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The Quick-N-Dirty Guide to Jails in FreeBSD

THIS PAGE IS OUT OF DATE and several of the commands will no longer work properly. Please see my updated page which has been tested on 9.2-RELEASE.

Please note. Lately, I have begun using the ezjail program to manage jails. I'm leaving this page intact, as I hope people may find it useful, but recommend that the reader wishing to run jails on FreeBSD use our ezjail guide instead.

Recently, I had to set up an apache server within a jail. Although the jail man page is quite good, there were a few tricks here and there that cost me some googling time. Remember, the qnd in our name stands for Quick 'N' Dirty so this is not going to be an exhaustive treatise on jails.

1.) Do you have jail?

At a command prompt type
which jail

You should get back the answer /usr/sbin/jail. If not, something is probably seriously wrong with your system. The jail program is a part of the basic FreeBSD system.

2.) Installing, Configuring and Testing Your Jail

In my case, I decided to put my jail in /usr. For purposes of example, we'll say I have a machine called On my home network, it has an IP address of The interface is fxp0. I'm going create a jail in /usr/jail, give it hostname of and an IP address of

So, first I look at the jail manpage which gives instructions.

man jail

I suggest that you do the same. After reading the instructions, I do the following.

cd /usr/src
mkdir -p $D
make world DESTDIR=$D
make distribution DESTDIR=$D
mount_devfs devfs $D/dev

As you can imagine, it takes a bit of time to do the make world. However, eventually this finishes and I have a virtual server in /usr/jail.

The man page then suggests adding various things to /etc/rc.conf. Following a page on the FreeBSDdiary site I didn't do this. There are various issues which I'll deal with shortly.

The next thing I wanted to do was a bit of configuration with the jail. So, still going with the man page I started a shell in the jail. At this point I haven't yet configured the jail's rc.conf however, using the made up hostname and IP address are necessary for the next step.

jail /usr/jail /bin/sh

This puts me in the jail with a sh shell. (It will also temporarily set the jail's hostname to There are several steps recommended by the man page. First, I create an empty fstab. I just type

vi /etc/fstab

That opens up a file with nothing in it. Then I save the file with :x or :wq, or whatever I wish to use to close vi. Next, I edit the jail's rc.conf. I use vi, you can use ee or whatever. You have a fully functional FreeBSD system in this jail, so both are available. Actually

touch /etc/fstab

would have done the same thing and been more efficient. I add the following to jail's /etc/rc.conf


The man page says to then run newaliases. If you do, you get an error message but I do it anyway. :) I then copy over's /etc/resolv conf to the jail. (I can do this from the host machine, not from within the jail) So, while logged on to

cp /etc/resolv.conf /usr/jail/etc/

Then, back in the jail, I set a root password, different than the password on, configure the timezone and add any users. In my case, I want to add a user called john, who will belong to the wheel group, so that he can perform any maintainance necessary. I might have user john on, and again, I'll use a different password on One can, while in the jail, run /usr/sbin/sysinstall and go to the post install configuration menu to do the timezone (and other aspects of configuration).

The whole purpose of this jail is to run a webserver. So, while within in the jail, I add apache. I can install the ports collection, (which is not in the jail by default) but in this case, I'm just going to use pkg_add. I'm also going to add bash, as I prefer a Bourne style shell, and the installed sh doesn't have tab completion. I'll also add sudo, so that john can do everything without having to su to root while in the jail.

pkg_add -r apache21 sudo bash

Next I configure apache. This is about jails, not apache, but I do add a line to start it in jail's /etc/rc.conf. I move my web pages to /usr/jail/usr/local/www/docs, edit my /usr/jail/usr/local/etc/apache21/httpd.conf and I'm basically set. I add the entry to /etc/hosts. I keep it as simple as possible, and as this works, all I do is add the line       jail

to jail's /etc/hosts.

With recent changes to the way rc.d scripts work, one may also have to add the following to the jail's /etc/rc.conf

If that line isn't in the jail's rc.conf then some programs that run from the jail's /usr/local/etc/rc.d may not start properly at boot.

Now, I exit out of the jail. I'm ready to restart it. I have a few things to do, as you'll see. We have to set the alias on interface fxp0, mount procfs on jail's /proc and then we begin. Note that I am back in, not in the jail, as I do the following.

ifconfig fxp0 inet alias
mount -t procfs proc /usr/jail/proc
jail /usr/jail /bin/sh /etc/rc

We also want to edit host's /etc/ssh/sshd_config. You'll find a line


That means it's listening on all interfaces. If, even after setting an alias for the jail, you then try to ssh into the jail, you'll wind up in So, we want to change this. Remember host is and jail So, we want host's ssh daemon to only listen on Edit the line to read (and don't forget to remove the # from the front of it)


Restart the sshd daemon, with a kill -SIGHUP or using /etc/rc.d/sshd restart or whatever you prefer. I should now be able to open a web browser to on my home network and be able to browse the website I've created in /usr/jail/usr/local/www/doc.

One can check the jail and its Jail ID with the command


The ordinary shutdown commands don't work within a jail. To kill it you can while in the jail use

kill -KILL -1


kill -TERM -1

To kill the jail from you can use the jexec command. First run jls to see the Jail ID. You might get something back like

JID   IP Address       Hostname             Path
2     /usr/jail

Now, you use jexec with the the JID

jexec 2 kill -TERM -1

You can then run jls again, which should show you that nothing is running.

While testing, I simply use a script to start jail. If all you wish to do is run apache, you don't have to mount devfs on /usr/jail/dev. The problem is that if you don't, you won't be able to ssh into the jail. However, as the jail manpage points out, you don't wish to mount the entire devfs on /dev because it opens up various vulnerabilites, possibly giving someone access to, for example, your hard drives. The jail man page sends you to the devfs manpage which has a few examples. While testing and using the script, I mount only enough so that I can ssh into it. I simply apply rules on the fly in the script

sudo ifconfig fxp0 inet alias
sudo mount -t procfs proc /usr/jail/proc
sudo jail /usr/jail /bin/sh /etc/rc
sudo devfs -m /usr/jail/dev rule apply hide
sudo devfs -m /usr/jail/dev rule apply path null unhide
sudo devfs -m /usr/jail/dev rule apply path random unhide

Once everything is working, I add various lines to /etc/rc.conf so that the jail will work upon reboot. There is already a default set of jail rules for devfs in /etc/defaults. It's part of the file /etc/defaults/devfs.rules. There is also a jail_list mentioned in man rc.conf that we can use for conveniently naming our jail. In this case I'm going to call it ujail, but I could name it Frank if I felt like it. There are some defaults that one can view in /etc/defaults/rc.conf. I'm going to set this jail so that root in the jail can't change the hostname. I'm going to add a devfs entry as well, so that I can ssh into the jail.

I'm also going to set the alias for the fxp0 interface, so that is running at startup as well. I will add all this to /etc/rc.conf

ifconfig_fxp0_alias0="inet netmask 0xffffffff"

Congratulations, you're done.


man jail, man devfs, man rc.conf

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