This Dracut module (dracut-serial) improves serial access to initramfs early userspace (on certain Linux distribution versions). With it enabled it's possible to unlock encrypted devices or deal with the emergency shell over a serial connection in addition to the system console.

2018, Georg Sauthoff mail@gms.tf, GPLv3+

2019-01-06: Note that this module isn't necessary anymore under Fedora 29. There it's sufficient to disable Plymouth and add something like console=tty0 console=ttyS0,115200 to the kernel command line to get early userspace output to both the system console and the serial one. That means that also unlocking encrypted volumes does work over the serial console, by default, with Fedora 29.

Problem

Dracut-serial solves the following problem: as of Fedora 28/CentOS 7, the Dracut early userspace just uses /dev/console for password prompting and starting the emergency shell. Thus, one can enable the serial console by adding something like

console=tty0 console=ttyS0,115200

to the kernel commandline, but this means that /dev/console is connected to /dev/ttyS0 and thus the emergency shell is only started on the serial console. The story is similar for unlocking encrypted devices, i.e. systemd-tty-ask-password-agent is just started on /dev/console (or via Plymouth - cf. systemd-ask-password-plymouth.service vs. systemd-ask-password-console.service ).

Note that /dev/console is whatever console declaration comes last at the kernel commandline, i.e. with

console=ttyS0,115200 console=tty0

the Dracut emergency shell is started on the system console instead of serial one. Log messages still go to both consoles.

This is suboptimal, as there is no backup console in case the primary one fails or is unavailable. It's also kind of unexpected as it's possible to enable both system and serial console with Grub2 and late userspace deals with multiple console as a core feature.

See also Fedora Bug 1031015 dracut shell doesn't start (2013).

Solution

Dracut-serial includes a getty (agetty) to start the dracut emergency shell (i.e. /bin/dracut-emergency) on /dev/ttyS0, as well. It overrides the dracut-emergency.servicesuch that it always uses/dev/tty0. In that way both services are independent of the ordering of console=kernel commandline parameters and there is always a backup console available. I still makes sense to addconsole=` parameters to get log messages on all consoles and to configure serial connection parameters (like baud).

Dracut-serial although starts the password prompting agent (/usr/bin/systemd-tty-ask-password-agent via systemd-ask-password-serial.service) on the serial console such that one is able to unlock encrypted devices either via the system or the serial console.

Install

Copy the 46sshd subdirectory to the Dracut module directory:

# cp -ri 05serial /usr/lib/dracut/modules.d

Disable Plymouth as it can't be told to just deal with tty0 and leave other ttys alone (--tty isn't sufficient on Fedora 28):

# cp dracut.conf.d/serial.conf /etc/dracut.conf.d
# sed -i 's/^\(GRUB_CMDLINE_LINUX="\)/\1plymouth.enable=0 /' /etc/sysconfig/grub
# grub2-mkconfig -o  /etc/grub2.cfg
# grub2-mkconfig -o  /etc/grub2-efi.cfg

Regenerate the initramfs:

# dracut -f -v

Verify that this serial module is included. Either via inspecting the verbose output or via lsinitrd. Reboot.

Emergency Shell

In the current design, leaving the emergency shell on the serial console yields a restart of that shell. Thus, to resume the system boot one has to explicitly terminate the emergency shell service, e.g.:

$ systemctl stop dracut-emergency.service

As before, leaving the emergency shell on the system console automatically resumes the boot.

A systemctl kill ... may also be used to terminate the emergency service, although one has to explicitly specify SIGHUP on CentOS 7 (systemctl kill --signal=HUP ...) to get the shell terminated.

As always, a simple way to unconditionally entering the emergency shell is to add rd.break to the kernel commandline.

Space Overhead

The agetty utility is very small:

System       file size
----------------------
CentOS 7     37 KiB
Fedora 27    57 KiB
Fedora 28    63 KiB

Development Notes

Dracut starts the dracut-emergency.service via _emergency_shell() in lib/dracut-lib.sh.

The Dracut module dracut-systemd (under 98dracut-systemd) contains the dracut-emergency.service and pulls the systemd-ask-password-{console,plymouth}.* service files from the system.

Tested Environments

• Fedora 28
• Fedora 27
• CentOS 7