Don't use rc.local. It has been obsolete since 1983. #FreeBSD has actually discontinued it twice in its history.
FreeBSD has had Mewburn rc for most of this century. It's what #NetBSD uses and #OpenBSD has a similar but NIH system. At minimum, use Mewburn rc scripts.
http://jdebp.info/FGA/rc.local-is-history.html
The rc(8) manual page has an example script. There's tutorial doco as well.