kernel: Entropy device is blocking

Joerg Sonnenberger joerg at
Fri Jan 28 07:36:35 PST 2005

On Fri, Jan 28, 2005 at 09:57:49PM +0800, sam wrote:
> Every now and then when I freshly install FreeBSD, I sometimes get an 
> error message saying "kernel: Entropy device blocking" when I turn on 
> sshd in rc.conf file. I have search around in google but coudln't find a 
>  right fix for it.

The problem is that during boot, only a small amount of entropy
is generated. Since sshd depends on full random bits, it can't
and doesn't the normal /dev/urandom. /dev/random can block, if
not enough random bits are available.

I'm not sure what triggers the omission of a normal entropy
level for you. It could happen in a low-irq environment like
pure ramdisks or so, it might be enough to just create some process
doing nothing. Very difficult to say. One idea to find out the 
amount of entropy needed is changing the rc.d script to run
sshd under ktrace and check for the read calls to /dev/random.


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