Signin URL does not respect reverse proxy headers

Environment

System:
  OS: Linux 6.6 Ubuntu 24.04.1 LTS 24.04.1 LTS (Noble Numbat)
  CPU: (24) x64 AMD Ryzen 9 3900XT 12-Core Processor
  Memory: 17.18 GB / 23.42 GB
  Container: Yes
  Shell: 5.9 - /usr/bin/zsh
Binaries:
  Node: 23.7.0 - ~/.nvm/versions/node/v23.7.0/bin/node
  Yarn: 1.22.19 - /usr/local/bin/yarn
  npm: 10.9.2 - ~/.nvm/versions/node/v23.7.0/bin/npm
  pnpm: 10.13.1 - /usr/local/bin/pnpm
  bun: 1.1.20 - ~/.bun/bin/bun
npmPackages:
  @auth/unstorage-adapter: ^2.0.0 => 2.10.0 
  next: latest => 15.4.5 
  next-auth: beta => 5.0.0-beta.29 
  react: ^18.2.0 => 18.3.1 

Reproduction URL

https://github.com/jeengbe/nextauth-reverse-proxy-bug

Describe the issue

While createActionUrl respects proxy headers such as x-forwarded-host, the signin URLs for providers themselves do not: https://github.com/nextauthjs/next-auth/blob/main/packages/core/src/lib/utils/providers.ts#L28

In the repro, signing in with Google links to http://0.0.0.0:3000/auth/... (since that's what Next.js is bound to), and running in Kubernetes (where a reverse proxy is the norm), you get links like https://cms-app-658c44c5d9-m2wx5:8080/api/auth/signin/google

How to reproduce

see repo

Expected behavior

It should also respect proxy headers.

This affects basically all .origin uses, e.g. https://github.com/nextauthjs/next-auth/blob/main/packages/core/src/index.ts#L194

While it does work with AUTH_URL, that is not something that should be required to configure, given the information is there at runtime (headers).

Start with packages/core/src/lib/utils/providers.ts, where provider sign-in URLs are built, and compare it with createActionUrl and the logic referenced in packages/core/src/index.ts. Reproduce the issue using the linked repository behind a reverse proxy, then verify that provider URLs and other affected .origin uses respect proxy headers without requiring AUTH_URL.