router: drop packets with invalid IPv4/6 destination address

[JordiSubira]

The router forwards packets relying on whatever the system call, i.e., unix.Sendmsg does with UDP messages with destination address 0.0.0.0. One could expect that the syscall returns an error, however, what it does is the following (based on a toy example I creted for trying this out):

• If internal BR address (or local_IP) uses IPv4 and destination address is 0.0.0.0: Sends a packet with destination_IP = local_IP
• If internal BR address (or local_IP) uses IPv6 and destination address is 0.0.0.0: Yields an error Error sending message: network is unreachable
• If internal BR address (or local_IP) uses IPv6 and destination address is [::]: Sends a packet with destination_IP = local_IP
• If internal BR address (or local_IP) uses IPv4 and destination address is [::]: Yields an error Error sending message: network is unreachable

(One can reproduce it using this simple code snippet https://gist.github.com/JordiSubira/4171baa5b0535878bebf4c93eb1688e5).

Based on an offline discussion, Anapaya's BR drops packets with 0.0.0.0, [::], loopback/localhost or ipv4-mapped-ipv6 destination addresses. Thus, the common conclusion is that we should reproduce this in the OS-BR.