Use `screen`

If the bug is a crash of the server, and it takes a long time for the crash to happen, perform the following steps:

Log in as root.
Open a screen session: $ screen bash.
Make sure FreeRADIUS is not running.
Make sure you have all the debug symbols about, or a debugable version of the server installed (one built with --enable-developer as above).
Configure screen to log to a file by pressing Ctrl+a, then h.
Type gdb /path/to/radiusd (or /path/to/freeradius on Debian).
At the (gdb) prompt, type run -X.
Detach from screen with Ctrl+a, d.
When you notice FreeRADIUS has died, reconnect to your screen session $ screen -D -r.
At the (gdb) prompt type where or for lots of info try thread apply all bt full.
Tell screen to stop logging, Ctrl+a, h.
Logout from screen.

Use screen