Finding the MAC address of the sender of a multicast UDP message in Python?

Question

I have some code that listens for "announcements" via UDP multicast. I can get the IP address of the sender, but what I really need is the MAC address of the sender (since the IP address can and will change).

Is there an easy way to do this in Python?

A code snippet is included for reference, but likely unnecessary.

sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM, socket.IPPROTO_UDP)

# Allow multiple sockets to use the same PORT number
sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)

# Bind to the port that we know will receive multicast data
sock.bind((self.interface, MCAST_PORT))

# Tell API we are a multicast socket
sock.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_TTL, 255)

# Tell API we want to add ourselves to a multicast group
# The address for the multicast group is the third param

status = sock.setsockopt(socket.IPPROTO_IP, 
          socket.IP_ADD_MEMBERSHIP, 
          socket.inet_aton(MCAST_ADDR) + socket.inet_aton(self.interface));

data, addr = sock.recvfrom(1024)

...

Answer 1

The protocol you need is ARP. Check this question/answer for details

Answer 2

You cannot, in general, get the mac address. You might succeed using ARP on a LAN, but across the Internet it's not possible.

Consider the case where the packet you receive has the IP address of the sender's NATting router. The packet may have traversed any number of intermediate machines along the way, each of which have mac addresses, too. Whose responsibility should it be to support the kind of lookup you're after? For all the machines along the way, the sender's mac address is completely useless, so why bother supporting that kind of lookup?

And, btw, changing the mac address is trivial on many network cards, so using it as some kind of unique ID is not a wise idea.

Answer 3

I'm not sure that it is possible to get the sender's MAC address since the MAC address is a link level address and not a network level address like IP. The MAC address will change at each hop in the network as the packet containing the UDP message is routed from the sender to the receiver.