Linux – Send Ctrl-Alt-Del over SSH

linuxremote-accessssh

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.

Best Answer

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

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You want to use expect for this. It's probably already on your machine. It's the standard tool for any kind of interactive command-line automation. It's a Tcl library, so you'll get some Tcl skills along the way for free. Beware; it's addictive.

#!/path/to/expect
spawn ssh user@example.com "mysql -u blub"
expect "*ssword:*"
send -- "mypass\r"
interact

Ssh – How to automate SSH login with password

Don't use a password. Generate a passphrase-less SSH key and push it to your VM.

If you already have an SSH key, you can skip this step… Just hit Enter for the key and both passphrases:

$ ssh-keygen -t rsa -b 2048
Generating public/private rsa key pair.
Enter file in which to save the key (/home/username/.ssh/id_rsa):
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/username/.ssh/id_rsa.
Your public key has been saved in /home/username/.ssh/id_rsa.pub.

Copy your keys to the target server:

$ ssh-copy-id id@server
id@server's password:

Now try logging into the machine, with ssh 'id@server', and check-in:

.ssh/authorized_keys

Note: If you don't have .ssh dir and authorized_keys file, you need to create it first

to make sure we haven’t added extra keys that you weren’t expecting.

Finally, check to log in…

$ ssh id@server

id@server:~$

You may also want to look into using ssh-agent if you want to try keeping your keys protected with a passphrase.

Best Answer

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Ssh – Bash-like command substitution in ssh or How to request things from ssh client