--- Comment #3 from Mark G. <[hidden email]> ---
It looks something like this:
If the jail is not identified by jid there is a possible race in between
the lookup of the jail and executing the command inside the jail. Giving
a jid has a similar race as another process can stop the jail and start
another one after the user looked up the jid.
If a forwarding information base (FIB, routing table) has been set within
a jail using the exec.fib parameter, the jexec utility does not honour
it, and command will be executed using the default fib (usually fib 0).
This can confuse users of jails when trying to troubleshoot jail packet
flows. To properly execute a command in a fib-configured jail using the
jexec utility, it must be prefixed with the setfib(1) utility as follows.