Tuesday, June 8, 2004

Java uses /dev/random; may block forever creating SSL connections

The software that we're developing creates SSL connections when it starts up, and it does so at S13 (has to be after network, but before other services start). The result is that on an NFS booted Linux machine, it sits there forever, and never completes the connection.

Clue #1: if you move the mouse or type on the machine's keyboard, eventually the connection will complete.

Of course the reason for it hanging is that Java is using /dev/random to generate the keys for the SSL connection. And /dev/random gets all of its entropy from the physical environment, and refuses to return random values until it gets some input from the outside world.

We don't see this on a machine that boots from disk; I assume that /dev/random gets entropy from the interaction with the drive, via interrupts and so forth. For some reason the network activity doesn't yield the same entropy data, or at least not enough.

I found this article that discusses the usefulness of /dev/random given its current design.

In order to work around this, we decided to use /dev/urandom. We could do this by a link in the file system, but a much superior solution is to set the following system property in Java:


Now all you have to worry about is attacks against your SSL connection from those who know that you are using the pseudo-random number generator...