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Building and Managing Thick Jails with VNET on FreeBSD

Introduction

Jails have been a defining feature of FreeBSD for decades. They provide a lightweight, secure, and flexible way to isolate services and applications at the operating system level. When paired with VNET networking and a thick jail approach, you gain complete control over customization, scalability, and performance. This post explores the advantages, disadvantages, potential use cases, reasons you might avoid jails, available tooling, native installations, efficient use of resources, scalability considerations, and the level of community support you can expect.

The insights here draw from practical implementation experiences and references to FreeBSD Jails using VNETs, FreeBSD Mastery: Jails (IT Mastery), and the FreeBSD Handbook Jail section.

Why Choose a Thick Jail with VNET and No Tooling

A thick jail contains a complete FreeBSD userland inside the jail, giving you more autonomy and a structure similar to a full installation. Using VNET assigns the jail its own network stack, including IP addresses, routing tables, and firewall rules. Skipping tooling such as iocage or ezjail means you configure everything manually through jail.conf, rc.conf, and network interfaces.

This approach offers maximum transparency and removes abstraction layers, making it ideal for advanced customization and scaling strategies.

Advantages

  • Complete control: Every configuration is transparent and adjustable, from filesystem layout to network parameters.
  • Network isolation with VNET: Each jail has its own TCP/IP stack, avoiding conflicts and enabling realistic network testing.
  • Security: Process, filesystem, and network isolation reduce attack surfaces.
  • Performance: Minimal virtualization overhead compared to full hypervisors.
  • Custom resource allocation: You choose exactly what system resources each jail gets.
  • Predictable behavior: No hidden defaults or unexpected tooling changes.

Disadvantages

  • Steeper learning curve: Manual setup requires a strong understanding of FreeBSD’s networking, storage, and system configuration.
  • Longer initial setup time: Thick jails require more initial work to provision than thin jails.
  • No tooling shortcuts: You must maintain all configurations yourself, which can be error-prone if not documented.
  • Version drift: Without automation, it’s easier to end up with jails running different patch levels or configurations.

Potential Use Cases

  • Network simulation: Using VNET to emulate complex topologies without extra hardware.
  • Application sandboxing: Running services in isolated environments to prevent cross-contamination.
  • Service consolidation: Hosting multiple independent services on one physical server.
  • Testing environments: Creating reproducible development and QA setups.
  • Lightweight hosting: Offering isolated environments to customers without full VM overhead.

Reasons to Avoid

  • High automation requirements: If your workflow demands rapid provisioning with minimal manual work, manual VNET jails may not be ideal.
  • Limited Linux compatibility: Jails run FreeBSD userlands, so Linux-specific applications may not work without other compatibility layers.
  • Small team or time constraints: If you cannot dedicate resources to maintain jail configurations, a tooling-assisted or container-based approach may be better.
  • Need for live migration: Unlike some container platforms, native jails do not have built-in live migration features.

Available Tooling

While this post focuses on a no-tooling approach, the following options exist for those who want more automation:

  • iocage: ZFS-centric jail manager with modern features.
  • ezjail: Older, script-based jail management.
  • bastille: Lightweight container framework for FreeBSD jails.
  • appjail: Template-driven jail deployment.

For native, tooling-free installs, you rely on:

  • /etc/jail.conf
  • jail command
  • rc.conf for startup configuration
  • ifconfig and pf for networking

Efficient Use of Resources

Jails share the FreeBSD kernel, avoiding the CPU and memory overhead of full virtual machines. With VNET, you can fine-tune bandwidth, IP ranges, and firewall rules per jail. ZFS datasets for each jail allow snapshotting, cloning, and efficient disk usage.

A thick jail approach does use more disk space than a thin jail, but it trades storage efficiency for configuration independence and fewer cross-jail dependencies.

Scalability

FreeBSD jails scale horizontally on a single host with minimal additional overhead. You can run dozens or even hundreds of jails on capable hardware, provided you monitor CPU, memory, and network load. Thick jails with VNET allow each instance to have a unique network presence, making them easier to scale into service clusters.

Community Support

The FreeBSD jail ecosystem has decades of history and an active community. You can find support through:

  • FreeBSD Forums: Peer-to-peer discussions and troubleshooting.
  • FreeBSD mailing lists: In-depth technical exchanges.
  • IRC and Matrix channels: Real-time chat with experienced admins.
  • Third-party guides: Books, blogs, and YouTube tutorials.

For advanced setups like thick jails with VNET, the best resources tend to be the FreeBSD Handbook, the man pages, and long-form guides like the ones linked above.

Step-by-Step Thick Jail with VNET Setup (No Tooling)

The following example creates a thick jail with VNET and a dedicated network interface. Adjust hostnames, IPs, and paths to match your environment.

1. Prepare the Base Jail Directory

mkdir -p /usr/local/jails/myjail

Where to obtain base.txz, lib32.txz, and src.txz

Choose one of the following methods. Place the files in /usr/freebsd-dist so the extraction commands in the next step work as written. Match the sets to your host release to avoid ABI mismatch.

A. From the installer media

# Mount your FreeBSD installer ISO or memstick
# Example for an ISO on /dev/cd0
mkdir -p /mnt
mount -t cd9660 /dev/cd0 /mnt

# Copy the distribution sets
mkdir -p /usr/freebsd-dist
cp /mnt/usr/freebsd-dist/base.txz /usr/freebsd-dist/
cp /mnt/usr/freebsd-dist/lib32.txz /usr/freebsd-dist/  # only for amd64 if needed
cp /mnt/usr/freebsd-dist/src.txz  /usr/freebsd-dist/

umount /mnt

B. Download from the official mirrors

# Replace REL with your host release, for example 14.1-RELEASE
REL=14.1-RELEASE
ARCH=$(uname -m)          # amd64 or aarch64 etc.

mkdir -p /usr/freebsd-dist
cd /usr/freebsd-dist

fetch https://download.freebsd.org/releases/${ARCH}/${ARCH}/${REL}/base.txz
# lib32 is only for amd64
[ "$ARCH" = "amd64" ] && fetch https://download.freebsd.org/releases/${ARCH}/${ARCH}/${REL}/lib32.txz
fetch https://download.freebsd.org/releases/${ARCH}/${ARCH}/${REL}/src.txz

C. Use the system tool to fetch sets to /usr/freebsd-dist

mkdir -p /usr/freebsd-dist
bsdinstall distfetch
# Follow prompts to select a mirror and fetch base, lib32 if applicable, and src

2. Install a Complete Userland into the Jail

cd /usr/local/jails/myjail
tar -xpf /usr/freebsd-dist/base.txz
tar -xpf /usr/freebsd-dist/lib32.txz   # Optional if you need 32-bit compatibility
tar -xpf /usr/freebsd-dist/src.txz     # Optional if you need source

3. Enable VNET in the Host

Edit /boot/loader.conf:

vnet.enable=1
if_bridge_load="YES"
if_epair_load="YES"

Reboot to apply changes.

4. Create a Bridge Interface for the Jails

ifconfig bridge0 create
ifconfig bridge0 addm em0 up

Replace em0 with your main network interface.

5. Configure rc.conf for Networking

cloned_interfaces="bridge0"
ifconfig_bridge0="addm em0 up"

6. Define the Jail in /etc/jail.conf

myjail {
    host.hostname = "myjail.local";
    path = "/usr/local/jails/myjail";
    persist;
    vnet;
    vnet.interface = "epair0b";
    exec.start = "/bin/sh /etc/rc";
    exec.stop = "/bin/sh /etc/rc.shutdown";
    exec.clean;
    allow.raw_sockets;
}

7. Create the epair Interface and Assign to the Jail

ifconfig epair0 create
ifconfig epair0a up
ifconfig bridge0 addm epair0a

8. Start the Jail

service jail onestart myjail

9. Configure Networking Inside the Jail

Inside the jail:

ifconfig epair0b inet 192.168.1.50/24 up
route add default 192.168.1.1
echo 'ifconfig_epair0b="inet 192.168.1.50 netmask 255.255.255.0"' >> /etc/rc.conf
echo 'defaultrouter="192.168.1.1"' >> /etc/rc.conf

10. Test

From the host:

jexec myjail ping -c 3 8.8.8.8

You now have a fully isolated thick jail with its own network stack using VNET, built entirely without jail management tooling.

VNET Jail Networking Diagram

                    +-------------------+
                    |     LAN / WAN     |
                    +-------------------+
                             |
                           em0
                             |
+--------------------------------------------------------------+
|                        Host System                           |
|                      FreeBSD Kernel                          |
|                                                              |
|   +-----------------+                                        |
|   |     bridge0     |  (members: em0, epair0a, epair1a, epair2a)
|   +-----------------+                                        |
|       |        |        |                                    |
|   epair0a   epair1a   epair2a                                |
+-------|--------|--------|------------------------------------+
        |        |        |
   +----+----+ +----+----+ +----+----+
   | epair0b | | epair1b | | epair2b |
   +----+----+ +----+----+ +----+----+
        |        |        |
  +------------+ +------------+ +------------+
  |  Jail A    | |  Jail B    | |  Jail C    |
  |  VNET      | |  VNET      | |  VNET      |
  +------------+ +------------+ +------------+

Conclusion

Thick jails with VNET and no tooling deliver unmatched customization and scalability for FreeBSD power users. They demand more time and expertise up front, but the result is a stable, flexible, and efficient environment that can evolve with your needs. If you value control over convenience, this approach will feel both empowering and future-proof.