How to read the MAC address of a peer that broadcast UDP packet

Question

Hi folks. I need to initialize an W5500 equipment. To do that , the configuration equipment will send an UDP broadcast packet to a certain receiver port , with a certain string and data, to trigger the w5500 equipment to initialize. The problem is that, to answer the initialization request, the w5500 needs to know the MAC address of the sender. With that the w5500 may be initialized to point the answer through the SEND-MAC Sn_DHAR . Using ARP method works only if the w5500 IP is in the same subnet of the sender. Using SEND-MAC command with Sn_DHAR filled with MAC address of the sender would work with any configured SIP of w5500. The question is: how to obtain the sender MAC address to fill Sn_DHAR? Is it possible or the sender should insert its own MAC address in the data field? That , although possible , is not desirable since I want to send the initializing command through a Linux socat manual procedure.

Eugeny · Answer

maurelio: the w5500 needs to know the MAC address of the sender The question is why your W5500 needs to know it, and why IP address is not enough. In general, if you need to know MAC, the information which is not route-able, you can supply it within the packet data.

Eugeny · Answer

Thank you so much for your knowledge sharing! maurelio: I spent a lot of time analyzing network malfunction until I discovered that my own MAC address was MULTICAST. I also caught it, and several people here on the forum did it too. maurelio: Configuring the SPI properly to mode 1 or setting the SCLK pin to level 0 during 5~10ms , the RST signal goes low reseting W5500. Very interesting idea. maurelio: UDP broadcast packet to port XXXX with ASCII trigger string , password, and data is sent through network Do not forget about security and safety. Not sure it is good idea sending sensitive information openly, and using broadcast. As I understand you make a kind of DHCP clone, but with slightly different functionality. I propose perform handshaking first, without using any sensitive information, and then use another type of (secure) connection to pass sensitive info.

maurelio · Answer

Eugeny: Do not forget about security and safety. Not sure it is good idea sending sensitive information openly, and using broadcast. As I understand you make a kind of DHCP clone, but with slightly different functionality. I propose perform handshaking first, without using any sensitive information, and then use another type of (secure) connection to pass sensitive info. There is a more secure channel for changing parameters. The initial setup is the problem since the equipment does not have any keyboard or display. In fact the equipment has 3 interfaces: 1- the initial setup (UDP server), used for initial setup or maintenance. In private networks. It also needs a password (clear text) but in private network. Maybe even on a table with a cable to a switch and/computer. 2- another TCP server interface for managing most of commands through a computer or App. Private network. Home network or condominium reception. Used with password (clear text) but in private network. 3- TCP client interface to connect to an internet cloud server with AES256 encryption. No password. Authentication is done by the AES encryption and a pre-provisioning of the equipment with the server inserting a particular and unique key/initial vector for the device. This is far more secure. And since the connection is started by the device (not answered by it) it would be very difficult to send hacked commands to it. The initial setup will be simplified as soon as I put the DHCP and DNS to work. Most of configuration would be solved without any initial setup with the default values of DHCP and DNS turned on.