Proxy with Flask

Two Flask routes relay the script and events with the requests library.

Set it up

1. Install requests

   terminal

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   pip install requests

2. Add the routes to your app

   metricraven_proxy.py

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   # metricraven_proxy.py · Flask 2.2+ with requests
   import requests
   from flask import Flask, Response, request
   
   app = Flask(__name__)
   # Behind a load balancer or CDN? Trust one proxy hop so remote_addr is the visitor:
   # from werkzeug.middleware.proxy_fix import ProxyFix
   # app.wsgi_app = ProxyFix(app.wsgi_app, x_for=1)
   
   
   def relay(url, cache_control):
       headers = {
           name: request.headers[name]
           for name in ("Content-Type", "Origin", "User-Agent")
           if name in request.headers
       }
       headers["X-Visitor-IP"] = request.remote_addr or ""
       try:
           upstream = requests.request(
               request.method,
               url,
               headers=headers,
               data=request.get_data() if request.method == "POST" else None,
               timeout=15,
           )
       except requests.RequestException:
           return Response(status=502)
       return Response(
           upstream.content,
           status=upstream.status_code,
           headers={
               "Content-Type": upstream.headers.get("Content-Type", "application/octet-stream"),
               "Cache-Control": cache_control,
           },
       )
   
   
   @app.get("/a/script")
   def metricraven_script():
       return relay("https://metricraven.com/js/script.js", "public, max-age=3600")
   
   
   @app.post("/a/event")
   def metricraven_event():
       return relay("https://metricraven.com/api/v1/collect", "no-store")

3. Trust your load balancer, if any

   Behind Nginx, Gunicorn on a platform router, or a CDN, request.remote_addr is that proxy. Enable the ProxyFix lines with the number of proxies in front of Flask.

4. Point your script tag at your own paths

   Replace your MetricRaven tag with this one. src loads the tracker from your domain and data-api sends its events there too.

   Your website's shared layout

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   <script defer src="/a/script" data-api="/a/event" data-site-id="YOUR_SITE_ID"></script>

Notes

• If a CDN or load balancer sits in front of this server, the connecting address is that proxy, not the visitor. Use the visitor IP header it sets (and only one it overwrites, so visitors can't fake it).

How the proxy works and which headers it must send: Use your own domain.