bind(2) issue (cont.)

FreeBSD
w/ ipv4-mapped address
allows both sockets bound to same port number
ipv6 sucked ipv4 traffic by default if ipv4 socket is not bound
ipv6 do not suck ipv4 traffic by default
Note
{Net,Free}BSD are vulunerable with ipv4-mapped addresses
ipv4 application can "steal" ipv4 traffic which used to be delivered to ipv6 sockets.