I just updated to a recent -current and tried the new nullfs. Sockets (e.g. the MySQL one) work now with nullfs. No need to have e.g. jails on the same FS and hardlink the socket to not need to use TCP in MySQL (or an IP at all for the jail).
I want to use a socket (mysql) in multiple jails. Using nullfs to mount it in each jail where you want to use it does not work (bug in nullfs?). Using a hardlink (if the jails reside on the same FS) works. I hope this info safes a little bit of time in case someone tries to do the same.
The hostname MySQL sees in such a connection is “localhost”. Maybe this is obvious to one or another, but because jails don’t have a 127.0.0.1 it was not obvious to me.