How to get MAC address of Ethernet interface of a UDP socket? - sockets

Question

I want to create a UDP server.
The server has several network interfaces : I want each interface to respond even if it is not in the same subnet as clients (clients send broadcast Ethernet packets).
How to do it ?
// The UDP server receives the client's request but fails to respond if it is not in the same subnet mask.
func udp_server(local_interface_ip net.IP){
p := make([]byte, 2048)
addr := net.UDPAddr{
Port: 1234,
IP: local_interface_ip,
}
ser, err := net.ListenUDP("udp", &addr)
if err != nil {
fmt.Printf("Some error %v\r\n", err)
return
}
for {
packetLen, remoteaddr, err := ser.ReadFromUDP(p)
if err != nil {
fmt.Printf("Some error %v", err)
continue
}
fmt.Printf("[RX %v] message(len : %d) : %s\r\n", remoteaddr, packetLen, p)
sendResponse(ser, remoteaddr)
}
}
func sendResponse(conn *net.UDPConn, addr *net.UDPAddr) {
data := []byte("1234")
_, err := conn.WriteToUDP(data, addr)
if err != nil {
fmt.Printf("[TX] Couldn't send response to %v (%v)\r\n", addr, err)
}
}
Then, for the server response, I want to put in the UDP data the MAC address of the server interface: How to do?
// This function get all MAC Address of the server but to know what is the good interface ?
func getMacAddr() ([]string, error) {
ifas, err := net.Interfaces()
if err != nil {
return nil, err
}
var as []string
for _, ifa := range ifas {
a := ifa.HardwareAddr.String()
if a != "" {
as = append(as, a)
}
}
return as, nil
}
It is a custom protocol based on UDP which is imposed to me which allows the clients to be able to discover IP address and MAC address of server interfaces.