Fixing Flask url_for when Behind mod_proxy

Imagine you have an Apache server set up with a virtualhost configuration listening to some domain, e.g.


You have a Flask app running at:


You want to add a mod_proxy directive so that:

http://my.domain:8080/app_location/  --->  http://localhost:5000

Most resources will suggest the following:

ProxyPass /app_location/  http://localhost:5000/
# Wrong:
ProxyPassReverse /app_location/ http://localhost:5000/

However, you should follow the advice from this post and use:

ProxyPass /app_location/ http://localhost:5000/
ProxyPassReverse /app_location/ http://my.domain:8080/app_location/

The first line remains the same (forward all requests at location app_location to this server over there), but the ProxyPassReverse-line changes changes (ProxyPassReverse will do a string replace in the HTTP Location headers). For Flask, however, you will have to use http://my.domain:8080/app_location/ as the target and not just http://my.domain:8080/ as the blog post above suggests.

Next up, you’ll might notice that url_for is not working as expected, and is rewriting URLs without the app_location appended to it, or using localhost and not my.domain. Solution as inspired by this snippet. First, create a class as follows:

from flask import Flask, flash, render_template, redirect, url_for, session, request, send_from_directory, abort
from flask import Response, Blueprint
from werkzeug.serving import run_simple
from werkzeug.wsgi import DispatcherMiddleware
import datetime

class ReverseProxied(object):

    def __init__(self, app, script_name=None, scheme=None, server=None): = app
        self.script_name = script_name
        self.scheme = scheme
        self.server = server

    def __call__(self, environ, start_response):
        script_name = environ.get('HTTP_X_SCRIPT_NAME', '') or self.script_name
        if script_name:
            environ['SCRIPT_NAME'] = script_name
            path_info = environ['PATH_INFO']
            if path_info.startswith(script_name):
                environ['PATH_INFO'] = path_info[len(script_name):]
        scheme = environ.get('HTTP_X_SCHEME', '') or self.scheme
        if scheme:
            environ['wsgi.url_scheme'] = scheme
        server = environ.get('HTTP_X_FORWARDED_SERVER', '') or self.server
        if server:
            environ['HTTP_HOST'] = server
        return, start_response)

Then set up your app using:

app = Flask(__name__)
app.wsgi_app = ReverseProxied(app.wsgi_app, script_name='/app_location')