Impact of deployment topology on rate-limiting and trust proxy

The trust proxy setting is an important concept in backend development, especially when implementing rate-limiting in our APIs. But deciding its accurate value depends heavily on our deployment topology. When we deploy our application in production, the client may not talk directly to our backend. There may be 1 or more proxies in between who forward the request to the next proxy or the backend server. Those proxies can be Load balancers, API gateways, reverse proxy like nginx or any custom service. So effectively, our request has to do some 'hops' over these proxies to reach backend. When we implement rate limiting in app to prevent the DOS attack, we generally intend this rate limit on the basis of client IP address. And this works fine when client request reaches our backend directly. But when we have multi-hop architecture, the simple setup won't work as expected. Because the most recent IP will be of the proxy and not the client. So all the traffic coming from different users will be considered from the single client(our own proxy) and thus there will be false positives as the rate limiting will trigger much often. In this scenario, we must tell our backend to ignore these extra hops(i.e. to trust our proxies). This is done by specifying trust proxy. If there is 1 proxy between client-server we set trust proxy to 1; if there are 2, or more, we set it accordingly. This will ensure our express app skips(trusts) these IPs, and accurately figures out actual client IP. The originating IP address of client is identified from 'X-Forwarded-For' header by the express app. But setting trust proxy is not that straightforward. The numerical value for trust proxy will not work in every case. If there are different paths from which our request reaches backend, there is a chance that the number of proxies may be different in each path. For example - internal vs external traffic: External(public) traffic: (Client -> Web Application Firewall -> Load balancer -> Reverse Proxy ->