The ResinStack is built on top of immutable machine images that are produced with LinuxKit. LinuxKit is a system for building immutable images that run containers.
If you’re curious how this differs from things like CoreOS or Flatcar Linux, the answer is simple: LinuxKit is a mechanism for building immutable images. Flatcar Linux has a concept of users, the ability to interact with it remotely and the ability to change it after it comes online. By default, LinuxKit packs your entire system image into a read-only CPIO archive with no mechanism for external interaction. Flatcar is designed to be installed to a disk, whereas LinuxKit images are designed to boot from a clean source every time. With the appropriate configuration, it is possible to boot an entire datacenter from the network using LinuxKit images, though this may be more or less difficult depending on your particular environment and network architecture.
LinuxKit has another main distinction between it and other dedicated container operating systems, and that is minimalism. Whereas other options have an entire SystemD init process running in them, LinuxKit goes for a very small amount of memory safe Go to bring the system up. The base system images are all based on Alpine, and this lends itself well to creating a very small boot image. An all in one image containing Nomad, Consul, Vault, CoreDNS, Docker, SSH, a getty process, and a handful of startup tasks is a mere 316M of space. Being small also comes with another advantage, and that’s security.
Often you hear about hardening servers after they’ve been installed. If you take a step back to think about what this implies, you realize that the concept is that your base image is simply insufficient for use in a secure environment. Typically this is a trade-off between convenience and security. Most environments are built on top of general purpose Linux distributions, and most general purpose distributions are meant to be convenient for their users to do things with. This convenience comes at a price, and sometimes that price is security. The hardening process often consists of turning off components, adding additional security measures, or trying to remove base functionality that’s not required to serve the function for which a machine was commissioned.
Wouldn’t it be much simpler to skip the hardening process and just start out with an image that was secure by default? This is the idea behind the use of LinuxKit. By starting at nothing and building up, we can add only the minimum number of components to the image needed to accomplish the goal of running Nomad. If only critical dependencies have been added, and they’ve been configured in a secure manner, then there’s nothing left to harden. ResinStack doesn’t need to have a package manager, so we can get rid of that. We don’t need to support a lot of complex things such as network accounts, so we don’t need NSS or PAM and can swap out glibc for the much smaller and better tested musl C library. We don’t need to log into these machines, so we don’t need getty or sshd in a production image.