FreeBSD thin jails using zfs snapshots

2025-09-05 freebsd jails

This is mostly documented already at https://docs.freebsd.org/en/books/handbook/jails/#thin-jail but that might go away?!

Done once to create zfs datasets:

zfs create -o mountpoint=/usr/local/jails zroot/jails
zfs create zroot/jails/containers
zfs create zroot/jails/media

Done once per release to create the base:

export FBSDREL=14.3-RELEASE
zfs create -p "zroot/jails/templates/$FBSDREL"
fetch "https://download.freebsd.org/ftp/releases/amd64/amd64/$FBSDREL/base.txz" -o "/usr/local/jails/media/$FBSDREL-base.txz"
tar -xf "/usr/local/jails/media/$FBSDREL-base.txz" -C "/usr/local/jails/templates/$FBSDREL" --unlink
cp /etc/resolv.conf "/usr/local/jails/templates/$FBSDREL/etc/resolv.conf"
cp /etc/localtime "/usr/local/jails/templates/$FBSDREL/etc/localtime"
freebsd-update -b "/usr/local/jails/templates/$FBSDREL/" fetch install
zfs snapshot "zroot/jails/templates/$FBSDREL@base"

To create a jail based on that release:

zfs clone "zroot/jails/templates/$FBSDREL@base" zroot/jails/containers/thinjail

Then configure the jail in /etc/jail.conf.