Distributing entropy
2 minutes read

Back at the Debian barbeque party at the end of August, I got myself an EntropyKey from the kind folks at Simtec. It has been working so well that I haven’t really had a big need to blog about it. Plug it in and watch /proc/sys/kernel/random/entropy_avail never empty.

However, Collabora, where I am a sysadmin also got one. We are using a few virtual machines rather than physical machines as we want the security domains, but don’t have any extreme performance needs. Like most VMs they have been starved from entropy. One problem presents itself: how do we get the entropy from the host system where the key is plugged in to the virtual machines?

Kindly enough the ekeyd package also includes ekeyd-egd-linux which speaks EGD, the TCP protocol the Entropy Gathering Daemon defined a long time ago. ekeyd itself can also output in the same protocol, so this should be easy enough, or so you would think.

Our VMs are all bridged together on the same network that is also exposed to the internet and the EGD protocol doesn’t support any kind of encryption, so in order to be safe rather than sorry, I decided to encrypt the entropy. Some people think I’m mad for encrypting what is essentially random bits, but that’s me for you.

So, I ended up setting up stunnel, telling ekeyd on the host to listen to localhost on a given port, and stunnel to forward connections to that port. On each VM, I set up stunnel to forward connections from a given port on localhost to the port physical machine where stunnel is listening. ekeyd-linux-egd is then told to connect to the port on localhost where stunnel is listening. After a bit of certificate fiddling and such, I can do:

# pv -rb < /dev/random > /dev/null  
17.5kB [4.39kB/s] 

which is way, way better than what you will get without a hardware RNG. The hardware itself seems to be delivering about 32kbit/s of entropy.

My only gripes at this point is that the EGD implementation could use a little bit more work. It seems to leak memory in the EGD server implementation. Also, it would be very useful if the client would reconnect if it was disconnected for any reason. Even with those missing bits, I’m happy about the key so far.

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