Python: Getting an IPv6 socket to receive packets destined for the Subnet-Routers Anycast address

Question

How do you get a socket to receive packets destined for the IPv6 Subnet-Routers Anycast address?

I haven't been able to find any informationn on how to do this.

In a fit of desparation, I've tried using socket.setsockopt as you would to join a multicast group:

# 7 is the interface number 
s = socket(AF_INET6, SOCK_DGRAM)
packed_iface_num = struct.pack("I", 7) 
group = inet_pton(AF_INET6, 'fd36:d00d:d00d:47cb::') + packed_iface_num

# socket.error: (22, 'Invalid argument')
s.setsockopt(IPPROTO_IPV6, IPV6_JOIN_GROUP,  group)

And also using bind

# socket.error: (99, 'Cannot assign requested address')
s.bind(('fd36:773e:6b4c:47cb::', 9876))

As expected, neither of these worked. Is there a way to do this?

Answer 1

Instead of IPV6_JOIN_GROUP, try passing IPV6_JOIN_ANYCAST to your s.setsockopt() code. Unfortunately the Python socket module doesn't define it but you should be able to pass the integer equivalent instead. In Linux IPV6_JOIN_ANYCAST is 27 and IPV6_LEAVE_ANYCAST is 28. (defined in /usr/include/linux/in6.h)

The best documentation I could find is from this lkml e-mail describing the anycast patch to the Linux kernel:

The application interface for joining and leaving anycast groups is 2 new setsockopt() calls: IPV6_JOIN_ANYCAST and IPV6_LEAVE_ANYCAST. The arguments are the same as the corresponding multicast operations.

May the dancing kame be with you!