Detect Origin Function is not configurable by the auth options which cause issues for multi tenancy across subdomains

Environment

Any

version 4.24.7

Reproduction URL

private

Describe the issue

I am trying to host a single nextauth server which is usable across multiple subdomains (ie a.app/api/auth/session & b.app/api/auth/session) however the catch being when you log in the redirect url is restricted to the next auth url which determined by the following function, when using a custom oauth client.

The following function is the reason for this behavior. utils/detect-origin.ts

You can override this behaviour in the core/lib/oauth/authorization-url.ts with AuthorizationParamaters however its not possible to override in core/lib/oauth/callback.ts where the custom defined token parameters are ignored. Which means the IDP rejects the token exchange as the redirect url does not match the authorization call redurect url.

One way around this is to do something unspeakable which is to do override process.env.NEXTAUTH_URL = dynamicUrl in code 🤮. This then forces the detechOrigin function to use the custom url. Allow muti tenancy with a single server

The providers I am using already restrict the redirects and in my service i can trust the host. Where i can use the undocumented AUTH_TRUST_HOST env to enable this feature. This feature should be configurable by the auth options and documented.

What would be even better though is the NEXTAUTH_URL was not taken directly from environment variables but if the url was defined in next auth option use that instead. This would allow the url to be restricted to a domain, but dynamic for a subdomain.

Additional it would be great if there was some example or documents on how to do multitenancy with a single instance for a non vercel hosted app.

How to reproduce

Run a nextjs app with next auth. With configured NEXTAUTH_URL which is 'localhost:4200'

in the provider configure

go to x.localhost:4200/api/auth/signin

This will fail

Expected behavior

authOption = { ... hostTrusted: true ... }

Or allow dyanmic URL authOption = { ... url: ${dynamic}.localhost:4200/api/auth/signin ... }

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nextauthjs / next-auth