Building for other Distributions and Architectures

NixOS is great! It gets you 99% to a reproducible and write-once-work-everywhere software chain. Sadly, it is not used everywhere so from time to time you might find yourself having to build for other distributions.

Now, for anyone who has ever tried to run executables built for let’s say Debian, will have found that it won’t just work. Debian’s file structure is way different to NixOS’ and as such the linker will not find the dynamic libraries it needs. They might not even be available!

Well, anything you build on NixOS has the reverse problem: None of the ‘classical’ other distributions do it like NixOS.

This means things are just incompatible. On top of that, the versions might be quite different, so it might be that either distributions uses ABI-incompatible versions.

This has conceptually an easy solution: statically link everything.

But that is boring, not easy, and maybe not something you always want. I’m not gonna go into pro/cons here. Nor how to statically link stuff.

Instead, I’ll explain how I made a pretty cursed setup to solve a problem I was facing.

I was tasked to compile and package software that has to run on Debian Bookworm, on a 64-bit ARM device. Aka, produce a deb file that can be installed there.

The problem was: I am running on a x86-64 NixOS system. So not only do I have to compile for another distribution, but I also have to cross-compile my software.

Prelude

Luckily I had several things going for me:

• The software in question is written in Rust. Rust has a great cross-compilation story! (Maybe I am saying this because I know other systems, but frankly its really not the worst).
• The software only had openssl as its more complicated library. glibc is pmuch always there in this scenario.
• I got it working in a docker container beforehand

The last part is important, as doing the whole iteration on required libraries etc… would have been extremely tedious in another situation.

The components

Conceptually nixpkgs gives us these components to realize what we need:

• The ability to specify packages from the debian repository
• The ability to splice them together as a FHS
• Start a container/virtual machine with a given set of debian packages
• Run commands in that container and copy it out again

![NOTE] TODO: Finish this part!