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Linux has the "magic sysrq key" to reboot frozen machines remotely, and it works over the serial console, but what about FreeBSD? Is there a way to send a "control-alt-delete" to FreeBSD servers on serial consoles?

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Assuming you have a kernel with the debugger option compiled in you can use ControlAltEscape. From there you can call boot(0) or panic.

Chapter 10 of the FreeBSD developers handbook explains this in a lot more detail.

So much for more or less the same as SysReq via a keyboard. On the serial console, you need to send the break signal and have the options BREAK_TO_DEBUGGER enabled. But "it is not the default since there are a lot of serial adapters around that gratuitously generate a BREAK condition, for example when pulling the cable".

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  • chapter 10 of the freebsd handbook is about printing, did you mean chapter 10 of the developers handbook?
    – anarcat
    Commented Oct 26, 2013 at 21:58
  • Aye. I did. I was linking to both the normal handbook and the lin you posted, then I cleaned it up. I guess I got sidetracked during that. Thanks for noticing.
    – Hennes
    Commented Oct 27, 2013 at 16:52
  • so i tried it, and it doesn't work from the serial console, it seems that you need to send a BREAK and have the options BREAK_TO_DEBUGGER in your kernel config.
    – anarcat
    Commented Oct 29, 2013 at 15:31
  • These day, you can also control the state of support for breaking into the kernel debugger with sysctl - the debug.kdb.break_to_debugger setting. There is also an alternate key sequence that is handy when your console connection does not support the serial break condition. See the debug.kdb.alt_break_to_debugger sysctl setting or options ALT_BREAK_TO_DEBUGGER in the kernel config. See also ddb(4) and gdb(4). The alt break method supports key sequences to enter the debugger, perform an immediate reboot, or generate a panic.
    – Juan
    Commented Mar 12 at 14:33

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