The worst thing that can happen¹ is losing contact with a server with no physical access to it. I haven’t found the root cause, but it’s fixed by restarting the sshd server.

1. Update: perhaps the root cause is solved by apt-get purge openssh-server && apt install openssh-server (seems to work?)
2. Update: perhaps the root cause is a race condition in systemd and can be solved by adding RuntimeDirectoryPreserve=yes to /usr/lib/systemd/system/ssh@.service (trying if above doesn’t work)

The issue ¶

I’ve now had this twice (sort of) when upgrading sshd on Debian, which resulted in

kex_exchange_identification: read: Connection reset by peer
Connection reset by 192.168.1.1 port 22

Fortunately I have an overly complex home networking setup and this Debian server runs inside proxmox, and I could still access proxmox (which is also based on Debian, so that’s a bit suprising), which allowed me to access the Debian server². It turns out sshd was logging errors as follows:

Dec 23 21:50:08 proteus sshd[419962]: fatal: Missing privilege separation directory: /run/sshd

which I had before once after upgrading sshd. I fixed it and forgot about it (which usually means I think I fixed it permanently), however it came back.

Resources ¶

Some other people also have this problem, some solved, some not:

• SSH failed to start - Missing privilege separation directory: /var/run/sshd [duplicate] (askubuntu.com) (2019) – unknown reason
• SSH failed to start - Missing privilege separation directory: /var/run/sshd (askubuntu.com) (2019) – unknown reason
• SSH Server stops working after reboot, caused by missing /var/run/sshd (askubuntu.com)
• sshd not starting because of missing /run/sshd (launchpad.net) (2023) – because of incorrect root fs permissions
• SSH server restart needed after package upgrades (proxmox.com) (2022)
• Missing privilege separation directory: /var/run/sshd (turnkeylinux.org) (2022) – on specific distro
• Container’s SSH Service Fails to Load on Startup (reddit.com) (2022) – unknown, solved by apt-get purge openssh-server && apt install openssh-server
• SSH server “crashing” (reddit.com) (2021) – race condition of services
• Debian bug report (debian.org) – bug report of race condition
• Debian bugfix (debian.org) – bug fix of race condition