Security: IPv4 mapped address

::ffff:127.0.0.1

Not distinguishable between the following two:
IPv4 packet goes into AF_INET6 socket (kernel translates the address)
IPv6 packet with ::ffff:127.0.0.1

Security holes
Malicious party could circumvent access control
Malicious party could make you generate unexpected IPv4 packet

Solution
Do not use IPv4 mapped address by setsockopt(IPv6_V6ONLY, 1)
Open AF_INET socket for IPv4