Ask HN: Does this exist? Hardware to inject html into http requests.

I was talking with a friend today about a project he was working on and was stumped by a question he had. He asked if there existed any sort of hardware which would let you intercept all HTTP requests made over a network which could also potentially add/inject custom html into whatever page was requested. It also seemed like he wanted it to be programmatic as well. Meaning based on the url, he wanted to inject different things. We talked about a few ways to create one (Arduino kit, Custom Router firmware, etc) but I wasn't sure if anything already existed. It sounded like a tiny pc with 2 network jacks sitting in between the internet and the WAN port on a router could work. Thoughts?