How to run a script before switch_root in initramfs?

Question

I'm starting dropbear ssh server in initramfs to be able to unlock a crypted partition over SSH*.

In Debian Jessie, it was all fine but in Buster, dropbear doesn't seem to stop after switch_root, which prevents OpenBSD ssh server from starting up.

apt-get purge dropbear does not help and which dropbear prints /usr/bin/dropbear and ps -ef | grep dropbear prints /sbin/dropbear; which tells that this process is the one which started at initramfs stage and it didn't stop.

As a workaround, I'm doing killall dropbear in SSH service script but it would be more appropriate to kill it right before switching the root partition.

How can I run the script at that point?

Answer 1

Create the cleanup script as /etc/initramfs-tools/scripts/init-bottom/cleanup.sh:

#!/bin/sh
echo "Killing dropbear"
killall dropbear
exit 0

...and make it executable:

chmod +x /etc/initramfs-tools/scripts/init-bottom/cleanup.sh

Update initramfs as usual:

update-initramfs -u 

Answer 2

You already have a script that calls switch_root, so just insert a line there to stop dropbear.

You can also stop it from within your ssh connection. As soon as you log in, there are two dropbear processes. The first is the process that accepts new connections, the secons is the process serving your connection. At this point you can kill the first process, as you don't want more than one connection. The second process will terminate once you disconnect.

At this early stage in the boot process, you can be sure that the process accepting new connections has the lower process number, although there are other ways to identify it.