I recently had to setup netconsole in order to diagnose some grsecurity-related suspend/resume problems. The idea is to have the broken machine send its kernel messages to a remote machine via the network.

As a prerequisite, the local machine (the one sending the console messages) must have the following kernel options turned on:

• CONFIGFS_FS
• NETCONSOLE
• NETCONSOLE_DYNAMIC

(The first and last ones are required in order to be able to configure netconsole after boot, through the configfs interface.)

The remote machine should be told (using netcat) to listen on a specific UDP port (64001 in this example):

netcat -l -p 64001 -u -s 192.168.1.1 2>&1 | tee /root/netconsole.log

Then, running this script on the local machine will turn netconsole on (don't forget to customize the appropriate parameters for your environment):

#!/bin/sh
modprobe configfs
umount /sys/kernel/config 2> /dev/null
mount -t configfs none /sys/kernel/config
modprobe netconsole
mkdir /sys/kernel/config/netconsole/hostname
echo "xx:xx:xx:xx:xx:xx" >| /sys/kernel/config/netconsole/hostname/remote_mac
echo 192.168.1.1 >| /sys/kernel/config/netconsole/hostname/remote_ip
echo 64001 >| /sys/kernel/config/netconsole/hostname/remote_port
echo 192.168.1.2 >| /sys/kernel/config/netconsole/hostname/local_ip
echo 64001 >| /sys/kernel/config/netconsole/hostname/local_port
echo eth1 >| /sys/kernel/config/netconsole/hostname/dev_name
echo 1 >| /sys/kernel/config/netconsole/hostname/enabled
dmesg -n 8

That's all you need to start seeing messages on the remote machine's screen.

Interested readers may want to look at other ways to configure and use netconsole.