Bug#298254: Diskless systems aren't random enough

[Tim Phipps <tim@phipps-hutton.freeserve.co.uk>]

I tracked this down on the KDE bug system and a temporary workaround is to put

RandomDevice=/dev/urandom

in the [General] section of /etc/kde3/kdm/kdmrc.

The reason for the failure to start is that by default /dev/random is used and 
that will block if there isn't enough randomness available. /dev/urandom will 
not block but will return less random results. I expect it's kdm that is 
trying to generate a random XAuth number before starting X which is why the 
XServer isn't even run and tweaking the timeouts won't help.

I don't know where to place the blame for this bug. /etc/init.d/urnadom sets 
up /dev/random to have some randomness at startup but a quick test on my 
system shows that it is easy to use up all the random numbers and they don't 
get replenished until I type or move the mouse. This script indicates how 
much randomness is available:

while true
do
  dd if=/dev/random of=/dev/null bs=512 count=1
done

Performing a find on a locally mounted disk gives around 10 bytes/s, waggling 
the mouse gets about 30 byte/s, find on the NFS mounted root disk gives 0 
bytes/s. Maybe that is the bug, if the NFS system or the networking stuff 
could add to the randomness this bug wouldn't happen. I'm using TCP for NFS 
and I used to use UDP and they both stop kdm starting.

If you have a local disk then a "find $mount-point > /dev/null &" just 
before /etc/init.d/kdm might help or just stick to /dev/urandom.

I think this is either a Debian bug in not getting enough randomness in the 
system at startup or a Linux bug in not generating randomness from the 
network.

Cheers,
TIm.