After yet another question asking how to bring hostname-awareness for proxying purposes to a protocol that doesn't have any support for it, is it time for a canonical question on hostname-aware proxying, plus a good answer on how DNS works, the few protocols that are hostname-aware and how they do it, and the impossibility of a general solution for all other protocols? I'm happy to take a stab at a short question, and longer answer, but wanted to take your minds on whether it was desirable before so doing.

Edit: thanks to HBruijn and upvoters for support, and especially to kasperd for some excellent suggestions. The QA now appears here, and people should feel free to amend/vote/mock as appropriate. I'll add it to the canonical canonical answer shortly.