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Sunday, September 2, 2018

Configure Chroot Jail for Apache in RHEL/CentOS 7

Chroot Jail the Apache Web Server in RHEL/CentOS 7

In Linux, Chroot is an operation to change the apparent root directory i.e. / for a running process and their child processes. The process running in Chroot can not access the files and commands outside that environmental directory tree. This modified environment is called a Chroot Jail.

By chrooting the Apache web server, we do not actually increase the security, rather we limit the access to files and commands by the httpd process and decreases the possible impact, when an Apache web server is compromised. Also potentially dangerous CGI scripts can only access the environmental root directory.

A chroot environment is relatively difficult to set up than a traditional install, especially if we run other software like MySQL, PHP, Python, etc.

In this post, we will set up a chroot jail for Apache Web Server in RHEL/CentOS 7, and create the systemd unit to autostart the httpd service.

Note: All demonstrations in this article are CLI based, therefore, it is recommended that you should have The Linux Command Line: A Complete Introduction for quick reference.

Chroot Jail the Apache Web Server in RHEL/CentOS 7

System Specification:

We are using a Linux machine with following specification for this demonstration.

  • Hostname - webserver-01.example.com
  • IP Address -
  • Operating System - RHEL 7.5

Read Also: Installation of CentOS 7 Server


Chrooting Apache Web Server:

Connect to our Linux machine using ssh and create the directory for setting up the chroot jail.

# mkdir -p /chroot/httpd/

We have already configured the local yum repository on the machine. Please refer to my previous article Configure Local Yum Repository using ISO in RHEL 7.

Now, we are installing Apache web server using yum, but in a non-default directory.

# yum install -y httpd --installroot=/chroot/httpd

Above command will install the Apache Web Server and its dependencies in the /chroot/httpd. You may observe that, it installs many packages that were already installed on the default root /. These packages are reinstalled in /chroot/httpd for the sake of chroot jail setup.

Create some required devices for the chroot environment.

# cd /chroot/httpd/dev
# rm -f null
# mknod /chroot/httpd/dev/null c 1 3
# mknod /chroot/httpd/dev/random c 1 8
# mknod /chroot/httpd/dev/urandom c 1 9

# ls -al
total 0
drwxr-xr-x.  2 root root   47 Sep  2 19:26 .
dr-xr-xr-x. 17 root root  224 Sep  2 19:17 ..
crw-r--r--.  1 root root 1, 3 Sep  2 19:26 null
crw-r--r--.  1 root root 1, 8 Sep  2 19:26 random
crw-r--r--.  1 root root 1, 9 Sep  2 19:26 urandom

Add a default homepage for Apache web server.

# cd /chroot/httpd/var/www/html/
# cat >> index.html << EOF
> <html>
> <head>
> <body>
> <h1>Apache Homepage from Chroot Jail.</h1>
> </body>
> </html>

Allow httpd service to communicate through Firewall.

# firewall-cmd --permanent --add-service=http
# firewall-cmd --reload

Configure systemd unit to autostart httpd service. First copy the required configuration files from chroot jail to host environment.

# cp /chroot/httpd/etc/sysconfig/httpd /etc/sysconfig/
# cp /chroot/httpd/usr/lib/systemd/system/httpd.service /usr/lib/systemd/system/

Now edit the httpd.service unit to start Apache in chrooted environment.

# vi /usr/lib/systemd/system/httpd.service

Search and update the following three lines.

ExecStart=/sbin/chroot /chroot/httpd /usr/sbin/httpd $OPTIONS -DFOREGROUND
ExecReload=/sbin/chroot /chroot/httpd /usr/sbin/httpd $OPTIONS -k graceful

For further clarification, the differences in files after editing are shown below.

# diff /chroot/httpd/usr/lib/systemd/system/httpd.service /usr/lib/systemd/system/httpd.service
< Type=notify
> Type=simple
< ExecStart=/usr/sbin/httpd $OPTIONS -DFOREGROUND
< ExecReload=/usr/sbin/httpd $OPTIONS -k graceful
> ExecStart=/sbin/chroot /chroot/httpd /usr/sbin/httpd $OPTIONS -DFOREGROUND
> ExecReload=/sbin/chroot /chroot/httpd /usr/sbin/httpd $OPTIONS -k graceful

Start and enable httpd service.

# systemctl daemon-reload
# systemctl start httpd ; systemctl enable httpd
Created symlink from /etc/systemd/system/multi-user.target.wants/httpd.service to /usr/lib/systemd/system/httpd.service.

Check root directory for running httpd service. First we obtain the PID of the httpd process and then check it’s root directory in the proc folder.

# systemctl status httpd
â httpd.service - The Apache HTTP Server
   Loaded: loaded (/usr/lib/systemd/system/httpd.service; enabled; vendor preset: disabled)
   Active: active (running) since Sun 2018-09-02 20:18:23 PKT; 1min 16s ago
     Docs: man:httpd(8)
 Main PID: 10620 (httpd)
   CGroup: /system.slice/httpd.service
           ââ10620 /usr/sbin/httpd -DFOREGROUND
           ââ10639 /usr/sbin/httpd -DFOREGROUND
           ââ10640 /usr/sbin/httpd -DFOREGROUND
           ââ10641 /usr/sbin/httpd -DFOREGROUND
           ââ10642 /usr/sbin/httpd -DFOREGROUND
           ââ10643 /usr/sbin/httpd -DFOREGROUND

Sep 02 20:18:23 webserver-01.example.com systemd[1]: Started The Apache HTTP ...
Sep 02 20:18:23 webserver-01.example.com systemd[1]: Starting The Apache HTTP...
Hint: Some lines were ellipsized, use -l to show in full.

# ls /proc/10620/root -al
lrwxrwxrwx. 1 root root 0 Sep  2 20:20 /proc/10620/root -> /chroot/httpd

Browse the URL http://webserver-01.example.com/ from a client’s browser to verify the httpd service.

Chrooted HTTP Homepage

We have successfuly set up a chroot jail for our Apache Web Server in RHEL/CentOS 7.

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  1. One issue... when I tried the initial yum install, I got this error:

    Cannot find a valid baseurl for repo: base/$releasever/x86_64

    When using --installroot=, the variable $releasever isn't populated. The fix is to include --releasever=7 in the command:

    yum install -y httpd --releasever=7 --installroot=/chroot/httpd

    After that it worked perfectly. Thanks!

  2. Hi, its good that you resolve the issue by yourself. But the problem does not related to Chroot, rather it is caused due to the yum configuration. Please refer to Configure Local Yum Repository using ISO in RHEL 7

  3. Hello, thanks a lot, very useful !
    Can you tell how to use php in that configuration please ? Does it need to be installed in the jail ?
    Thanks a lot

    1. There are different ways to install PHP in chrooted environment. It depends upon the user requirement. But if you are following the above article, then it is better that you install PHP in the jail as well.


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