Eric Campbell Feb 20, 2025 0 min read FreeBSD

FreeBSD Jails: Lightweight Containerization

FreeBSD jails predate Docker by a decade and provide OS-level virtualization with minimal overhead.

Key Insights

Jails provide process isolation, filesystem isolation, and network isolation without a hypervisor
VNET jails get their own full network stack including routing tables
ZFS + jails enable instant cloning and snapshot-based rollback

Creating a Basic Jail

# Create the jail filesystem
fetch https://download.freebsd.org/releases/amd64/14.0-RELEASE/base.txz
mkdir -p /jails/web
tar -xf base.txz -C /jails/web

# /etc/jail.conf
web {
    host.hostname = "web.jail";
    path = "/jails/web";
    ip4.addr = "lo1|10.0.0.1/24";
    mount.devfs;
    exec.start = "/bin/sh /etc/rc";
    exec.stop = "/bin/sh /etc/rc.shutdown";
}

ZFS Integration

zfs create zpool/jails/template
# ... set up base jail ...
zfs snapshot zpool/jails/template@base
zfs clone zpool/jails/template@base zpool/jails/newjail

VNET for Full Network Isolation

VNET jails get their own network interfaces, routing tables, and firewall rules — true network namespace isolation.