# mount -t devfs devfs /var/jails/arepo/dev

# devfs -m dev rule -s 4 applyset
devfs rule: ioctl DEVFSIO_SAPPLY: No such process

# sudo devfs -m dev rule -s devfsrules_jail applyset
devfs rule: error converting to number: devfsrules_jail

# grep devfsrules_jail /etc/devfs.rules
[devfsrules_jail=4]