Tag Archives: Firejail

Securing a Web Server Using a Linux Namespaces Sandbox

The goal of this article is to isolate a small public web server on a simulated demilitarized zone (DMZ) network, and to restrict the local network access in case the server is breached. It is an extra security layer added to an existing home server setup.

Internal DMZ network setup

Internal DMZ network setup

The DMZ consists of an internal network 10.10.20.0/24 connected to br0 bridge device. On this network I place a Linux namespaces security sandbox at 10.10.20.10, running a web server. In case an intruder gets control of the web server, he will be running with low privileges as a generic www-data user. The host firewall configuration will not allow him to open connections anywhere outside DMZ network.

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How to Restrict a Login Shell Using Linux Namespaces

Firejail is a SUID sandbox program that reduces the risk of security breaches by restricting the running environment of untrusted applications using Linux namespaces. It allows a process and all its descendants to have their own private view of the globally shared kernel resources, such as the network stack, process table, mount table.

Started as a simple sandbox for Mozilla Firefox, Firejail was expanded to work on any type of executable, such as servers, graphic programs, and even as login shell.

The program is written in C and only needs libc and POSIX threads (libpthreads), available by default on any Linux platform. The download page provides source code (./configure && make && sudo make install), deb (dpkg -i firejail.deb) and rpm (rpm -i firejail.rpm) packages. Once installed, you can start a program in sandbox as:

$ firejail [options] program and arguments
Example:
$ firejail --debug firefox

Default sandbox

To login into a Firejail sandbox, you need to set /usr/bin/firejail as user shell in /etc/passwd. You can change the shell for an existing user with chsh command:

# chsh --shell /usr/bin/firejail

Another option is to define the shell when the user account is created:

# adduser --shell /usr/bin/firejail username

Below is a ssh login session into a sandboxed account:

SSH login into a default Firejail sandbox

SSH login into a default Firejail sandbox

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