An incredibly underused but oh-so-amazing feature in Linux is capabilities. Everyone should use it to make their lives simpler.

By default, a user does not have many permissions on a system. Some are granted automatically through different means, such as reading and writing files in your home directory. As developers, we may regularly have to reach for sudo to allow us to do what we want. Writing our password over and over again, or constantly forgetting to prefix your command with sudo. It can get tiring.

On the other end, you could do like ye olden days and start a shell as a superuser; run everything with sudo. However, sudo was specifically invented to not have users run everything as a superuser, as it is inherently a bad idea. Using sudo gives you access to everything. Not using it gives you access to very little. But what if there was a middle ground?

Enter capabilities. All of the actions that normally require sudo have one or more capabilities associated with them. Sudo grants you all capabilities, allowing for any kind of mayhem when running commands with it. However, you can also grant yourself only some of these capabilities. It is assigned per user and application. This means that to run a command that normally requires sudo without it, both the user and the command must have the required capabilities. This gives the bonus of safety, an application cannot do actions you explicitly haven’t given it access to. It also allows system administrators to give users access to run certain applications without the need to give them sudo rights.

Using capabilities

As mentioned above, capabilities are given to both users and applications. Let’s start with users. This is stored in the file /etc/security/capability.conf. Edit the file and add a line like this but with your username.

cap_net_raw,cap_net_admin	casan

This gives the user casan the two capabilities cap_net_raw and cap_net_admin and should take effect immediately. Next, we need some application. Let’s use the ip command. To find where the command is located we can use which ip. An important detail here is that you can’t give capabilities to symlinks, so if a command is symlinked the capability must be given to the actual executable file.

But before we add the capability, let’s try doing a command without sudo. It should fail with ioctl(TUNSETIFF): Operation not permitted.

ip tuntap add tap99 mod tap

Now add the capability cap_net_admin to the command.

sudo setcap cap_net_admin+ep /bin/ip

Try the previous command again and it should work. And just to clean up our mess we can delete the newly created tap99 interface with ip link del dev tap99 (which we can also do without sudo).

Now you know how to add capabilities to avoid using sudo. But how do we know which capabilities a command needs? It can depend on which arguments you pass the command, depending on what the arguments do. If you use the commands of ip netns you need different capabilities than the example above. You can always read the manpage capabilities(7) to get a better understanding of the individual capabilities. But if you don’t fully understand how a command works it can be difficult to figure out. It can also be the case of one command starting another subprocess, where the subprocess is the one that needs the capability.

Capmon: figuring out capabilities

Introducing Capmon - a Linux capabilities monitor. A tool that monitors your process and shows a report over which capabilities it looked for. You simply pass your command as an argument to Capmon and it will execute it. But before you get started, Capmon does require the capabilities cap_dac_override and cap_sys_admin, running it with sudo does not work correctly because that bypasses certain checks. Now that you’ve added those capabilities to yourself and Capmon, let’s try it out.

capmon 'ip tuntap add tap99 mod tap'

Assuming you did everything correctly, this should output the following text indicating that the ip application accessed the cap_net_admin capability successfully.


Now take what you’ve learnt here and use more commands without sudo.

Some things to keep in mind

  • If a command is failing but you are not seeing any FAIL output from Capmon, try running with the flag -a. This adds some additional monitoring points.
  • It is recommended to use quotes around your command, otherwise dash-arguments arguments will be interpreted as arguments to Capmon.
  • Commands that require multiple capabilities will usually stop and return an error on the first failed check. In this case, add the first capability, then run it again and it will fail on the second.


The title of this post is a homage to a blogpost by a former colleague who taught me about capabilities and a ton of other things, which in turn inspired me to create Capmon. You can read his post at A life without sudo.