FreeBSD Jails (Tested on 6.3)

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FreeBSD Jails (Tested on 6.3)

It seems that almost every year I need to update this tutorial. Some things change, and some things don’t. This version is being done on FreeBSD 6.1, 6.2, and now tested with 6.3. If you are using FreeBSD 5.x, feel free to look at my 5.x tutorial on Screaming Electron.

To start out, make sure you have your source tree installed for FreeBSD. If you do not have this, run /stand/sysinstall, go to Configure, Distributions, and then “src”. After you have your system prepped and ready for jails to be added, start planning out your virtual network.

Preparing for FreeBSD Jails

To start out, make sure you have your source tree installed for FreeBSD. If you do not have this, run /stand/sysinstall, go to Configure, Distributions, and then “src”. After you have your system preped and ready for jails to be added, start planning out your virtual network.

In this tutorial, we are going to use our main interface as a switch. Each jail will be in the same collision domain, but they can be subnetted, or even put as their own networks. For now, we’ll stick with the same network, and after this tutorial, you shouldn’t have a problem changing the network to your needs.

First I’m going to set the IP to, and then run a jail off the main interface.

# ifconfig em0
# ifconfig em0 inet alias netmask
# ifconfig em0
inet6 fe80::204:5aff:fe6f:1d0c%dc0 prefixlen 64 scopeid 0x1
inet netmask 0xffffff00 broadcast
inet netmask 0xffffffff broadcast
ether 00:04:5a:6f:1d:0c
media: Ethernet autoselect (100baseTX <full-duplex>)
status: active

Ok. So now we have our main interface at and our aliased IP at Since we don’t want any inetd services listening on the aliased IP, we need to add a line to the /etc/rc.conf file.

inetd_flags="-wW -a"

Creating the Jail

Now to make the jail. First make the directory you want the jail in.

# mkdir /usr/jail/server1

Then we make the virtual system.

# cd /usr/src

# make world DESTDIR=/usr/jail/server1

# cd etc

# make distribution DESTDIR=/usr/jail/server1

# cd /usr/jail/server1

# ln -sf /dev/null kernel

Once this is all done your jail is built and ready to be set. First, a few minor details to the setup.

# touch /usr/jail/server1/etc/fstab

# cat /etc/resolv.conf > /usr/jail/server1/etc/resolv.conf

So now we are ready to configure the jail for the first time. We start the jail with the jail command, like so:

# jail /usr/jail/server1 /bin/sh

You will then be dropped to a new shell inside the jail. First, set your root password with ‘passwd’ or with ‘sysinstall’. Also setup a user account to use when using SSH to enter. Make sure to also run ‘newaliases’ while in this environment, otherwise you will have issues with your jail starting with sendmail. Next, add the SSH enable line into your /etc/rc.conf (in the jail).


Once you have this done, type ‘exit’ to leave the jail. Now, we need to mount the proc and dev filesystems for our jail.

# mount -t procfs proc /usr/jail/server1/proc

# mount -t devfs dev /usr/jail/server1/dev

NOTE: If you are having trouble with SSH finding a console, make sure this is mounted

and now to start it all up:

# jail /usr/jail/server1 /bin/sh /etc/rc

Testing the Jail Environment

Now you should be able to ssh to your new jail, and start configuring the services you wish to run inside.

To double check that your jail is running, run ‘jls’

# jls
    JID  IP Address      Hostname                      Path
1    /usr/jail/server1

Well, now we have a fully functioning jail. Now we need to set it up so we don’t have to start it manually. We are going to add a few more values to the rc.conf. You can probably put this anywhere in the rc.conf, but in good practice, you should put it behind main host specific entries (i.e. hostname, ifconfig, defaultroute, etc).

ifconfig_em0_alias0="inet netmask 0xffffffff"







jail_jail1_exec_start="/bin/sh /etc/rc"



Here we set the jail config name inside rc.conf, and the settings for that jail “jail_jail1*”. We also set the global setting, “jail_socket_unixproute_only”, to only allow TCP/IP to be used inside the jail.

Now, if we want to administer the jail without using SSH, we can always use the jexec command.

jexec <JID> <command>

# jexec 1 /bin/sh

This will drop you right into a Bourne in the jail (If it doesn’t, run jls, and make sure the JID is correct). Now that we have the environment setup, you might want to start installing some ports. Before you go and drop the ports collection into your jail, why not make it to where you don’t have to use all of that space? Drop out of your jail shell, if you are in one, to setup the ports from the main to the jail. Since symlinks do not work in a jail (and we don’t want them to), login with your root user, or use sudo to create a ports dir, and mount a nullfs copy of the ports.

# mkdir /usr/jail/server1/usr/ports

# mount_nullfs /usr/ports /usr/jail/server1/usr/ports

If for any reason that you need to find processes in the jail, do a “ps ax | grep J” in a shell of the main system.

Now you should be all ready to go with your jail, and many more to come. Be sure that your network service applications in the jail are listening to the aliased IP, otherwise you might have issues connecting to the server with those services. In this sort of setup, you shouldn’t have too much of a problem.

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