[TriLUG] Re: console access through serial port?

[M. Mueller]

On Thursday 18 April 2002 11:16 am, lfwelty wrote:

> Ummmmm, that's kind of a stupid question, of course it will allow
> you to run headless.  I guess what I mean is, if it passes your vga
> to the tty, will you be able to boot headless?  I don't suppose it
> changes your bios... so not likely... :/

You won't see the bios stuff.

> > > To get kernel info output to the serial line (and lose it on the VGA
> > > line), in lilo.conf add:
> > > serial=0,38400n8
> > >         image=blah
> > >         append="console=ttyS0,38400"

I used:
              append="console=ttyS0,38400 console=tty0"

> > > and to get a vty on your serial line, in inittab add something like:
> > >         s0:12345:respawn:/sbin/mingetty ttyS0 DT38400 ansi

I downloaded and compiled mgetty+sendfax 1.1.27 so I could control some 
compile time options.  The level 9 debug messages in this version of mgetty 
are very helpful in debugging.

> > >
> > > Then connect up the proper cable (Null modem for computer to computer)
> > > and run minicom or some other terminal program on it.
> > >
> > > > 2. Provided I can get in, is there any way I can *crash* the
> > > >    system so I can take a look at a dump to figure out WIH was
> > > >    going on to wedge it?
> > >
> > > There's a few different things you can do (gdb?) to crash a system, you
> > > should be able to find that on google... but I wonder if that kernel
> > > config option "SysRq key" would help you here.  Check the kernel stuff
> > > on that one.

go to /usr/src/linux; make xconfig; select kernel hacking and read help on 
Magic SysReq;  

It seems that you are pursuing serial console access to your Linux system in 
hopes of having some back door through which you can enter a crashed system 
and inspect the damage and prescribe a remedy.  You may be dissapointed.  If 
a system is so badly off that it won't let you use keyboard and VGA I/O, then 
there is a good chance that the serial I/O drivers will not function either.  

If you pursue using kernel hacking tools like SysReq and KDB you face a steep 
learning curve on learning to manipulate the kernel environment.  The 
granularity of information you gain in this approach is too fine to be of 
much use to you as a system adminstrator or system user.

And for the final disappointment, when I was running null modems, my systems 
did not respond to the 3-finger salute.  Maybe they would have if I spent 
more time trying to figure out the reason.  Part of the problem is that 
serial control is passed around during the various stages in a system's 
lifecycle.  So if the 3-salute problem is solved in one stage, it may require 
another solution in another stage.  You don't want to solve that problem 
right now. Also, there's no guarantee the salute will work on a crashed 
system.

-- 
Michael Mueller
Signalnetware, Inc.
www.signalnetware.com
919.621.6090