Working with the serial console

An Arch Linux machine can be configured for connections via the serial console port, which enables administration of a machine even if it has no keyboard, mouse, monitor, or network attached to it.

Installation of Arch Linux is possible via the serial console as well.

A basic environment for this scenario is two machines connected using a serial cable (9-pin connector cable). The administering machine can be any Unix/Linux or Windows machine with a terminal emulator program (PuTTY or Minicom, for example).

The configuration instructions below will enable boot loader menu selection, boot messages, and terminal forwarding to the serial console.

Configure console access on the target machine

Boot loader

GRUB

When using GRUB with a generated grub.cfg, edit /etc/default/grub and enable serial input and output support:

/etc/default/grub

...
GRUB_TERMINAL_INPUT="console serial"
...
GRUB_TERMINAL_OUTPUT="gfxterm serial"
...

Next add the GRUB_SERIAL_COMMAND variable and set the options for the serial connection. For COM1 (/dev/ttyS0) with baud rate of 115200 bit/s:

/etc/default/grub

...
GRUB_SERIAL_COMMAND="serial --unit=0 --speed=115200"

Read GRUB's manual on Using GRUB via a serial line and the serial command for detailed explanation of the available options.

GRUB Legacy

Edit the GRUB Legacy configuration file /boot/grub/menu.lst and add these lines to the general area of the configuration:

serial --unit=0 --speed=9600
terminal --timeout=5 serial console

Note: When the terminal --timeout=5 serial console line is added to your menu.lst, your boot sequence will now show a series of Press any key to continue messages. If no key is pressed, the boot menu will appear on whichever (serial or console) appears first in the terminal configuration line.

rEFInd

rEFInd supports serial console only in text mode. Edit refind.conf and uncomment textonly.

Syslinux

To enable serial console in Syslinux, edit syslinux.cfg and add SERIAL as the first directive in the configuration file.

For COM1 (/dev/ttyS0) with baud rate of 115200 bit/s:

SERIAL 0 115200

The serial parameters are hardcoded to 8 bits, no parity and 1 stop bit.[1]. Read Syslinux Wiki:Config#SERIAL for the directive's options.

Kernel

Kernel's output can be sent to serial console by setting the console= kernel parameter. The last specified console= will be set as /dev/console.

console=tty0 console=ttyS0,115200

See https://www.kernel.org/doc/html/latest/admin-guide/serial-console.html.

getty

At boot, systemd-getty-generator(8) will start a getty instance for each console specified in the kernel command line.

If you have not configured console= in kernel command line start serial-getty@device.service. For /dev/ttyS0 (COM1) that would be serial-getty@ttyS0.service. Enable the service to start it at boot.

Unless specified otherwise in the kernel command line, getty will be expecting 38400 bit/s baud rate, 8 data bits, no parity and one stop bit-times.

Making Connections

Connect using a terminal emulator program

Note: Before making a connection, it is recommended to add your user to the uucp group. Otherwise you will need root's permission to make a connection:

# gpasswd -a username uucp

See Users and groups#User groups for details.

Perform these steps on the machine used to connect the remote console.

Command line

dterm

dterm^AUR is a tiny serial communication program. If you invoke it without parameters, it will connect to /dev/ttyS0 at 9600 baud by default. The following example connect to /dev/ttyS0 at 115200 baud, with 8 data bits, no parity bit and 1 stop bit-times:

$ dterm 115200 8 n 1

See its homepage[2] for more examples.

Minicom

minicom can be obtained from the official repositories. Start Minicom in setup mode:

$ minicom -s

Using the textual navigation menu, change the serial port settings to the following:

Serial Device: /dev/ttyS0
Bps/Par/Bits: 9600 8N1

Press Enter to exit the menus (pressing Esc will not save changes). Remove the modem Init and Reset strings, as we are not connecting to a modem. To do this, under the Modem and Dialing menu, delete the Init and Reset strings. Optionally save the configuration by choosing save setup as dfl from the main menu. Restart minicom with the serial cable connected to the target machine. To end the session, press Ctrl+A followed by Ctrl+X.

picocom

picocom is a tiny dumb-terminal emulation program that is very like minicom, but instead of mini, it is pico. The following example connect to ttyS0 at 9600 bps:

$ picocom -b 9600 /dev/ttyS0

Note: if the backspace key will not work properly try out this option: '--omap delbs'

See its manual for detailed usage.

Screen

GNU Screen is able to connect to a serial port. It will connect at 9600 baud by default:

$ screen /dev/ttyS0

A different baud rate (e.g. 115200) may be specified on the command line.

screen /dev/ttyS0 115200

To end the session, press Ctrl+a followed by K. Alternatively, press Ctrl+a, type :quit and confirm it by pressing Enter.

Serialclient

Serialclient[3] is a CLI client for serial connection written in ruby. Install ruby package, then install it with the following:

# gem install serialclient

Then, you can use like this:

$ serialclient -p /dev/ttyS0

And, for Windows

On Windows machines, connect to the serial port using programs like PuTTY[4] or Terminalbpp[5].

Graphical front-ends

cutecom^AUR is another gui enabled serial monitor.

putty is also available for Linux.

moserial is a gtk-based serial terminal, primarily intended for technical users and hardware hackers who need to communicate with embedded systems, test equipment, and serial consoles.

Installing Arch Linux using the serial console

Note: The Arch Linux monthly release(i.e. the installation CD)'s boot loader has been configured[6] to listen on 0 port(ttyS0/COM1) at 115200 bps, with 8 data bits, no parity bit and 1 stop bit-times.

1. Connect to the target machine using the method described above.
2. Boot the target machine using the Arch Linux installation CD.
3. When the bootloader appears, select Boot Arch Linux (<arch>) and press Tab to edit
4. Append console=ttyS0,115200 and press Enter.
5. Now systemd should detect ttyS0 and spawn a serial getty on it. Login as root and start the installation as usual.

Note: After setup is complete, the console settings will not be saved on the target machine; in order to avoid having to connect a keyboard and monitor, configure console access on the target machine before rebooting.

Note: While a port speed of 9600 is used in most of the examples in this document, working with higher values is recommended (List of available speeds is displayed in Minicom by pressing 'Ctrl-A' and then 'P')

Debugging an unresponsive machine using a serial console

Even though [7] has only raw and terse instructions, it presents the full scene. It is important to note that here, the machine under test got unresponsive in a reproducible manner. And that it happened during normal operation. So it could be accessed normally before it needed debugging. However, in general, the serial console is also useful for debugging boot issues. Perhaps by configuring the boot loader by hand at machine startup time. Also note the mentioned netconsole within the P.S paragraph of the external link from this section.

Troubleshooting

Ctrl+c and Minicom

If you are having trouble sending a Ctrl+c command through minicom you need to switch off hardware flow control in the device settings (minicom -s), which then enables the break.

Resizing a terminal

Unlike ssh, serial connections do not have a mechanism to transfer something like SIGWINCH when a terminal is resized. This can cause weird problems with some full-screen programs (e.g. less) when you resize your terminal emulator's window.

Resizing the terminal via stty is a workaround:

$ stty rows lines cols columns

However, this requires you to manually input the proper geometry. The following methods should be simpler.

1. There is a lesser-known utility called resize, shipped with xterm, that can solve this problem. Invoke it without parameters after you resize the terminal emulator's window:

$ resize

2. If you do not want to install xterm, it is possible to do the same work via a shell function. Put the following function into your zshrc and invoke it without parameters after resizing the terminal emulator's window:

rsz() {
	if [[ -t 0 && $# -eq 0 ]];then
		local IFS='[;' escape geometry x y
		print -n '\e7\e[r\e[999;999H\e[6n\e8'
		read -sd R escape geometry
		x=${geometry##*;} y=${geometry%%;*}
		if [[ ${COLUMNS} -eq ${x} && ${LINES} -eq ${y} ]];then
			print "${TERM} ${x}x${y}"
		else
			print "${COLUMNS}x${LINES} -> ${x}x${y}"
			stty cols ${x} rows ${y}
		fi
	else
		[[ -n ${commands[repo-elephant]} ]] && repo-elephant || print 'Usage: rsz'  ## Easter egg here :)
	fi
}

Missing ports on multi-port expansion cards

The number of serial ports using the generic 8250 driver on the default kernel configuration is set to 4 at runtime with a maximum of 32. This will prevent the creation of /dev/ttyS4 and above. Counting the typical built in serial port on the motherboard this prevents the use of the 4th serial port on a 4 port expansion card.

This can be overridden with the kernel parameter 8250.nr_uarts. E.g.:

8250.nr_uarts=5