Re: console to com1

[Jeff Licquia <jeff@licquia.org>]

On Sun, 2002-03-31 at 17:23, Charles Menzes wrote:
> my company's systems group has an interesting setup with their linux 
> servers whereby each machine sends its console's stdout to the machine's 
> com1. each machine has a cisco 2511 attached to its com1 so that by 
> telnetting to the cisco, you can gain console access to the linux box 
> through network access to the cisco device.
> has anyone messed with this type of configuration, or something similar in 
> which output was sent to something alternate than the video out?

Yes; some of Progeny's work has involved supporting serial consoles in
our software.

The Linux console is handled via drivers, and three major types exist:
the VGA console, the framebuffer console, and the serial console.  The
first two interact with the video hardware, while the last sends console
I/O via the serial port.  Of course, you have to compile support for the
console type into your kernel, and for serial consoles, you have to
compile the serial drivers into your kernel as well.  (I believe it's
possible to compile them as modules, but there isn't much point to that;
how do you get your console if your initrd blows up to find out why?)

Generally, you pick your console at boot time with kernel args.  For a
serial console, you do this:

  console=ttyS0,9600n8

(That's the first serial port as the console at 9600 bps, no parity, and
8 data bits.)

What's really nice is that, if you don't specify a console device and
you have support for them all compiled in, the system will try VGA
first, then serial.  So, if you build a headless box and take out the
video card, it will start doing serial console by default.

Obviously, you can't run an X server, svgalib, or fb apps.  It's a
console in every other way, though.  Combine this with support in LILO
and GRUB for serial consoles, and you've almost got as good console
support as Sun machines have.  (All that's left is BIOS support for
serial consoles.)

An interesting point I learned while looking up the exact kernel command
line: you can also do parallel port consoles.  Why you'd want to do such
a thing is beyond me.


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