Sepherosa Ziehau has posted a note that V4-mapped addressing is no longer supported in DragonFly.  You will need to do a full buildworld/buildkernel if you are running master.  Also, TCP MTU path discovery is on by default.  Also also, he’s added a SOL_SOCKET/SO_CPUINT socket option for use to reduce load in heavy network activity.  As usual, I don’t quite comprehend.