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I've managed to find one case, the net-snmp package uses MSG_PEEK in the SMUX subagent system, heaven knows why. But since the smux socket is practically always bound to localhost, that is not a problem.

As far as I can tell, a packet from a non-localhost source may be received, but (obviously) the udp_recvmsg does not get called since the system call is not triggered.

Unfortunately, there is a large amount of devices such as home routers, televisions, phones, etc, that are completely impossible to update. So, being aware of cases of software that actually use MSG_PEEK might be useful.