Netconsole

zh-CN:Netconsole netconsole is a kernel module that sends all kernel log messages (i.e. dmesg) over the network to another computer, without involving user space (e.g. syslogd). Name "netconsole" is a misnomer because it's not really a "console", more like a remote logging service.

It can be used either built-in or as a module. Built-in netconsole initializes immediately after NIC cards and will bring up the specified interface as soon as possible. The module is mainly used for capturing kernel panic output from a headless machine, or in other situations where the user space is no more functional.

Documentation is available in the Linux kernel tree under Documentation/networking/netconsole.txt

Installation

Install gnu-netcat from the official repositories.

Build-in Configuration

Netconsole and other modules' Kernel parameters can be passed from a bootloader to kernel at its startup via kernel command line by modifying the bootloader environment, which is type and version specific. Example for Uboot, where 1st address is Plug Computer ArchLinux device's netconsole Out Port & IP, and 2nd address is your PC's netconsole In Port & IP & adapter MAC:

fw_setenv usb_custom_params 'loglevel=7 netconsole=6666@192.168.1.28/eth0,6666@192.168.1.19/00:13:32:20:r9:a5'

Logging is done by your ArchLinux set logger like syslog-ng, so available loglevels (output details) are defined in that logger docs, and may differ for each log type. One can also pass netconsole string parameters at kernel runtime (no config file required), then start two netconsole instances on the monitoring PC (one to read output, another for input), and restart it on the PC or device you are logging as shown in Dynamic Configuration:

# set log level for kernel messages
dmesg -n 8

netconsole=6666@192.168.1.28/eth0,6666@192.168.1.19/00:13:32:20:r9:a5

nc -l -u -p 6666 &
nc -u 192.168.1.28 6666

One may need to switch off PC and router firewall, and setup proper router port forwarding to monitor and input data in Netconsole.

Dynamic configuration

Netconsole can be loaded as one of kernel modules manually after boot or auto during boot depending on this module config. See kernel modules for configuring it to load at boot. For loading manually any time after boot:

# set log level for kernel messages
dmesg -n 8

modprobe configfs
modprobe netconsole
mount none -t configfs /sys/kernel/config

# 'netconsole' dir is auto created if the module is loaded 
mkdir /sys/kernel/config/netconsole/target1
cd /sys/kernel/config/netconsole/target1

# set local IP address
echo 192.168.0.111 > local_ip
# set destination IP address
echo 192.168.0.17 > remote_ip
# find destination MAC address
arping `cat remote_ip` -f |grep -o ..:..:..:..:..:.. > remote_mac

echo 1 > enabled

netconsole should now be configured. To verify, run 'dmesg |tail' and you should see "netconsole: network logging started". Check available log levels by running 'dmesg -h'.

Receiver

nc -u -l 6666

or

nc -u -l -p 6666

Starting at Boot

Just add the netconsole to the kernel cmd line. It takes a string configuration parameter "netconsole" in the following format:

  netconsole=[src-port]@[src-ip]/[<dev>],[tgt-port]@<tgt-ip>/[tgt-macaddr]
        src-port      source for UDP packets (defaults to 6665)
        src-ip        source IP to use (interface address)
        dev           network interface (eth0)
        tgt-port      port for logging agent (6666)
        tgt-ip        IP address for logging agent
        tgt-macaddr   ethernet MAC address for logging agent (broadcast)

Examples:

linux /vmlinuz-linux root=UUID=a322511e-b028-4f11-87b6-e48b5d99bbd8 ro netconsole=514@10.0.0.1/eth1,514@10.0.0.2/12:34:56:78:9a:bc
linux /vmlinuz-linux root=/dev/disk/by-label/ROOT ro netconsole=514@10.0.0.2/12:34:56:78:9a:bc

From: Net Console for Boot Debugging.