fix: set x-forwarded-host based on request (#58500)

Co-authored-by: @brkalow <bryce@clerk.dev>

### What?

A number of our customers have been experiencing issues stemming from an
`x-forwarded-host` header that doesn't match the `host` header.

### Why?

[This PR](#57815) removes
functionality which sets `x-forwarded-host` to `req.headers['host']` and
relies solely on the server's hostname and port.

This can be seen locally when visiting the app via a localhost
subdomain.

The `x-forwarded-host` header will remain as `localhost:${port}` while
the actual requested host will contain the subdomain.

### Related 

- #57815 (comment)

---------

Co-authored-by: BRKalow <bryce@clerk.dev>
Co-authored-by: Zack Tanner <zacktanner@gmail.com>

packages/next/src/server/base-server.ts

@@ -910,7 +910,7 @@ export default abstract class Server<ServerOptions extends Options = Options> {
         )
       }
 
-      req.headers['x-forwarded-host'] ??= `${this.hostname}:${this.port}`
+      req.headers['x-forwarded-host'] ??= req.headers['host'] ?? this.hostname
       req.headers['x-forwarded-port'] ??= this.port?.toString()
       const { originalRequest } = req as NodeNextRequest
       req.headers['x-forwarded-proto'] ??= (originalRequest.socket as TLSSocket)

test/e2e/app-dir/x-forwarded-headers/x-forwarded-headers.test.ts

@@ -5,6 +5,7 @@ createNextDescribe('x-forwarded-headers', { files: __dirname }, ({ next }) => {
     const res = await next.fetch('/')
     const headers = await res.json()
     const url = new URL(next.url)
+
     expect(headers['x-forwarded-host']).toBe(url.host)
     expect(headers['x-forwarded-port']).toBe(url.port)
     expect(headers['x-forwarded-proto']).toBe(url.protocol.replace(':', ''))
@@ -14,4 +15,53 @@ createNextDescribe('x-forwarded-headers', { files: __dirname }, ({ next }) => {
       url.protocol.replace(':', '')
     )
   })
+
+  describe('host header exists', () => {
+    it('should include x-forwarded-* headers relative to host', async () => {
+      const url = new URL(next.url)
+      const reqHeaders = {
+        host: `subdomain.localhost:${url.port}`,
+      }
+      const res = await next.fetch('/', {
+        headers: reqHeaders,
+      })
+      const headers = await res.json()
+
+      expect(headers['x-forwarded-host']).toBe(reqHeaders.host)
+      expect(headers['x-forwarded-port']).toBe(url.port)
+      expect(headers['x-forwarded-proto']).toBe(url.protocol.replace(':', ''))
+      expect(headers['middleware-x-forwarded-host']).toBe(reqHeaders.host)
+      expect(headers['middleware-x-forwarded-port']).toBe(url.port)
+      expect(headers['middleware-x-forwarded-proto']).toBe(
+        url.protocol.replace(':', '')
+      )
+    })
+  })
+
+  describe('already assigned', () => {
+    it('should not override existing x-forwarded-* headers', async () => {
+      const url = new URL(next.url)
+      const reqHeaders = {
+        host: `subdomain.localhost:${url.port}`,
+        port: '1234',
+        proto: 'https',
+      }
+      const res = await next.fetch('/', {
+        headers: {
+          host: 'override.localhost',
+          'x-forwarded-host': reqHeaders.host,
+          'x-forwarded-port': reqHeaders.port,
+          'x-forwarded-proto': reqHeaders.proto,
+        },
+      })
+      const headers = await res.json()
+
+      expect(headers['x-forwarded-host']).toBe(reqHeaders.host)
+      expect(headers['x-forwarded-port']).toBe(reqHeaders.port)
+      expect(headers['x-forwarded-proto']).toBe(reqHeaders.proto)
+      expect(headers['middleware-x-forwarded-host']).toBe(reqHeaders.host)
+      expect(headers['middleware-x-forwarded-port']).toBe(reqHeaders.port)
+      expect(headers['middleware-x-forwarded-proto']).toBe(reqHeaders.proto)
+    })
+  })
 })