[staticassertion]

OK and presumably it does that by taking a request for an address A and rewriting part of it so that it actually goes to B with some additional headers to indicate how to forward it.

edit: Apparently I've hit my HN rate limit so I can't reply! Thanks Dang.

As for the proxying, obviously something has to rewrite the address that the request is going to. Depending on the type of proxy (ex: HTTP CONNECT) you may also have an x-forwarded-by header set. It sounds like Chrome never allowed you to do this manually, cool TIL.

[TedDoesntTalk]

I mean, you are just completely wrong. It does not rewrite the request. It does not change headers. There is an API that allows the extension to set a proxy server destination:

https://developer.chrome.com/docs/extensions/reference/proxy...

And that API does not change the request, either. That is not how proxying works.