14.5. Updating Multiple Jails

Contributed by Daniel Gerzo.
Based upon an idea presented by Simon L. B. Nielsen.
And an article written by Ken Tom.

The management of multiple jails can become problematic because every jail has to be rebuilt from scratch whenever it is upgraded. This can be time consuming and tedious if a lot of jails are created and manually updated.

This section demonstrates one method to resolve this issue by safely sharing as much as is possible between jails using read-only mount_nullfs(8) mounts, so that updating is simpler. This makes it more attractive to put single services, such as HTTP, DNS, and SMTP, into individual jails. Additionally, it provides a simple way to add, remove, and upgrade jails.


Simpler solutions exist, such as ezjail, which provides an easier method of administering FreeBSD jails but is less versatile than this setup. ezjail is covered in more detail in Section 14.6, “Managing Jails with ezjail.

The goals of the setup described in this section are:

  • Create a simple and easy to understand jail structure that does not require running a full installworld on each and every jail.

  • Make it easy to add new jails or remove existing ones.

  • Make it easy to update or upgrade existing jails.

  • Make it possible to run a customized FreeBSD branch.

  • Be paranoid about security, reducing as much as possible the possibility of compromise.

  • Save space and inodes, as much as possible.

This design relies on a single, read-only master template which is mounted into each jail and one read-write device per jail. A device can be a separate physical disc, a partition, or a vnode backed memory device. This example uses read-write nullfs mounts.

The file system layout is as follows:

  • The jails are based under the /home partition.

  • Each jail will be mounted under the /home/j directory.

  • The template for each jail and the read-only partition for all of the jails is /home/j/mroot.

  • A blank directory will be created for each jail under the /home/j directory.

  • Each jail will have a /s directory that will be linked to the read-write portion of the system.

  • Each jail will have its own read-write system that is based upon /home/j/skel.

  • The read-write portion of each jail will be created in /home/js.

14.5.1. Creating the Template

This section describes the steps needed to create the master template.

It is recommended to first update the host FreeBSD system to the latest -RELEASE branch using the instructions in Section 23.5, “Updating FreeBSD from Source”. Additionally, this template uses the sysutils/cpdup package or port and portsnap will be used to download the FreeBSD Ports Collection.

  1. First, create a directory structure for the read-only file system which will contain the FreeBSD binaries for the jails. Then, change directory to the FreeBSD source tree and install the read-only file system to the jail template:

    # mkdir /home/j /home/j/mroot
    # cd /usr/src
    # make installworld DESTDIR=/home/j/mroot
  2. Next, prepare a FreeBSD Ports Collection for the jails as well as a FreeBSD source tree, which is required for mergemaster:

    # cd /home/j/mroot
    # mkdir usr/ports
    # portsnap -p /home/j/mroot/usr/ports fetch extract
    # cpdup /usr/src /home/j/mroot/usr/src
  3. Create a skeleton for the read-write portion of the system:

    # mkdir /home/j/skel /home/j/skel/home /home/j/skel/usr-X11R6 /home/j/skel/distfiles
    # mv etc /home/j/skel
    # mv usr/local /home/j/skel/usr-local
    # mv tmp /home/j/skel
    # mv var /home/j/skel
    # mv root /home/j/skel
  4. Use mergemaster to install missing configuration files. Then, remove the extra directories that mergemaster creates:

    # mergemaster -t /home/j/skel/var/tmp/temproot -D /home/j/skel -i
    # cd /home/j/skel
    # rm -R bin boot lib libexec mnt proc rescue sbin sys usr dev
  5. Now, symlink the read-write file system to the read-only file system. Ensure that the symlinks are created in the correct s/ locations as the creation of directories in the wrong locations will cause the installation to fail.

    # cd /home/j/mroot
    # mkdir s
    # ln -s s/etc etc
    # ln -s s/home home
    # ln -s s/root root
    # ln -s ../s/usr-local usr/local
    # ln -s ../s/usr-X11R6 usr/X11R6
    # ln -s ../../s/distfiles usr/ports/distfiles
    # ln -s s/tmp tmp
    # ln -s s/var var
  6. As a last step, create a generic /home/j/skel/etc/make.conf containing this line:

    WRKDIRPREFIX?=  /s/portbuild

    This makes it possible to compile FreeBSD ports inside each jail. Remember that the ports directory is part of the read-only system. The custom path for WRKDIRPREFIX allows builds to be done in the read-write portion of every jail.

14.5.2. Creating Jails

The jail template can now be used to setup and configure the jails in /etc/rc.conf. This example demonstrates the creation of 3 jails: NS, MAIL and WWW.

  1. Add the following lines to /etc/fstab, so that the read-only template for the jails and the read-write space will be available in the respective jails:

    /home/j/mroot   /home/j/ns     nullfs  ro  0   0
    /home/j/mroot   /home/j/mail   nullfs  ro  0   0
    /home/j/mroot   /home/j/www    nullfs  ro  0   0
    /home/js/ns     /home/j/ns/s   nullfs  rw  0   0
    /home/js/mail   /home/j/mail/s nullfs  rw  0   0
    /home/js/www    /home/j/www/s  nullfs  rw  0   0

    To prevent fsck from checking nullfs mounts during boot and dump from backing up the read-only nullfs mounts of the jails, the last two columns are both set to 0.

  2. Configure the jails in /etc/rc.conf:

    jail_list="ns mail www"

    The jail_name_rootdir variable is set to /usr/home instead of /home because the physical path of /home on a default FreeBSD installation is /usr/home. The jail_name_rootdir variable must not be set to a path which includes a symbolic link, otherwise the jails will refuse to start.

  3. Create the required mount points for the read-only file system of each jail:

    # mkdir /home/j/ns /home/j/mail /home/j/www
  4. Install the read-write template into each jail using sysutils/cpdup:

    # mkdir /home/js
    # cpdup /home/j/skel /home/js/ns
    # cpdup /home/j/skel /home/js/mail
    # cpdup /home/j/skel /home/js/www
  5. In this phase, the jails are built and prepared to run. First, mount the required file systems for each jail, and then start them:

    # mount -a
    # service jail start

The jails should be running now. To check if they have started correctly, use jls. Its output should be similar to the following:

# jls
   JID  IP Address      Hostname                      Path
     3    ns.example.org                /home/j/ns
     2    mail.example.org              /home/j/mail
     1    www.example.org               /home/j/www

At this point, it should be possible to log onto each jail, add new users, or configure daemons. The JID column indicates the jail identification number of each running jail. Use the following command to perform administrative tasks in the jail whose JID is 3:

# jexec 3 tcsh

14.5.3. Upgrading

The design of this setup provides an easy way to upgrade existing jails while minimizing their downtime. Also, it provides a way to roll back to the older version should a problem occur.

  1. The first step is to upgrade the host system. Then, create a new temporary read-only template in /home/j/mroot2.

    # mkdir /home/j/mroot2
    # cd /usr/src
    # make installworld DESTDIR=/home/j/mroot2
    # cd /home/j/mroot2
    # cpdup /usr/src usr/src
    # mkdir s

    The installworld creates a few unnecessary directories, which should be removed:

    # chflags -R 0 var
    # rm -R etc var root usr/local tmp
  2. Recreate the read-write symlinks for the master file system:

    # ln -s s/etc etc
    # ln -s s/root root
    # ln -s s/home home
    # ln -s ../s/usr-local usr/local
    # ln -s ../s/usr-X11R6 usr/X11R6
    # ln -s s/tmp tmp
    # ln -s s/var var
  3. Next, stop the jails:

    # service jail stop
  4. Unmount the original file systems as the read-write systems are attached to the read-only system (/s):

    # umount /home/j/ns/s
    # umount /home/j/ns
    # umount /home/j/mail/s
    # umount /home/j/mail
    # umount /home/j/www/s
    # umount /home/j/www
  5. Move the old read-only file system and replace it with the new one. This will serve as a backup and archive of the old read-only file system should something go wrong. The naming convention used here corresponds to when a new read-only file system has been created. Move the original FreeBSD Ports Collection over to the new file system to save some space and inodes:

    # cd /home/j
    # mv mroot mroot.20060601
    # mv mroot2 mroot
    # mv mroot.20060601/usr/ports mroot/usr
  6. At this point the new read-only template is ready, so the only remaining task is to remount the file systems and start the jails:

    # mount -a
    # service jail start

Use jls to check if the jails started correctly. Run mergemaster in each jail to update the configuration files.