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Re: [vox-tech] magic sysreq key
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Re: [vox-tech] magic sysreq key

Thanks Mike, Bill, and Pete for some helpful comments. I actually found my answer with a google search (http://www.tldp.org/HOWTO/Remote-Serial-Console-HOWTO/security-sysrq.html). Here it clearly states that "Writing a 0 into /proc/sys/kernel/sysrq will disable the magic SysRq key", so my initial guess was correct; my default RH8 install disabled the magic sysrq keys!

It turns out there is a line in /etc/sysctl.conf which enables or disables magic sysrq:

kernel.sysrq = [0 or 1]

I now have magic sysrq enabled and have tested it succesfully from console with ctl-alt-h. Now I am ready the next time Linux freezes on me.


btw Pete: my init level 3 is multiuser mode without X. I had thought this was generally true, but I guess not from your comments. I now know that I can get a non-X console by simply pressing ctl-alt-f1, and can return to X with ctl-alt-f7. That is also useful to know.

Peter Jay Salzman wrote:
note: subject change

hi jonathan,

when not latexing or playing games, i'm usually not using X.

runlevels are not meaningful since they mean different things for
different operating systems. i'm guessing you don't use debian, so i
don't know what runlevel 3 means.

when doing anything of any importance, like trying to recover gracefully
from a kernel error, do it in console. don't run X. direct logging by
the kernel and modules may not be seen from within X. furthermore, X
can steal the key sequence like it does with ctl-alt-backspace.

i know *quite literally* nothing about KDE or GNOME other than you can
have cute little icons on your desktop, so i'm not sure of any gotchas
with kernel logging, magic keystrokes and desktop managers.

so try it in console.

one more question, and i hope this isn't insulting -- can you tell me
*precisely* the keystrokes you're hitting in excruciating detail?
something along the lines of:

i press down the alt key. while keeping the alt key pressed down, i
then press down "print screen". etc

lastly, have you read the part in sysreq.txt about "i press the sequence
and nothing happens"?


begin Jonathan Stickel <jjstickel@netscape.net>
When I try alt-sysrq-h (to display magic-sysrq help) in KDE, my cpu runs for awhile but nothing seems to happen. I wanted to avoid testing something more drastic, but since you asked, I entered init level 3 and tried a few (alt-sysrq-'h', 's', 'u', and 'b'). Nothing happens at all.


Peter Jay Salzman wrote:

redirected to vox-tech

begin Jonathan Stickel <jjstickel@netscape.net>

I was previously unaware of the magic sysreq keys. They do look useful, and I read the documentation as you suggest. From what I can tell, it is compiled in my kernel (CONFIG_MAGIC_SYSRQ=y), but I am wondering if it is disabled with the run-time command: "echo "0" > /proc/sys/kernel/sysrq", as suggested by the documentation. In fact, my /proc/sys/kernel/sysrq contains the single character "0". Where would I find this run time command? Can I just delete it or comment it out? Thanks,


have you tried to use sysreq and it didn't work?


Peter Jay Salzman wrote:

hi doug,

check out /usr/src/linux-2.4.20/Documentation/sysrq.txt

it's very short. basically, you make the kernel:

1. flush buffers ("sync disks")
2. remount all partitions as read-only
3. reboot

print screen == sysrq

alt-sysrq-s flush
alt-sysrq-u remount partitions as read-only
alt-sysrq-b reboot

you can actually send TERM and KILL to all processes. this is the kind
of thing you should really read about before using. it's short and
well written.


begin R. Douglas Barbieri <doug@dooglio.net>

Okay, so explain to me the magic sysreq keys... :-) I experience lockups
sometimes when I'm trying to start up Win4Lin. It hoses my machine so
badly that Ctrl-Alt-Backspace won't even kill X (and Ctrl-Alt-Fn doesn't
even work. I have to hard restart when that happens).

On Sat, Mar 15, 2003 at 09:38:11PM -0800, Bill Kendrick wrote:

Today's Linux demo was pretty successful. Mike showed up (a bit late ;^) )
with the Demo machine. Before that, I just had the flyers set up, and
a couple of kids came by, and I gave them Tux Paint CDs. :^)

When the computer showed up, we started getting many more visitors.
Questions ranged from "I use Linux at work, and the other day it /completely/
locked up!" (Mike explained magic sysreq keys) to "Why would I want Linux?"
(I said "for the good of humanity"; she really digged that answer, and
stuck around and talked for a long time)

Towards the latter half, Jeff Newmiller came by and set up his laptop,
so we had one desktop, one laptop, and one PDA (which didn't get too much use).

We worked on getting Apache configured to show off the LUGOD.org website,
demo'd Tux Paint a lot, and played a little with getting OCR software to
work (in response to someone's complaint about Windows).

Henry - we should set one up for April or May!

bill@newbreedsoftware.com Hire me!
http://newbreedsoftware.com/bill/ http://newbreedsoftware.com/bill/resume/
vox mailing list
R. Douglas Barbieri

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