Improve redirection handling (#62561)

Currently to redirect to another RSC route of the same app, we initiate
another request to that route. This PR changes that request to directly
use the internal hostname of the current server handler, rather than
relying on other headers.

Closes NEXT-2614

packages/next/src/server/app-render/action-handler.ts

@@ -179,9 +179,13 @@ async function createRedirectRenderResult(
     const forwardedHeaders = getForwardedHeaders(req, res)
     forwardedHeaders.set(RSC_HEADER, '1')
 
-    const host = originalHost.value
     const proto =
       staticGenerationStore.incrementalCache?.requestProtocol || 'https'
+
+    // For standalone or the serverful mode, use the internal hostname directly
+    // other than the headers from the request.
+    const host = process.env.__NEXT_PRIVATE_HOST || originalHost.value
+
     const fetchUrl = new URL(
       `${proto}://${host}${basePath}${parsedRedirectUrl.pathname}`
     )

packages/next/src/server/lib/start-server.ts

@@ -246,6 +246,7 @@ export async function startServer(
 
       // expose the main port to render workers
       process.env.PORT = port + ''
+      process.env.__NEXT_PRIVATE_HOST = `${actualHostname}:${port}`
 
       // Only load env and config in dev to for logging purposes
       let envInfo: string[] | undefined