I have a page that covers this at
http://home.roadrunner.com/~computertai ... djail.html. (Just search for jail.conf on the page, it's around the middle of it.)
The man page, even for 9.x, states that you should use
/etc/jail.conf, but in practice, I found that it seemed to be ignored. (I didn't investigate it thoroughly, it worked with /etc/rc.conf, so that's what I did.).
However, in 10.x, it's apparently necessary to use
/etc/jail.conf. This has been my experience, and again, not heavily tested. My procedure was to try the conf file in 9.x, find it didn't work, put the lines in
/etc/rc.conf, and continue to use that in 9.x. On FreeBSD-10, once again tried with
/etc/jail.conf, found that it worked, with
as the only jail reference in
/etc/rc.conf, and that was the extent of my testing.