On starting (and stopping) rngd

[Thorsten Glaser]

Hi *,

I’m copying this eMail to those who requested various starting
methods for rngd and those who can probably help me with it.

Background: I took over the heavily patched 2.x series of
rng-tools as “rng-tools-debian”, which is currently started
from a sysvinit script only.

Now I have got requests for starting it different ways:

• my own arngc (not packaged in Debian itself) starts it as
  client of an stunnel (for entropy distribution over the
  network), so it must avoid starting rngd from the init
  script… but might make use of stopping at system shutdown?
  (arngc is always started manually or late, it needs network
  and possibly additionally VPN set up)

• some people wish for a systemd script

• according to some people, the /dev/hwrng device sometimes
  was not online when rngd was started, so we need a way to
  wait for that (the submitter suggested a systemd unit with
  multiple delayed restart attempts, which is… too crude)

• some people use a HWRNG that only comes online after some
  activity (e.g. plugging in a USB device, or powering up
  the WLAN chip), and so want it started from udev (but how
  to stop in that case?)

My first question is to rngd upstream: does it, at all, make
sense to start rngd multiple times? If so, how would I best
manage that? If not, how would we best deal with possibly
having multiple entropy sources?

Then I need init system-related help. I need to write a
systemd unit and a udev start script; I’ll probably factor
out what the sysvinit script does and put that into a new
shell script and start it from both sysvinit and systemd
unit and document that this is the script to start from
one’s udev rules somewhere?

I also need information on how I can delay the rngd start
to occur after udev has set up /dev/hwrng but not if there
is nothing to set up for it, that is, no source, or already
set up earlier / built into the kernel.

Then, I have a report in #466946 indicating that stopping
rngd on system shutdown might not be needed; I asked on
http://www.chiark.greenend.org.uk/pipermail/debian-init-diversity/2020-November/thread.html#3753
for comments on that. On one hand, not stopping rngd until
the bitter end ensures it stays available; on the other
hand, it can negatively affect things like umounting,
cryptsetup unconfiguration, etc; in fact, on my own systems,
I always get “process still running” messages from insserv(?)
for the rngd instances started from arngc (see above).

So I was considering letting the init script stop all rngd
instances (a simple pkill would do the trick, I assume) on
system shutdown, but don’t know if that’s a good idea and
if it would work with systemd.

There’s also the question whether/how to stop the instances
started by udev.

The question of whether to allow multiple rngd processes
(see above) also comes into play.

Finally: yes, I’m aware of rng-tools5 and its version 6.x,
but the version “traditionally shipped with Debian” contains
a lot of new functionality that never made it upstream and
as such has many users; arngc, for example, requires this
functionality, as do others (cf. #951799).

Thanks in advance,
//mirabilos
-- 
22:20⎜<asarch> The crazy that persists in his craziness becomes a master
22:21⎜<asarch> And the distance between the craziness and geniality is
only measured by the success 18:35⎜<asarch> "Psychotics are consistently
inconsistent. The essence of sanity is to be inconsistently inconsistent