netconsole

Yet another mechanism to get the kernel debug messages is netconsole which sends the messages over Ethernet. You would not be able to get those early printks or crashes but it is useful nonetheless.

Setting up netconsole is fairly easy. You can either pass the netconsole parameters at the boot time or enable it after the machine has booted using modprobe.

Setting netconsole using boot parameters

Add the following parameter to the kernel boot string

netconsole=[s-port]@[s-ip]/[dev],[t-port]@[t-ip]/[t-mac]

s-port = source port, default is 6665

s-ip = source IP address

dev = device (eth0, eth1 ...)

t-port = target port, default is 6666

t-ip = target  IP address

t-mac = target MAC address

For example:

netconsole=1234@10.1.1.152/eth0, 1235@10.1.1.153/11:22:33:44:55

Setting netconsole using modprobe

It is preferred to use modprobe instead of insmod

$ sudo modprobe netconsole netconsole="@/,@t-ip/"

For example:

$ sudo modprobe netconsole netconsole="@/,@10.1.1.153/"

Target (receiver) side 

Use netcat to get the messages

$ nc -l -u [t-port]

The full documentation on using the netconsole can be found in the "Documentation/networking/netconsole.txt" file in the kernel folder.

Online resources -  1, 2, 3 (some of these might be dated ... )