Linux Kernel Updates: When and How to Reboot

Package updates take effect when a service restarts, but a new kernel only runs after a reboot. Until then your VPS keeps running the old, possibly vulnerable kernel. This guide shows how to tell when a reboot is due and how to do it safely.

Before you start

sudo access over SSH and access to your VPS control panel or console.
Services set to start at boot (systemctl is-enabled, see systemctl).

Check the running and installed kernels

uname -r

dpkg --list | grep linux-image

rpm -q kernel

If the newest installed kernel is newer than uname -r, you need a reboot to use it.

Is a reboot required

On Ubuntu and Debian:

cat /var/run/reboot-required 2>/dev/null
cat /var/run/reboot-required.pkgs 2>/dev/null

On AlmaLinux and Rocky Linux:

sudo dnf needs-restarting -r

To find services that still run old libraries without rebooting, use sudo needrestart (Ubuntu) or sudo dnf needs-restarting -s.

Reboot safely

Install all pending updates: see apt, dnf and yum.
Check /etc/fstab is valid with sudo findmnt --verify, so the server boots.
Pick a quiet time and warn users if needed.
Reboot: sudo reboot.
Reconnect after a minute or two and confirm with uname -r and systemctl --failed.

Tip: Schedule a reboot for later with sudo shutdown -r 03:00; cancel with sudo shutdown -c.

Automatic reboots

Ubuntu's unattended-upgrades can reboot automatically at a set time. In /etc/apt/apt.conf.d/50unattended-upgrades:

Unattended-Upgrade::Automatic-Reboot "true";
Unattended-Upgrade::Automatic-Reboot-Time "04:00";

On AlmaLinux and Rocky Linux, dnf-automatic installs updates; reboots are planned separately. See automatic security updates.

Remove old kernels

sudo apt autoremove --purge

dnf keeps three kernels by default (installonly_limit in /etc/dnf/dnf.conf).

Troubleshooting

VPS does not come back: open the console from your control panel to see boot errors; a broken fstab line is the most common cause (see mount a disk with fstab). Our team can help through a ticket.
uname still shows the old kernel: on some virtualisation types the kernel is set by the host; check with support.
A site is down after reboot: its service was not enabled; run sudo systemctl enable --now name.

Next steps

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