Messages in this thread | | | Date | Sun, 14 Jun 2009 21:04:17 +0200 | From | Folkert van Heusden <> | Subject | Re: issue with /dev/random? gets depleted very quick |
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> [cc:ed to lkml]
> > On an idle system (no gui, no daemons, nothing) system, /dev/random gets > > empty in a matter of 20 seconds with a 2.6.26 kernel. > > My test: > > add 1000 bits to the device: > > zolder:/tmp# cat test-RNDADDENTROPY.c ... > > } > > and then check whayt is in it: > > zolder:/tmp# ./a.out ; while true ; do echo `date` `cat /proc/sys/kernel/random/entropy_avail` ; sleep 1 ; done > > 0 > > Sun Jun 14 14:50:44 CEST 2009 1117 ... > > Sun Jun 14 14:50:55 CEST 2009 157 > > Is there something wrong with it? > Does it go below 128? If not, that's the behavior of something depleting > the pool down to the anti-starvation threshold via either /dev/urandom > or get_random_bytes.
No, it stays above 128. Sometimes around 13x, sometimes 151, so not always close to 128.
> On my system, I'm seeing that behavior as well. fuser reports a bunch of > processes hold /dev/urandom open, but stracing them doesn't reveal a > culprit. Which means there's now probably something in the kernel > calling get_random_bytes continuously.
Yes. On the systems I tried, nothing had /dev/*random open, also no cronjobs that could use it. And still it gets lower.
> Is this a problem? It really shouldn't be. Everyone should be > using /dev/urandom anyway. And the anti-starvation threshold guarantees
Well, if I understood correctly how /dev/*random works, urandom is fed by /dev/random. So if there's almost nothing left in the main pool and urandom demands bits then we have an issue. Also, if you frequently want to generate keys (thing gpg, ssl), I think you want bits from /dev/random and not urandom.
> that if there's entropy being collected, readers of /dev/random can > always make forward progress.
Also if it is used so heavily, you need quit an entropy-source to keep it filled.
Folkert van Heusden
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