Tell Flask it is Behind a Proxy

When using a reverse proxy, or many Python hosting platforms, the proxy will intercept and forward all external requests to the local WSGI server.

From the WSGI server and Flask application’s perspectives, requests are now coming from the HTTP server to the local address, rather than from the remote address to the external server address.

HTTP servers should set X-Forwarded- headers to pass on the real values to the application. The application can then be told to trust and use those values by wrapping it with the werkzeug:middleware/proxy_fix middleware provided by Werkzeug.

This middleware should only be used if the application is actually behind a proxy, and should be configured with the number of proxies that are chained in front of it. Not all proxies set all the headers. Since incoming headers can be faked, you must set how many proxies are setting each header so the middleware knows what to trust.

from werkzeug.middleware.proxy_fix import ProxyFix

app.wsgi_app = ProxyFix(
    app.wsgi_app, x_for=1, x_proto=1, x_host=1, x_prefix=1
)

Remember, only apply this middleware if you are behind a proxy, and set the correct number of proxies that set each header. It can be a security issue if you get this configuration wrong.