>> Also, it would be nice if you could ask the kernel what address an
>> incoming UDP packet had "connected" with.

> You can.

> You have to set a socket option and then use recvmsg(2). You get an
> additional sockaddr in the control structure.  I can never remember
> the name of the option, or I'd point you right at it.

UTSLing makes me think it's IP_RECVDSTADDR, at level IPPROTO_IP.
Search for RECVDSTADDR in /sys/netinet if you want to see for yourself.

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