random(4): Attempt to persist entropy promptly
The goal of saving entropy in Fortuna is two-fold: (1) to provide early
availability of the random device (unblocking) on next boot; and (2), to
have known, high-quality entropy available for that initial seed. We know
it is high quality because it's output taken from Fortuna.
The FS&K paper makes it clear that Fortuna unblocks when enough bits have
been input that the output may be safely seeded. But they emphasize
that the quality of various entropy sources is unknown, and a saved entropy
file is essential for both availability and ensuring initial
In FreeBSD we persist entropy using two mechanisms:
- The /etc/rc.d/random shutdown() function, which is used for ordinary shutdowns and reboots; and,
- A cron job that runs every dozen minutes or so to persist new entropy, in case the system suffers from power loss or a crash (bypassing the ordinary shutdown path).
Filesystems are free to cache dirty data indefinitely, with arbitrary flush
policy. Fsync must be used to ensure the data is persisted, especially for
the cron job save-entropy which is oriented at power loss or crash safety.
Ordinary shutdown may not need the fsync because unmount should flush out
the dirty entropy file. But it is always possible power loss or crash
occurs during the short window after rc.d/random shutdown runs and before
the filesystem is unmounted, so the additional fsync there seems harmless.