On Fri, Feb 22, 2008 at 09:28:35AM +0100, Oliver Fromme wrote:
> Bartosz Giza wrote:
> > I have found quite interesting feature on one of router that lately i have
> > taken to administer.
> > What i knew was that file /var/run/dmesg.boot holds data from kernel buffer
> > that is taken right after file system(s) are mounted.
> > Lately i have found that one router writes to this file data from kernel
> > buffer when system is going to reeboot. Below are few lines from this file.
> > What you can see are lines from kernel right before reeboot. I have never seen
> > before such lines in this file. And this is quite interesting. Could anyone
> > tell me how can i achieve such funcionality on other systems ? I have tried
> > to find on google about this but i couldn't find anything similar to this.

> Upon a reboot, the kernel is usually loaded to the same
> physical addresses in RAM where it was before, so the
> dmesg buffer will be at the same location, too (unless
> you built a new kernel, of course). So all the contents
> from before reboot are still there -- *IF* the system
> BIOS didn't clear the RAM. Then the old contents will
> end up in /var/run/dmesg.boot, too.
> You could try looking at your BIOS setup. Some have an
> option called "Quick POST" or similar. If you enable
> it, the BIOS will skip the RAM test (which is rather
> useless anyway) which clears the RAM. It might help,
> but it depends very much on your mainboard and BIOS.

There is also kern.msgbuf_clear. However, this is a sysctl, which
means if set to 1 in /etc/sysctl.conf, you'd lose your dmesg output
after the OS had started. Bummer.

It would be useful if there was a loader.conf variable which was the
equivalent of msgbuf_clear. In fact, I'm wondering why the message
buffer isn't cleared on shutdown/immediately prior to reboot...

Interesting tidbit: We have one production machine which when booted
into single-user via serial console for a world install, retains all of
the output from that single-user session even once rebooted and brought
back into multi-user mode. This poses a substantial security risk,
especially during the mergemaster phase (we can discuss why if anyone is

| Jeremy Chadwick jdc at parodius.com |
| Parodius Networking http://www.parodius.com/ |
| UNIX Systems Administrator Mountain View, CA, USA |
| Making life hard for others since 1977. PGP: 4BD6C0CB |

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