alt Hacker News> I remember seeing a linux firewall/gateway set up to run with just the kernel, without any userspace at all. Completely unhackable.
Do you remember any details that would let me search for it? Because that does sound cool, and even maybe useful; the thought has certainly crossed my mind that a router or VPN box doesn't really get a lot of use out of userspace... Although maybe it's worth keeping for control/configuration/debugging.
> To print some text or run a simple program, I belive DOS without a memory manager would be even faster.
Or just make your code boot directly. It's not hard to make a .efi, or use https://github.com/jart/cosmopolitan to make a binary that runs in many places including bare metal.
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That's a halted firewall setup. Normally as part of shutdown you would tear down networking in SysVinit or systemd but you don't actually have to do that. When shutting down you can choose whether to power off or just to halt. It's basically like the old Windows "It is now safe to power off your PC".
> control/configuration/debugging
This is one of several major arguments made against unikernels in that famous Triton rant from a decade ago:
https://tritondatacenter.com/blog/unikernels-are-unfit-for-p...
Basically, even if your application _can_ run as the kernel, and it's desirable for it to run with kernel-level permissions, do you really want production to be a world without strace and iotop and the like?
IIRC, it ran a script as init process that set up the network cards, set up iptables, etc. and then just exited. Kernel would panic (the "init was killed" panic), but the network would still be functional. Automatic reboot on panic was disabled.
To reconfigure, the admin would simply reset it and start the system with "init=/something/else" as kernel parameter that booted to a normal userspace.