OpenBSD's Philosophy —

OpenBSD's Philosophy

by Joshua Branson — April 17, 2024

I have talked about OpenBSD before in this blog, and I recently watched a talk by OpenBSD's leader Theo de Raadt about pledge () and arc4random(). He described OpenBSD's design philosophy so well, that I wanted to write it down. You should definitely take some time to hear Theo speak. He's entertaining, and his talks are super awesome!

So a long time ago (early 90s ?), a cracker (1) broke into Theo's OpenBSD's syslog. This got Theo to examine OpenBSD's source code to fix any lingering bugs, and he found a lot. He realized that trying to constantly examine source code to prevent bugs is a never ending process. He wanted to ensure code quality got better over time. He envision an operating system that that enforced code correctness like the below ASCII art shows.

| Poorly written programs |
| crash on OpenBSD.       |
| ----------------------  |
| | Correctly written  |  |
| | programs run well  |  |
| | on OpenBSD.        |  |
| ----------------------  |

He wondered if he could create features to slightly narrow the things that applications could do. When poorly written programs run on OpenBSD, they crash in deterministic ways, but correct programs work just fine. Theo also made sure that "mitigations", or checks to ensure a program's correctness, cannot be turned off on OpenBSD. If a user or a project manager has a problem with an application not working on OpenBSD due to a toggleable security feature, then the user or project manager will just turn off the feature. So to ensure that programs abide by the mitigations, OpenBSD enforces their security policies.

OpenBSD also makes it easy to use their security features. If they introduce a policy that is manatory, they try to make the API easy to use. If you create a security policy that is hard for the programmer to use, then the security policy won't be used.

When OpenBSD creates a new mitigation, their application porters port 3rd party software packages to OpenBSD. Typically these changes find their way into the upstream packages. What's awesome is that usually other operating systems start to use OpenBSD mitigations on their systems 5 years after OpenBSD introduced them. Windows, Mac, and Linux applications all benefit from the strict standards that OpenBSD creates. Theo's talks gave two examples for this: pledge () and arc4random ().

Here's a question for you. How do you get random data? Well you read /dev/random of course. Simple. Easy. Done. That used to be how things were handled. Apparently reading from /dev/random has some limitations that Theo mentioned. Can you read from /dev/random inside the kernel? Inside a library? In your libc? Reading from /dev/random is not perfect. Theo wanted to make something better.

OpenBSD created arc4random () as a better source of entropy. It is a C function that almost any application can call (even the kernel), most of the time. This lets many applications, libraries, etc. easily use random numbers. What's surprizing to me is that many operating systems use arc4random () or a function like it, so that more applictions can easily request random data. This function (or one like it) exists on your Android phone, iPhone, iMac, and sort of on Linux thanks to OpenBSD.

What's pledge () ? Oh ho, let me tell you! Let's take a look at a typical program.

int main () {

  for (;;) {
    ;; let's run the program


Most programs have an initialization phase followed by a loop, in which the application runs. The OpenBSD team realized that the initialize phase uses most of the system calls. After the initialize phase, the program typically needs less system calls. OpenBSD's pledge () was created in response to this pattern present in most programs.

pledge () is a security call, by which an application tells the kernel, "I pledge to only do these things and no other." For example, "I pledge to only output text". Or "I pledge to only access the internet and output text." If the application tries to do something that it has pledged not to do, then OpenBSD kills the application.

A really interesting blog post that talks about this is at justine's blog.

After watching two of Theo's talks, I am fairly convinced that his design goals are slowly working to make all POSIX operating systems more correct and secure. I used to dual boot Guix System and OpenBSD, but unfortunately my spare SSD busted. So for now I am only using Guix System. I personally prefer to use Guix System for my linode server (that powers this blog), because Guix makes it easy to manage servers.

I wish that the Guix developers could one day create Guix OpenBSD System, but I have been told that Guix System assumes your libc is glibc and that OpenBSD cannot currently make isolated build environments as well as Linux can. Also fun fact OpenBSD is not currently working on reproducible builds. :)

My two only minor complaints with OpenBSD currently are:

  • I wish OpenBSD supported Wayland. This may happen in 6 months to a year. Fingers crossed.
  • A better filesystem: OpenBSD's FFS (fast file system) works, but it is possible to lose data in a crash. Filesystems are really hard to get right. Kent Overstreet has been working on bcachefs for almost a decade now, and it might soon become one of Linux's best filesystems (my opinion), so I don't blame OpenBSD for not trying to create a new next generational filesystem. Anybody want to spend 10 years of their life creating an awesome filesystem that will probably only work well on OpenBSD? One might be able to port HammerFS to OpenBSD, and a basic read-only port does exist.

If you want to try OpenBSD, give it a shot! It works really well on most lenovo laptops and most desktop machines.

  1. Nerdy computer people like me use the word "hacker" to mean someone who builds computer programs, and a "cracker" is someone who breaks into computer systems.