Verifying that the jail works

Once a process has been jailed, the chroot directory will become the new root directory for the process. In our case, /chroot becomes the new /, and we can verify that things are working as expected by attempting to access a file in the root directory and see where the retrieved file is actually located in the real file system.

Let's create two files—both called testfile, but one located in the real root directory and the other located in /chroot:

$ echo "Inside the jail" > /chroot/testfile
$ echo "Outside the jail" > /testfile

To see if we are running inside or outside the jail, we want to create a web page that will display the contents of the file /testfile. If the text "Inside the jail" is displayed, we will know Apache was successfully jailed.

Apache comes with a feature called Server Side Includes (SSI), and one of the commands that is provided by this feature has the ability to execute a command and include the output of it in the web page. The syntax...