# Push the old seed into the kernel, create a future seed  and create a seed
  # file for the boot-loader.
  random_seed() {
        dd if=/var/db/host.random of=/dev/random bs=65536 count=1 status=none
        chmod 600 /var/db/host.random
        dd if=/dev/random of=/var/db/host.random bs=65536 count=1 status=none
        dd if=/dev/random of=/etc/random.seed bs=512 count=1 status=none
        chmod 600 /etc/random.seed
  }

FreeBSD:

* https://svnweb.freebsd.org/base/head/libexec/save-entropy/

* https://www.freebsd.org/cgi/man.cgi?loader.conf(5) (see "entropy_cache_load")

OpenBSD's boot loader also injects entropy into the kernel.

This business of /var/ not being writable early enough is probably to do with the wonderful complexities of modern Linux, and is just not a problem that OpenBSD has.

They also do it at shutdown, before /var is R/O or unmounted.

One can also have multiple files, and have a regular cron job that replaces one of them occasionally while the system is running. This is what the FreeBSD script does.

* https://svnweb.freebsd.org/base/head/libexec/save-entropy/

* https://svnweb.freebsd.org/base/head/libexec/rc/rc.d/random

Other systems have solved this problem, so I'm not sure why all the high drama with Linux.

FreeBSD runs a regular cron job so that there are multiple files. If /var goes R-O, you still have a bunch of files from when it wasn't and they are not the same as on initial boot (assuming that (a) /var was mounted R-W at some point, and (b) cron managed to run as well).

Also, on shutdown there is an attempt to write 4096B to both /entropy and /boot/entropy:

* https://svnweb.freebsd.org/base/head/libexec/rc/rc.d/random?...

Further, all of these various seed files are not the only sources of entropy (timers, RDRAND, etc).

Not a good assumption if the initial r/w mount is the one that fails...

But for >99% of cases where the system comes up cleanly, and runs cleanly for a period of time, you'll have a bunch of seed files ready to go for the next boot. This configuration optimizes for the common case.

https://svnweb.freebsd.org/base/head/libexec/rc/rc.d/random?...

Similar idea to what OpenBSD does.