Woah, I'm lost here. The idea is that it uses /etc/ssh/sshd_config as its input? (How do you handle upgrades, then?) What is d4ys2c8kzzcp3g4fv3ivy7a5nkayg7w2?
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.
