Rewriting the server side

Port number space
What is the relationship of port number space between TCP over v4 versus TCP over v6?
Does bind(2) ordering matter?
Take the safest side, do not assume kernel behavior
getaddrinfo(AI_PASSIVE)
bind(2) to all addresses returned
Die only if all bind(2) fails

IPv6 wildcard bind...
may receive IPv4 traffic as IPv4 mapped addr (RFC2553/3493)
Leads to mistakes/vulnerabilities, low portability, don't use it
Use setsockopt(IPV6_V6ONLY, 1) to avoid mistake