linux Security Server

Converting Privileged LXC Containers to Unprivileged

Not long ago, I was looking through my container configurations in the Proxmox GUI and noticed that one very important container had been running as privileged. I must’ve forgotten to click the “Unprivileged” checkbox when I was creating it. For security sake, I try making all of my containers unprivileged. It makes things like sharing files between the host and containers slightly more difficult, but if that particular container is ever compromised by someone with malicious intent, it makes it much more difficult for that malicious actor to compromise the entire host. See the Proxmox documentation on unprivileged containers for more information.

To make this particular container more secure, and to avoid having to set everything up again, I thought it might be easier to simply try and converting it to an unprivileged container. While you can’t just shut the container down, go into the GUI and mark it unprivileged, you can create a backup and make a new container from that backup unprivileged. If you clicked the link to the Proxmox documentation from earlier, you’d see just what I was talking about. In it, you can see under the Creation section, that all you need to do is run

pct restore 1234 var/lib/vz/dump/vzdump-lxc-1234-2016_03_02-02_31_03.tar.gz
-ignore-unpack-errors 1 -unprivileged 

where the first 1234 is your new container ID, and the second (in the backup file) is the old container ID. You can overwrite the previous container with the restore, but it might be a safer bet to just create a new container and then shutdown your old one.

You can also do this through the GUI by navigating to the backups of your container, selecting your backup, and clicking restore. However, when I ran it through the GUI, it gave errors and destroyed the container. Thank goodness for backups, right? Even when running the above command in the CLI, I received errors. Fortunately, they were easy enough to troubleshoot. If you see something like

400 Parameter verification failed.
storage: storage 'local' does not support container directories

then you’ll need to specify your storage. This is easy enough to get around by providing the –storage option and selecting the proper storage location. In my case, the entire command looked like

pct restore 1234 /var/lib/vz/dump/vzdump-lxc-1234-2018_05_25-10_29_59.tar.lzo
-ignore-unpack-errors 1 -unprivileged --storage local-zfs

With that done, you can start up your new container and use it the same way you were before, but this time, it’s a little more secure.

By Dylan Hildenbrand

10+ years experience in web development, homelab enthusiast, tinkerer

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