[systemd-devel] Why we need to read/save random seed?

[cee1]

2015-06-17 23:38 GMT+08:00 Reindl Harald <h.reindl at thelounge.net>:
>
>
> Am 17.06.2015 um 17:08 schrieb cee1:
>>
>> 2015-06-17 22:03 GMT+08:00 Lennart Poettering <lennart at poettering.net>:
>>>
>>> On Wed, 17.06.15 20:21, cee1 (fykcee1 at gmail.com) wrote:
>>>>
>>>>
>>>> What I means is:
>>>> 1. Load a saved seed to /dev/urandom.
>>>> 2. The service read /dev/random, which will block until kernel thinks
>>>> there's enough entropy - then the Random Number should be good?
>>>> 3. Save the random number returned in step 2 on disk.
>>>
>>>
>>> Blocking at boot for this doesn't really sound like an option. But the
>>> kernel does not provide us with any nice notifications about when the
>>> RNG pool is complete. If we want to do this kind of polishing, then
>>> that'd be great, but we'd need sane notifiers for that, blocking
>>> syscalls are not an option.
>>
>>
>> That don't mean blocking boot, but a service, let's say
>> systemd-random-seed.service:
>> 1. systemd-random-seed.service loads a seed from disk to /dev/urandom
>> 2. systemd-random-seed.service tells systemd "I'm ready" (sd_notify())
>> 3. Instead of quitting immediately, systemd-random-seed.service tries
>> to read /dev/random, and it blocks ...
>> 4. systemd-random-seed.service at last gets a 'good random number',
>> and saves it on disk
>
>
> * the purpose of systemd-random-seed.service is to seed
>   /dev/random realy at boot so that other services like
>   sshd, vpn, webservers have a random source

First it seeds /dev/urandom
Second, seed /dev/random will not increase the entropy without using
ioctl (please see
https://www.mail-archive.com/systemd-devel@lists.freedesktop.org/msg32555.html)

Though, some other services may read /dev/random, and the suggested
logic may exhaust the very little entropy, hence blocks "those other
services"?

May use getrandom(as mentioned in http://www.2uo.de/myths-about-urandom):
"""
This syscall does the right thing: blocking until it has gathered
enough initial entropy, and never blocking after point.
"""



-- 
Regards,

- cee1