Behavioral Differences: Proxy Server vs. Firewall

The other day, an HTTP request sent to the outside via a firewall came back with a 403 error.

I assumed the firewall itself was returning the 403, but it turned out to be coming from an AWS ELB on the other side of the internet.

When I mentioned this to a senior colleague, they explained that a firewall simply drops packets — so if a request is blocked by a firewall, the HTTP request would time out. On the other hand, if a proxy server blocks it, you would get an HTTP status like 403 or 500. That made perfect sense to me, and I learned something I had never really thought carefully about before.

So, when you are sending HTTP requests to the outside through a firewall in your own network and you get an HTTP error back, you can conclude that the firewall is not the one causing the error (a WAF might respond with an HTTP error, but a plain firewall should behave as described above).