[squarefoot]

> MSG_PEEK is only used AFAIK only in specific applications that need to "peek" at the message without consuming it

I never used the MSG_PEEK flag, but over 15 years ago (maybe it wasnt implemented yet?) I accomplished a similar task by using the SO_REUSEADDR and SO_REUSEPORT flags which worked nicely on Windows too. The goal was to have several UDP "agents" listening on the same port on the same machine (which would normally raise an EADDRINUSE error), all of them examining a packet and acting upon a header contained in that packet. All them would receive a copy of the packet on their respective socket (that was the flags purpose), so that they could filter it and act only if the header contained data relative to that agent. This allowed a very modular approach where once the protocol was defined I could compile only the agent I needed without touching the others.