It is possible to make DHCP requests to a server outside of one's LAN*, but only because DHCP allows you to specify your MAC address inside of your request. The DHCP request looks a bit like "My MAC address is 00:00:00:00:00:00. Please give me an IP address." In other words, the client must explicitly place the MAC address in the DHCP request. There's no IP or UDP (or even TCP) solution to your problem.

You can do the same as DHCP — have the clients send their MAC address along with their request — with your UDP-based protocol if you're still developing the protocol.

* — That requires the co-operation of the gateway, but only because the machine doesn't have an IP address yet.

In reply to Re^3: (OT) Getting the source hardware address of received UDP packets? by ikegami
in thread (OT) Getting the source hardware address of received UDP packets? by Anonymous Monk

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