Starlette behind reverse proxy, url_for does not return externally valid urls

Suppose I have my application reverse proxied, such that http://my.domain/foo (e.g. Traefik or Nginx) has the upstream http://localhost:8000 (the Starlette app). url_for will generate urls relative to the upstream url. How do I make it produce urls relative to the external url (with prefix /foo)?

For other projects, e.g. Flask/Werkzeug, this can be handled by setting SCRIPT_NAME in the request environment (scope) if the ‘X-Forwarded-Prefix’ header is set. It does not look like there is an equivalent option in Starlette.

Am I missing something? Otherwise, consider this a feature request.

[sm-Fifteen]

I really just think Starlette shouldn’t be trying to generate URLs with authority, here. The rule seems to be that, unless the authority you want to use is known to you in advance (such as having it stored in an environment variable or a config file), you should just specify the URL as an absolute path and leave it at that. It’s what Flask’s url_for does and what Django’s HttpRequest.build_absolute_uri(location) does if you omit the location parameter. Now that root_path is properly acknowledged (see #690), URLs generated this way should be valid for a client making requests to a Starlette app from behind proxy.

Trying to automatically build URLs with authority using the routing headers like that is just going to lead to issues. One should still be able to build authority-form URLs, but it shouldn’t be the default.

[jonathanunderwood]

I am confused as to why this, and #592 , have both been closed, @tomchristie - do you regard this as something that should not be fixed in Starlette?