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
If not included with OS, Netconsole can be installed from the depository:
pacman -Sy netcat
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 email@example.com/eth0,firstname.lastname@example.org/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 email@example.com/eth0,firstname.lastname@example.org/00:13:32:20:r9:a5 nc -l -u -p 6666 & nc -u 192.168.1.28 6666
One may need to switch off firewall and setup proper port forwarding to monitor and input data in Netconsole.
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'.
nc -u -l 6666
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)
linux /vmlinuz-linux root=UUID=a322511e-b028-4f11-87b6-e48b5d99bbd8 ro email@example.com/eth1,firstname.lastname@example.org/12:34:56:78:9a:bc linux /vmlinuz-linux root=/dev/disk/by-label/ROOT ro email@example.com/12:34:56:78:9a:bc