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Author: Solène
Date: 14 February 2021
Tags: linux security sandbox
Firejail is a program that can prepare sandboxes to run other programs. This is an efficient way to keep a software isolated from the rest of the system without need of changing its source code, it works for network, graphical or daemons programs.
You may want to sandbox programs you run in order to protect your system for any issue that could happen within the program (security breach, code mistake, unknown errors), like Steam once had a "rm -fr /" issue, using a sandbox that would have partially saved a part of the user directory. Web browsers are major tools nowadays and yet they have access to the whole system and have many security issues discovered and exploited in the wild, running it in a sandbox can reduce the data a hacker could exfiltrate from the computer. Of course, sandboxing comes with an usability tradeoff because if you only allow access to the ~/Downloads/ directory, you need to put files in this directory if you want to upload them, and you can only download files into this directory and then move them later where you really want to keep your files.
On most Linux systems you will find a Firejail package that you can install. If your distribution doesn't provide a Firejail package, it seems the installing from sources process is quite easy, and as the project is written in C with limited dependencies it may be easy to get the build process done.
There are no service to enable and no kernel parameters to add. Apparmor or SELinux features in kernel can be used to integrates into Firejail profiles if you want to.
The simplest usage is to run a command by adding Firejail before the command name.
$ Firejail firefox
Firejail has a neat feature to allow starting software by their name without calling Firejail explicitly, if you create a symbolic link in your $PATH using a program name but targeting Firejail, when you call that name Firejail will automatically now what you want to start. The following example will run firefox when you call the symbolic link.
export PATH=~/bin/:$PATH $ ln -s /usr/bin/firejail ~/bin/firefox $ firefox
There is a Firejail --list command that will tell you about all sandboxes running and what are their parameters. As a first column the identifier is available for more Firejail features.
$ firejail --list 6108:solene::/usr/bin/firejail /usr/bin/firefox
Firejail also has a neat feature that allows to limit the bandwidth available only for one sandbox environment. Reusing previous list output, I will reduce firefox bandwidth, the number are in kB/s.
$ firejail --bandwidth=6108 set wlan0 1000 40
You can find more information about this feature in the "TRAFFIC SHAPING" section of the Firejail man page.
If for some reason you want to start a program with absolutely no network access, you can run a program and deny it any network.
$ firejail --net=none libreoffice
Firejail is a neat way to start software into sandboxes without requiring any particular setup. It may be more limited and maybe less reliable than OpenBSD programs who received unveil() features but it's a nice trade off between safety and required work within source code (literally none). It is a very interesting project that proves to work easily on any Linux system, with a simple C source code with little dependencies. I am not really familiar with Linux kernel and its features but Firejail seems to use seccomp-bpf and namespace, I guess they are complicated to use but powerful and Firejail comes here as a wrapper to automate all of this.
Firejail has been proven to be USABLE and RELIABLE for me while my attempts at sandboxing Firefox with AppArmor were tedious and not optimal. I really recommend it.
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