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.

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](http://lxr.linux.no/linux/Documentation/sysrq.txt)
The three-fingered reboot is only available from a local console.

It is handled by the following inittab line:

```
# What to do at the "Three Finger Salute".
ca:12345:ctrlaltdel:/sbin/shutdown -r now

```

You would need to use the `shutdown`, `halt` or `reboot` commands from a shell with root privileges instead, if you don't have physical access to the machine.