Remote console setup

In case you are experiencing a kernel crash (oops) and have already tested your hardware, you should report what kernel says to the console (i.e. an oops text) to Bugzilla. Sometimes kernel crashes so badly that syslogd is not working and what kernel says is never written to a file. If this is the case, you have to catch what kernel says. There are several ways possible.

Manual/Photo

If kernel backtrace is not long enough there are chances that it can fit into a single screen. In that case, you can just make a photo of the kernel crash screen and attach it to the bug report. If you do not have a camera, you still can carefully write down (using a piece of paper and a pen, that is) what you see on the screen, and later type it into the bug report.

Serial console

Here is a description of a common routine that is necessary to set up a serial console.

Hardware setup

First of all you should make sure that your node has a serial port. If there is no such port then unfortunately this way is not for you.

Then you need to find a second machine with a serial port on it. This machine will be used to collect logs from your primary machine. Further you need to acquire so-called null modem cable (a.k.a. serial cable) and it must be long enough to connect these two machines.

Software setup

Sending side

In your boot loader add the following kernel parameters:

console=ttyS0,115200 console=tty0

For example, in GRUB boot loader configuration file /boot/grub/grub.conf it looks like this:

title Fedora Core (2.6.16-026test014.1-smp)
        root (hd0,0)
        kernel /vmlinuz-2.6.16-026test014.1-smp ro root=LABEL=/ console=ttyS0,115200 console=tty debug silencelevel=8
        initrd /initrd-2.6.16-026test014.1-smp.img

Kernel loaded with such parameters will send all kernel messages to /dev/ttyS0 (first serial port, a.k.a. COM1). If you have several ports, make sure that your null modem cable is connected to the appropriate port.

Receiving side

On the second node you should run any software that can log from /dev/ttyS0.

It can be usual

cat /dev/ttyS0 > /var/log/serial.log &

or something more sophisticated: syslogd, watchtty etc.

Port setup

One more important thing. 115200 in the example above is the rate of emitting port. Receiving port must also work at the same rate. For example, to tune ttyS0 rate use stty program like this:

stty 115200  < /dev/ttyS0

Some other serial port parameters, like parity, number of stop bits etc. should also be the same on both sides.

Netconsole

Kernel recompilation

If you use binary kernel from openvz.org, it already has netconsole module compiled in, so just skip to next section.

If you build the kernel yourself, you might need to check if netconsole is compiled. To that effect, change to your kernel source directory and grep your kernel .config for NETCONSOLE:

# cd /usr/src/openvz/linux-2.6.16
# grep NETCONSOLE .config

If you see nothing or "# CONFIG_NETCONSOLE is not set" you need to recompile the kernel.

Edit your kernel configuration file .config with a text editor (nano .config or vi .config). Set netconsole to Y or M (depending on whether you want it as a module or built into the kernel; I have compiled it as a module):

CONFIG_NETCONSOLE=m

Save the file, then recompile the kernel:

# make bzImage && make modules && make modules_install

Update your bootloader for the new updated kernel. In my case I use LILO so I just type lilo at the prompt.

Reboot into new kernel.

Setting up OpenVZ side

Next you want your netconsole to send the request to somewhere. Load netconsole module, specifying the remote server parameters:

# modprobe netconsole netconsole=4444@10.0.2.1/eth0,6666@10.0.2.2/00:05:5D:34:11:AF

This will load the module with your settings. Replace your local IP address with where 10.0.2.1 is, eth0 with your network interface card device, 6666 with the remote netconsole port (UDP), and 10.0.2.2 with your remote netconsole server IP. Also add in the mac address of your remote netconsole server, which in my case was 00:05:5D:34:11:AF. You can get the MAC address using arp utility:

# ping -c 1 10.0.2.2
# /sbin/arp -i 10.0.2.2
? (10.0.2.2) at 00:05:5D:34:11:AF [ether] on eth2

Netconsole documentation is available from Documentation/networking/netconsole.txt file under your kernel source directory.

Setting up remote side

Setup netcat on your console server to listen on port 6666 UDP:

netcat -u -l -p6666

When your kernel prints something on the console, the text will be also captured on this netconsole server.