[jbboehr]

In NixOS, typically the config files will be generated from scratch using the Nix language, see for example sshd [0] or bind [1].

`d4ys2c8kzzcp3g4fv3ivy7a5nkayg7w2` is a hash of the inputs to a derivation (a package in Nix terms).

In a sense, at it's lowest level, a derivation is a function `f(x) -> y` where `x` is some Nix expressions (including the inputs and how to build it, often in bash) and `y` is a nix store path. The nix store path includes the hash which is a hash of `x`.

For bind, the config file itself is a derivation, it just uses a plain string (interpolated with variables via Nix) into the writeText wrapper.

[0]: https://github.com/NixOS/nixpkgs/blob/de493bd74921139860624e... [1]: https://github.com/NixOS/nixpkgs/blob/de493bd74921139860624e...