Expected Behavior

Deployed a flask application which is reachable over multiple domains and ports:

• external via load balancer: client - Host: example.org -> LB -> flask app
• internal via DNS service discovery without load balancer: client - Host: instance-1231.example.org -> flask app

If the client connects directly (Host: instance-1231.example.org) the app should be able to return absolute and stable URLs like http://example.org/path/to/my/view as the URL (http://instance-1231.example.org/path/to/my/view) with the internal DNS name is ephemeral.
Therefore I configured the SERVER_NAME config key and url_for generates the intended absolute URL by using _external=True within and without request context. But the app should be still able to route requests coming with Host: instance-1231.example.org.

Actual Behavior

Flasks creates the werkzeug.routing.MapAdapter with server_name=app.config['SERVER_NAME'] and therefore no view method will match to incoming requests with Host: instance-1231.example.org.

Environment

• Python version: 2.7.13 (I'm sorry)
• Flask version: 1.0.2
• Werkzeug version: 0.14.1

Applied workaround:

Overwrite Flask.create_url_adapter and create MapAdapter for request context without server_name parameter. Routing and URL generation works fine.