Serial console access
When a boot fails, the firewall locks you out or the network stack will not come up, the serial console is the way in that does not depend on any of them.
Below the network
A serial console attaches to the machine at a level beneath its operating system's networking. That is exactly what you want when the problem is the networking: a misconfigured firewall, a kernel that will not bring up the interface, or a boot that stalls before SSH ever starts. It is text-only and unglamorous, and it works when nothing else does.
How it differs from remote KVM
Remote KVM and the BMC give you screen, keyboard and power control, including the graphical early-boot screens. A serial console is lighter: a pure text stream, scriptable, and ideal for watching a kernel boot or catching a panic. Many operators use both.
Set it up before you need it
Configure the kernel and boot loader to emit console output to the serial line, and enable a login on it. Done ahead of time, the console is waiting the day a change locks you out. Ask us to confirm serial access for your machine — it is the first thing you will want during a bad rescue.
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