diskless ubuntu serial console

I’m using Ubuntu servers with qemu-kvm as a virtualization solution. The software included in 10.04LTS includes a variety of annoyances, such as broken PXE, odd bridge behavior, and “general weirdness.” Although 10.10 is not supported in the long term, I decided to try it.

The good news is, the 10.10 virtualization stack works much better. The bad news is, 10.10 didn’t want to run on my diskless hardware. Boot attempts all died with many lines of:

ipconfig: no devices to configure

and a message about killing init. The server was quite explicit that it was dead, and how it was dying, but didn’t leave any clues as to what had killed it. I’m sure that the console showed useful error messages, but they had scrolled off the top of the screen.

The manual says that if you hit shift-PageUp, Ubuntu should page up through the console messages. That should be amended to read “unless init is dead and your keyboard LEDs are blinking slowly but steadily.”

The only way to resolve this problem is to see the error messages that say why the machine crashed. So, a serial console. I want PXE messages, initrd messages, and kernel boot messages sent to serial console. These are all controlled by the /tftpboot/pxelinux.cfg/machine file. The actual file name is the MAC address of the booting NIC.

If you want to get messages from the pxe and initrd boot stages, the pxelinux.cfg file’s first line must include the SERIAL statement. If you want to get console messages from the booting kernel and/or log into the running system over the serial console, you must append a serial statement to the kernel boot command. The end result for a serial console looks like this:

SERIAL 0 115200
LABEL linux
KERNEL vmlinuz-2.6.35-27-server
APPEND root=/dev/nfs initrd=initrd.img-2.6.35-27-server-pxe nfsroot=192.0.2.1:/nfsroot ip=dhcp rw console=tty0 console=ttyS0,115200n8
TIMEOUT 0


The Web site will probably wrap the APPEND statement around, but that line and everything beneath it down to TIMEOUT is a single line.

If you want a serial login in multiuser mode, you need to create a script to activate the terminal. Here’s the Ubuntu default terminal script:
/etc/init/ttyS0.conf

# ttyS0 - getty
#
# This service maintains a getty on ttyS0 from the point the system is
# started until it is shut down again.

respawn
exec /sbin/getty -L 115200 ttyS0 vt102

The next time you reboot your diskless box, you should have a full serial console.

Some time soon, more on the actual error and how I fixed it.