Creating Jails

This feature is generally available in TrueNAS CORE and supported by the TrueNAS Community. iXsystems customers with TrueNAS Enterprise hardware and an iXsystems Support contract can contact Support about accessing these features.

Jails are a lightweight, operating-system-level virtualization. One or multiple services can run in a jail, isolating those services from the host TrueNAS system. TrueNAS uses iocage for jail and plugin management. The main differences between a user-created jail and a plugin are that plugins are preconfigured and usually provide only a single service.

Why use a Jail instead of a VM? By default, jails run the FreeBSD operating system. These jails are independent instances of FreeBSD. The jail uses the host hardware and runs on the host kernel, avoiding most of the overhead usually associated with virtualization. The jail installs FreeBSD software management utilities so FreeBSD packages or ports can be installed from the jail command line. This allows for FreeBSD ports to be compiled and FreeBSD packages to be installed from the command line of the jail.

It is important to understand that users, groups, installed software, and configurations within a jail are isolated from both the TrueNAS host operating system and any other jails running on that system.

The ability to create multiple jails offers flexibility regarding software management. For example, an administrator can choose to provide application separation by installing different applications in each jail, to create one jail for all installed applications, or to mix and match how software is installed into each jail.

Setting Jail Storage

You must create a data storage pool before using jails. Make sure the pool has enough storage for all the intended jails. The Jails screen displays a message and button to CREATE POOL if a pool doesn’t exist on the TrueNAS system.

If pools exist, but not chosen to use with jails or plugins, a dialog displays prompting you to choose a pool. Select a pool and click CHOOSE.

To select a different pool for jail and plugin storage, click the icon. You can select a different pool from the dropdown list.

Jails and downloaded FreeBSD release files are stored in a dataset named iocage/.

The iocage dataset
  • It should have at least 10 GiB of free space (recommended).
  • It cannot be located on a share.
  • It, the iocage dataset, automatically uses the first pool that is not a root pool for the TrueNAS system.
  • A defaults.json file contains default settings used when a new jail is created. The file is created automatically when not already present. When the file is present but corrupted, iocage shows a warning and uses default settings from memory.
  • Each new jail installs into a new child dataset of iocage/. For example, with the iocage/jails dataset in pool1, a new jail called jail1 installs into a new dataset named pool1/iocage/jails/jail1.
  • FreeBSD releases are fetched as a child dataset into the /iocage/download dataset. This datset is then extracted into the /iocage/releases dataset to use in jail creation. The dataset in /iocage/download can then be removed without affecting the availability of fetched releases or an existing jail.
  • They, the iocage/ datasets on activated pools, are independent of each other and do not share any data.

iocage jail configs are stored in /mnt/poolname/iocage/jails/jailname. When iocage is updated, the config.json configuration file is backed up as /mnt/poolname/iocage/jails/jailname/config_backup.json. You can rename the backup file to config.json to restore previous jail settings.

Creating Jails

TrueNAS has two options to create a jail. The Jail Wizard makes it easy to create a jail or the ADVANCED JAIL CREATION alternate method, where every possible jail option is configurable. See [Jails Screen](/core/uireference/jailspluginsvms/jails/jailsscreens/ for more information on the Jails screens and configuration settings.

To add a new jail, click Jails > ADD. The wizard provides the simplest process to create and configure a new jail. The advanced configuration method is recommended for only advanced users with very specific use applications.

Enter a name for the jail. Names can contain letters, numbers, periods (.), dashes (-), and underscores (_).

Select the jail type. Default (Clone Jail) or Basejail. Clone jails are clones of the specified FreeBSD release. They are linked to that release, even if they are upgraded. Basejails mount the specified release directories as nullfs mounts over the jail directories. Basejails are not linked to the original release when upgraded.

Specify the release to use. Jails can run FreeBSD versions up to the same version as the host TrueNAS system. Newer releases are not shown. Versions of FreeBSD are downloaded the first time they are used in a jail. Additional jails created with the same version of FreeBSD are created faster because the download has already been completed.


Click Next to display the second Wizard screen with a simplified list of networking options.


Jail Supported Networking Options

Jails support several different networking solutions:

  • VNET adds a virtual network interface to the jail. This interface can select NAT, DHCP, or static jail network configurations. Since VNET provides the jail with an independent networking stack, it can broadcast an IP address, which is required by some applications.

  • NAT (Network Address Translation) uses the TrueNAS IP address and selects a unique port for the jail to use. VNET is required when NAT is selected.

  • DHCP Autoconfigure IPv4 selected for the jail to receive its IP address from a DHCP server.

  • Configure networking by entering values for the IPv4 Address or IPv6 Address fields. You can configure any combination of these fields. Multiple interfaces are supported for IPv4 and IPv6 addresses. To add more interfaces and addresses, click ADD.

    Setting the IPv4 Default Router and IPv6 Default Router fields to auto automatically configures these values. You must select VNET to enable the IPv4 Default Router field. When manually configuring IP addresses and if no interface is selected, TrueNAS automatically assigns the given jail IP address to the current active interface of the host system.

Leaving all checkboxes cleared and fields empty initializes the jail without any networking abilities.
Add networking to the jail after creation by going to Jails, clicking for a jail, then > Basic Properties.

Selecting a proxy in the TrueNAS network settings also configures new jails to use the proxy settings, except when performing DNS lookups. Make sure a firewall is properly configured to maximize system security.

When pairing the jail with a physical interface, edit the network interface and select Disable Hardware Offloading. This prevents a network interface reset when the jail starts.
Additional VMware Requirements

Jail Networking

If you have installed TrueNAS in VMware, you need functional networking to create a jail.

For the jail to have functional networking, you have to change the VMware settings to allow Promiscuous, MAC address changes, and Forged Transmits.

Promiscuous ModeWhen enabled at the virtual switch level, objects defined within all portgroups can receive all incoming traffic on the vSwitch.
MAC Address ChangesWhen set to Accept, ESXi accepts requests to change the effective MAC address to a different address than the initial MAC address.
Forged TransmitsWhen set to Accept, ESXi does not compare source and effective MAC addresses.

Click NEXT to view a summary screen of the chosen jail options. Click SUBMIT to create the new jail. After a few moments, the new jail is added to the primary jails list.

Advanced Jail Creation

Click Jails > ADD, then ADVANCED JAIL CREATION to open the advanced jail creation form.


Creating a Jail without Networking

Click for details

You can create a usable jail without any networking by selecting only the required Jail Name and Release. Configure the remaining Basic Properties when the jail needs to communicate over the local network or out to the internet.

If you are an experienced user you can access additional advanced configuration settings in the Jail Properties, Network Properties, and Custom Properties sections.

For more information on the configuration screens, see Jails Screens.

Creating Template Jails

Click for details

Template jails are basejails that can efficiently create jails with the same configuration. These steps create a template jail:


  2. Select Basejail as the Jail Type. Configure the jail with desired options.

  3. Select Template in the Custom Properties section.

  4. Click SAVE.

  5. Click ADD.

  6. Enter a name for the template jail.

    Leave Jail Type as Default (Clone Jail).

    Enter Release as the basejailname(template), where basejailname is the name of the base jail created earlier.

  7. Complete the jail creation wizard.