antipaucity

fighting the lack of good ideas

improve your entropy pool in linux

A few years ago, I ran into a known issue with one of the products I use that manifests when the Red Hat Linux server it’s running on has a low entropy pool. And, as highlighted in that question, the steps I found 5 years ago didn’t work for me (turns out modifying the t parameter from ‘1’ to ‘.1’ did work (rngd -r /dev/urandom -o /dev/random -f -t .1), but I digress (and it’s no longer correct in CentOS 7 (the ‘t’ option, that is))).

In playing around with the Mozilla-provided SSL configurator, I noticed a line in the example SSL config that referenced “truerand”. After a little Googling, I found an opensource implementation called “twuewand“.

And a little more Googling about adding entropy, and I came across this interesting tutorial from Digital Ocean for “haveged” (which, interestingly-enough, allowed me to answer a 6-month-old question on Server Fault about CloudLinux).

Haveged “is an attempt to provide an easy-to-use, unpredictable random number generator based upon an adaptation of the HAVEGE algorithm. Haveged was created to remedy low-entropy conditions in the Linux random device that can occur under some workloads, especially on headless servers.”

And twuewand “is software that creates hardware-generated random data. It accomplishes this by exploiting the fact that the CPU clock and the RTC (real-time clock) are physically separate, and that time and work are not linked.”

For workloads that require lots of entropy (generating SSL keys, SSH keys, PGP keys, and pretty much anything else that wants lots of random (or strong pseudorandom) seeding), the very real problem of running out of entropy (especially on headless boxes or virtual machines) is something you can face quite easily / frequently.

Enter solutions like OpenRNG which are hardware entropy generators (that one is a USB dongle (see also this skh-tec post)). Those are awesome – unless you’re running in cloud space somewhere, or even just a “traditional” virtual machine.

One of the funny things about getting “random” data is that it’s actually very very hard to get. It’s easy to describe, but generating “truly” random data is incredibly difficult. (If you want to have an aneurysm (or you’re like me and think this stuff is unendingly fascinating), go read the Wikipedia entry on “Cryptographically Secure Pseudo Random Number Generator“.)

If you’re in a situation, though, like I was (and still am), where you need to maintain a relatively high quantity of fairly decent entropy (probably close to CSPRNG level), use haveged. And run twuewand occasionally – at the very least when starting Apache (at least if you’re running HTTPS – which you should be, since it’s so easy now).