Send and receive IPv6 link-local multicast UDP datagrams in Python?

Question

This following is a straightforward IPv4 UDP broadcast, followed by listening on all interfaces.

sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
sock.setsockopt(socket.SOL_SOCKET, socket.SO_BROADCAST, True)
sock.bind(("", 1337))
sock.sendto("hello world", ("255.255.255.255", 1337))
while True:
    data, addr = sock.recvfrom(0x100)
    print "received from {0}: {1!r}".format(addr, data)

I want to adjust this to send and receive both IPv4 and IPv6.

I've poked around and read as much as I can and believe the following is roughly the route I need to take:

1. Create an IPv6 socket.
2. Add the socket to a link or site local multicast group.
3. Send the UDP packets to the multicast address for the group in use.

Further info I have is that I may need to bind to several interfaces, and tell the socket using setsockopt() that it should also receive multicast packets. Lastly getaddrinfo() can be used across the board to gracefully "drop back" to IPv4 where IPv6 isn't available.

I have much of this implemented, but stumble primarily on the multicasting parts. A complete code example in Python, or vivid description of the constants and addresses needed are preferred.

Answer 1

Here's a link to python mcast demo, does both IPv4 and IPv6.

Answer 2

I'm currently asking a question over here that involves getting the multicast address of a received message, but the source code answers your question!

To listen:

# Initialise socket for IPv6 datagrams
sock = socket.socket(socket.AF_INET6, socket.SOCK_DGRAM, socket.IPPROTO_UDP)

# Allows address to be reused
sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)

# Binds to all interfaces on the given port
sock.bind(('', 8080))

# Allow messages from this socket to loop back for development
sock.setsockopt(socket.IPPROTO_IPV6, socket.IPV6_MULTICAST_LOOP, True)

# Construct message for joining multicast group
mreq = struct.pack("16s15s".encode('utf-8'), socket.inet_pton(socket.AF_INET6, "ff02::abcd:1"), (chr(0) * 16).encode('utf-8'))
sock.setsockopt(socket.IPPROTO_IPV6, socket.IPV6_JOIN_GROUP, mreq)

data, addr = sock.recvfrom(1024)

and to send:

# Create ipv6 datagram socket
sock = socket.socket(socket.AF_INET6, socket.SOCK_DGRAM)
# Allow own messages to be sent back (for local testing)
sock.setsockopt(socket.IPPROTO_IPV6, socket.IPV6_MULTICAST_LOOP, True)
sock.sendto("hello world".encode('utf-8'), ("ff02::abcd:1", 8080))

This is for python3.6, with python 2.7 I don't think the encodes are necessary. Also in the struct.pack line, I've seen variations of "16s15s" such as "4s", but I don't know what it is and what I have worked for me!