Configuring unattended-upgrades for automatic security patching on Debian and Ubuntu

Goal

Enable automatic installation of security (and optionally all) package updates on a Debian or Ubuntu host, with a safe way to preview what it would do first.

Prerequisites

apt-get install unattended-upgrades (Ubuntu also ships update-notifier-common, needed for the reboot-notification mechanism); root access; a maintenance window if reboots will be allowed.

Steps

1. Enable the periodic timer by writing /etc/apt/apt.conf.d/20auto-upgrades:

   APT::Periodic::Update-Package-Lists "1";
   APT::Periodic::Unattended-Upgrade "1";
   

   On Ubuntu, dpkg-reconfigure --priority=low unattended-upgrades creates this file non-interactively when run with DEBIAN_FRONTEND=noninteractive.
2. Edit /etc/apt/apt.conf.d/50unattended-upgrades. The Unattended-Upgrade::Allowed-Origins block "specifies which repositories will be used to gather" the packages that may be auto-installed; on a default install it already lists the security pocket/suite for your distribution.
3. To also reboot automatically when an upgrade requires it, set Unattended-Upgrade::Automatic-Reboot "true"; and, on Ubuntu, keep update-notifier-common installed; optionally set Unattended-Upgrade::Automatic-Reboot-Time "02:00"; for a fixed time.
4. Blacklist anything that must never be silently upgraded with Unattended-Upgrade::Package-Blacklist { "linux-image*"; }; inside the same file, adjusted to your package names.
5. Before trusting the configuration, run it by hand: unattended-upgrade --dry-run --debug. The --dry-run option only simulates installing updates and does not actually do it, while --debug (or -d) writes extra detail to /var/log/unattended-upgrades/unattended-upgrades.log.
6. Check /var/log/unattended-upgrades/unattended-upgrades.log and unattended-upgrades-dpkg.log after the first real run for the list of upgraded packages and any reboot-required marker.

Expected result

unattended-upgrade --dry-run --debug prints the packages it would install without changing the system; after enabling the timer, the log files show a run at the scheduled time.

Limits and test basis

Automatic reboots can interrupt long-running work with no warning beyond the log; a fixed reboot time and a package blacklist for anything reboot-sensitive are the mitigations documented above, not a guarantee. To back out, set both periodic options back to "0" and remove or comment the automatic-reboot line.