[daper]

This is a flag that the application has to explicitly set when receiving data. Looks like this flag has limited usage based on the description. I've checked with the strace tool few programs that use UDP on my Linux boxes: unbound resolver, ntpd, openvpn. None seems to use it.

MSG_PEEK This flag causes the receive operation to return data from the beginning of the receive queue without removing that data from the queue. Thus, a subsequent receive call will return the same data.

[aduitsis]

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.