There are two schools of thought on how to start xdm. One school starts xdm from
/etc/ttys using the supplied example, while the other
simply runs xdm from rc.local or
from a X.sh script in /usr/local/etc/rc.d.
Both are equally valid, and one may work in
situations where the other doesn't. In both cases the result is the
same: X will popup a graphical login: prompt.
The ttys method has the advantage of documenting which vty X will start on and passing the responsibility of restarting the X server on logout to init. The rc.local method makes it easy to kill xdm if there is a problem starting the X server.
If loaded from rc.local, xdm should be started without any
arguments (i.e., as a daemon).
A previous version of the FAQ said to add the
vt you want X to use to the
/usr/X11R6/lib/X11/xdm/Xservers file. This is not necessary:
X will use the first free vt it finds.