Symptoms

Affected computers show long delays during boot, especially with applications that need randomness for initialization - including openssh, so it can leave a machine apparently dead after a Stretch-to-Buster upgrade.

If you're sitting in front of a machine that might be suffering from this problem then you may be able to speed up its recovery by jiggling the mouse. If it's a remote system, ping a network interface.

Underlying Problem

Linux is being stricter about getting urandom initialized with properly random randomness, and systemd... well, let's just say it isn't helping. As a result the system has to slowly collect entropy from other sources.

Solutions

For kernel versions < 5.4:

With Linux 5.4, an in kernel jitter entropy collector has been added that should collect enough entropy at boot time to avoid entropy-related hangs at system boot. So the issues described here should generally be resolved if you're running backports kernels on buster, or reasonably up-to-date bullseye or sid installations.

https://daniel-lange.com/archives/152-hello-buster.html

debian-devel thread: https://lists.debian.org/debian-devel/2018/12/msg00184.html, https://lists.debian.org/debian-devel/2019/01/msg00079.html

bug #912087 (bug #912616) bug #916690 bug #923675 bug #931644 bug #948519

Buster release-notes

Keywords

boot-time delay, crng, hang, getrandom, randomness, systemd