The man page on rc.conf states that "AUTO" can be given to dumpon variable and it will save the coredump to first suitable swap device. I tried this today and it doesn't work - during boot, there's a line like "dumpon: 'AUTO' is not a valid device". By quick grepping in /etc/rc.d it seems that 'AUTO' is not implemented. So, this should probably be implemented or removed from man page :) (this is 5.4-PRERELEASE, apologies if it's fixed already)