cool feature of dmesg.boot file
olli at lurza.secnetix.de
Fri Feb 22 09:53:02 UTC 2008
Jeremy Chadwick wrote:
> Oliver Fromme wrote:
> > 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...
So you can see panic messages and other useful things that
happened before the reboot. It's a _feature_, not a bug.
> 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
Oliver Fromme, secnetix GmbH & Co. KG, Marktplatz 29, 85567 Grafing b. M.
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start asking why software is ignoring 30 years of accumulated wisdom.'
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