on the cloud
Container setup with Docker

Issue Description

I'm facing an issue with NextAuth.js when my Next.js application is running behind an Nginx reverse proxy. The authentication flow works fine when accessing the application directly through its internal port, but fails when accessing it through the Nginx proxy.

What works:

When setting NEXTAUTH_URL=http://localhost:3000/ekeweb/api/auth and accessing the app directly, authentication works fine (but redirects to port 3000).

What doesn't work:

When setting NEXTAUTH_URL=http://localhost/ekeweb/api/auth and accessing through Nginx proxy, I get an invalid_grant (Code not valid) error.

Error logs:
2025-04-16 10:00:46 name: 'OAuthCallbackError',
2025-04-16 10:00:46 code: undefined
2025-04-16 10:00:46 providerId: 'keycloak',
2025-04-16 10:00:46 message: 'invalid_grant (Code not valid)'

Questions:

1. Has anyone successfully configured NextAuth with Nginx proxying to work correctly?
2. Is there a way to properly configure NEXTAUTH_URL_INTERNAL to handle this scenario?
3. Are there specific Nginx configuration changes needed for NextAuth to work properly?

Any guidance would be greatly appreciated!