Jtdylktuy

There is Linux kernel boot option to enable system console, the console=, for example, console=ttyS0,115200 or console=ttyS0,115200n8
http://www.linuxtopia.org/online_books/linux_kernel/kernel_configuration/re01.html

After passing this option, linux kernel will print all (or some) kernel messages to the console, which is useful for debugging.

Is there a way of changing system console in Linux in runtime, without rebooting (with some special sysctls/ioctls/or in /proc or /sys filesystems)?

First case: I had no console= option in current booting (checked in /proc/cmdline) and I want to enable dmesg printing to /dev/ttyS0 serial port to debug the situation.

Second case: I had wrong settings in my console=ttyS0,9600 and want to change speed without reboot (remove old console setting, add new console with good 115200 setting; or just change speed).

(Where console= option is parsed, where in /sys or /proc current settings are stored?)

Not a full real hardware console added in runtime, but something better for some cases: meet a "netconsole". It is kernel console emulation over udp in Ethernet networks, one-way, only for collecting kernel logs. No more usb-uart adapters or null-modem cables or overpriced com-over-ethernet "console server" devices. And no more transmit (You may want to use classic kernel console to send magic SysRq, but not with netconsole).

Documentation: https://www.kernel.org/doc/Documentation/networking/netconsole.txt

Examples:

• https://wiki.archlinux.org/index.php/Netconsole


• https://wiki.ubuntu.com/Kernel/Netconsole

Netconsole should be enabled (or built as module) in the kernel configuration to work. (Sometimes it may be possible to build netconsole module for running kernel and modprobe it into linux without reboot).

To enable with linux command line (with reboot), add:

netconsole=[src-port]@[src-ip]/[<dev>],[tgt-port]@<tgt-ip>/[tgt-macaddr]

Everything need to be specified: source udp port, source ipv4 address, network interface name like "eth0" (will it work with udev renaming the eth0 into something position-related?), target udp port and ipv4, and also target macaddress in format of six bytes 00:13:32:20:r9:a5.

Receiver is nc -u -l $tgt_port.

Example of runtime configuration from the Arch wiki - https://wiki.archlinux.org/index.php/Netconsole

# 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

# set local network device name (find it trough ifconfig, examples: eth0, eno1, wlan0)

echo eno1 > dev_name

# find destination MAC address

arping -I $(cat dev_name) $(cat remote_ip) -f | grep -o ..:..:..:..:..:.. > remote_mac



echo 1 > enabled