Not after every update. Restart services that are still using old libraries, and reboot when an update replaces the kernel or a package requests a reboot. Check Debian’s reboot signal, but don’t treat its absence as proof that a reboot is never needed.
When does a Debian security update require a reboot?
The answer depends on what the update changed. An update to a library may require restarting the services that use it; a kernel update requires a reboot to start the server with the new kernel. A package may also signal that a reboot is requested.
Library and service updates
A daemon that was already running during an update can continue using the old shared-library code until it is restarted. Debian’s Securing Debian Manual explains that security updates may therefore require restarting some system services. This usually calls for targeted service restarts, not a whole-host reboot.
Kernel updates
Installing a new kernel does not make the running system use it immediately. Reboot the server to boot into the updated kernel. Debian’s Securing Debian Manual describes this requirement.
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A package-requested reboot
Some packages use /run/reboot-required to indicate that a reboot is requested. Debian Policy documents this convention, but explicitly says it provides no guarantee about when or whether the requested reboot will happen. The file is a useful prompt to investigate, not a complete decision system.
How to check what needs restarting
- Finish the package update. Review APT’s output and any package-specific instructions before deciding what to restart.
- Check for the reboot signal. Run
test -e /run/reboot-required && echo "Reboot requested". If the file exists, inspect/run/reboot-required.pkgsfor package names recorded by maintainers. If it does not exist, continue checking: the convention does not guarantee that every situation needing a reboot will be signalled. - Identify affected services. Use
needrestartafter an APT upgrade to identify services that may still need restarting after library updates. Debian’s Security Manual says it can prompt for affected service restarts. For older releases, the manual mentionscheckrestart, available indebian-goodies. - Restart affected services where appropriate. Debian’s default service manager is systemd, so manage the relevant service through the host’s service manager. For example, the command is
sudo systemctl restart SERVICE; replaceSERVICEwith the service identified for your host. - Schedule a reboot if the kernel was updated or a package requests one. Choose a maintenance window that fits the workload and arrange monitoring and access before rebooting.
- Verify recovery. After the reboot, check that the server is reachable, critical services are healthy, and the running kernel is the expected one.
How to reboot safely over SSH
A reboot can interrupt remote access, and a service restart can disrupt users even when the host stays online. Before acting, consider the service’s impact, the maintenance window, and how you will recover if networking does not return.
- Keep a working access path and, where possible, arrange provider console, serial console, or other recovery access before rebooting a remote server.
- If restarting SSH, keep the current SSH session open. Start a second connection and confirm it works before closing the first. Debian’s Security Manual recommends this check.
- For kernel updates, do not assume a remote server will reconnect automatically. Debian’s Trixie release notes warn that a remotely managed machine may need local-console recovery if networking fails to return. Their specific advice to arrange remote serial-terminal access applies to the Bookworm-to-Trixie upgrade; the same recovery concern is worth considering before other remote kernel reboots.
What the reboot signal does—and does not—tell you
Debian Policy Manual v4.7.4.1 describes /run/reboot-required as a package convention: maintainers can use it to request a reboot and record package names in /run/reboot-required.pkgs. The policy warns that there are no guarantees about when or whether the requested reboot occurs. Treat the signal as an additional clue alongside the update details and service checks, rather than as a definitive yes-or-no test.
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