net: WriteMsgUDPAddrPort should accept IPv4-mapped IPv6 destination addresses on IPv4 UDP sockets #74737
Description
Go version
all versions since go1.18
Output of go env in your module/workspace:
see aboveWhat did you do?
Create an IPv4 UDP socket listening on 0.0.0.0. Call WriteMsgUDPAddrPort with an IPv4-mapped IPv6 destination address.
What did you see happen?
write udp4 0.0.0.0:48509->[::ffff:192.168.0.162]:20221: address ::ffff:192.168.0.162: non-IPv4 address
What did you expect to see?
The write succeeds, just like WriteMsgUDP.
Back when Go 1.18 was just out, I opened #52264 for sending to IPv4 addresses on IPv6 sockets.
This issue is about the other way around: sending to IPv4-mapped IPv6 addresses on IPv4 sockets. It's less common, as IPv4 UDP sockets are less used compared to dual-stack IPv6 sockets. But it's just as painful, as APIs like net.DefaultResolver.LookupNetIP happily return IPv4-mapped IPv6 addresses.
I'm sending a CL to address it.