sysrqd: small daemon to manage Linux SysRq over network

January 3rd, 2007 edited by ana

Entry submitted by Julien Danjou. DPOTD needs your help, please contribute !

sysrqd is a tiny daemon aiming to control sysrq over network.

SysRq stands for System Request, and this are functions mapped to keyboard shortcuts by the kernel. You can use them by pressing Alt+SysRq+[key] (SysRq might be named “Print Screen” on your keyboard), where key can be s (sync), k (sak), 0 to 9 (logging level), b (reboot), etc.
The goal of this keyboard sequences is too be available even if you can’t do anything on your box, because you screwed up everything, or because it’s crashed or under very heavy load.

sysrqd can help you to use SysRq keys when you are not in front of your box,and you need to do Emergency Sync, R/O remount and reBoot.

It is designed to respond under heavy load or half-crashed box, so it might work even if you can’t access to your box for example. In this case, you will be happy to telnet to your box on sysrqd port, enter your password, and then press s, u, b and wait for your server to reboot. You would not have been able to do that since ssh was not responding.

The connection is password protected but not cyphered so you might want to firewall it or to run it on a trusted network.

You can find latest version 8 packaged in both Debian and Ubuntu.

Posted in Debian, Ubuntu |

6 Responses

  1. Juraj Says:

    well, what makes you think, that you will be able to telnet to user-space daemon on half-crashed machine?

  2. Neil McGovern Says:

    > well, what makes you think, that you will be
    > able to telnet to user-space daemon on
    > half-crashed machine?

    From what I can tell:

    a) it should be run with a lower nice level
    b) it doesn’t require a shell

  3. Don Jackson Says:
    is an interesting addition to the topic of “sysrq” key usage in Linux.

  4. Yuriy Says:

    “Rate” doesn’t work

  5. Eric Windisch Says:

    Why not just get a switched PDU instead? Another poster was right, if the box is half-crashed, there are many cases where this won’t work either. If you are actually in legitimate need to solve the problem that this supposedly addresses, look elsewhere. There are plenty of better (hardware) solutions out there.

  6. Constantine Evans Says:

    I’m not sure how the RSEIUB mnemonic started, but it is incorrect and shouldn’t be spread (R is unraw, not raw, the situations where the author recommends its use can usually be solved without rebooting by using K, and so on). The sysrq kernel documentation has instructions for performing the same task properly.