Send Ctrl-Alt-Del over SSH?

Question

I know that Linux may, depending on configuration, allow the interactive user to reboot the server by pressing Ctrl-Alt-Del even without logging in. I have a remote console session over SSH to a remote server that I cannot log into, but I want to reboot it. Is it possible to somehow send the key combination Control-Alt-Delete over SSH (or telnet)? If so, how?

Edit: to clarify, I am not just connected to the server's sshd over ssh - I am connected via a KVM over IP switch, so it's a serial console. But this connection itself is over SSH and my question is how to send the Ctrl-Alt-Del key combination over it.

Solution 1

If you are connected to a serial console and your kernel has CONFIG_MAGIC_SYSRQ and /proc/sys/kernel/sysrq enabled (default on my Ubuntu machines), you can try BREAK, ALT-BREAK or CTRL-BREAK (depends on the SSH to serial converter).

If that works, you can press h (within 5 seconds) to get a list of available commands ("s = sync filesystems" and "b = force immediate reboot" are maybe what you are looking for).

For more information, have a look at linux/Documentation/sysrq.txt

Solution 2

You can use the Sysrq trigger in order to make an immediate reset (it's like pressing ALT-BREAK-b):

echo b >/proc/sysrq-trigger

BEFORE that, you might want to sync all filesystems, to not lose any data:

echo s >/proc/sysrq-trigger

This might only work if CONFIG_MAGIC_SYSRQ is enabled in your kernel. The possible keys are explained at Wikipedia btw.

Solution 3

In a comment you mention (ttyS0) - this isn't the local console, it's a serial console.

I don't think you can do Ctrl-Alt-Del from the serial console.

http://osdir.com/ml/linux.hardware.dell.poweredge/2003-01/msg00200.html suggests a way getting magic-sysrq working over serial - maybe this would be sufficient for rebooting a server?

Comments

[kmarsh]

Read your KVM documentation. It should have a way to invoke keychords, especially given you have to use Ctrl-Alt-Del to log into Windows.

[Sam Cogan]

He said that he is unable to login to the the server, so I don't think that will work. I also don't think he will be ablt to use ctrl-alt-del either

[Brian Knoblauch]

Allowing ctrl-alt-del over a non-logged in SSH session sounds like a horrible idea! Just think, you'd be able to reboot any server you could open an SSH connection with!

[ceyko]

I am connected over a remote KVM device - but I don't know how to make it reboot the server. I don't have any menu or anything, I just see the server's screen (in text mode, of course).

[Dan Carley]

Ahh, I see. Do you know the make/model of KVM? I would have thought it provided a break-out menu for such escape sequences.

[ceyko]

No, I don't know the model - it's provided by the DC. I suspect there's one KVM for multiple machines, too, so I don't have full access to it, only to my server.

[Dan Carley]

I would hazard then that it is probably not possible.

[Travis Campbell]

SSH's built-in "send break" may work too. It's "~B" (tilde-b).

[ceyko]

Yep, sorry - serial console. Updated.

[ceyko]

Good to know about this, but didn't work in this particular case. :( I guess it's not enabled.

[ceyko]

I doubt that a Windows utility would have worked on a Linux server. :)