100tb.nix

# A NixOS module which configures a system that is hosted by 100TB.  Each of
# our servers hosted with 100TB will probably import this module and pass it
# the minimum system configuration to get the server to boot and accept
# administrative ssh connections.
#
# A NixOS module is defined as a Nix expression language function.
{
  # This contains generally useful library functionality provided by nixpkgs.
  # These are things like string manipulation and, notably for us, a library
  # for defining options for configuring moduless.
  lib,

  # This is all of the configuration for a particular system where this module
  # might be instantiated.  For any system where we want the 100TB module to
  # be active, this should have the 100TB configuration details (IP, gateway,
  # etc).
  config,

  # More parameters exist and are accepted but we don't need them so we ignore them.
  ...
}:
let
  # Pull out the configuration for this module for convenient use later.  The
  # module name is quoted because `1` makes `100tb` look an awful lot like it
  # should be a number.
  cfg = config."100tb".config;

  # Define the API to this module.  Everything in `options` is about
  # specifying what kind of values we expect to be given.  This is both
  # human-facing documentation as well as guidance to NixOS about acceptable
  # values (mainly by type) so it can automatically reject certain bogus
  # values.  This value is in the `let` to make the code below a little easier
  # to read.  See below where we use it.
  options = {
    interface = lib.mkOption
    { type = lib.types.str;
      example = lib.literalExample "eno0";
      description = "The name of the network interface on which to configure a static address.";

    };
    publicIPv4 = lib.mkOption
    { type = lib.types.str;
      example = lib.literalExample "192.0.2.0";
      description = "The IPv4 address to statically assign to `interface`.";
    };
    prefixLength = lib.mkOption
    { type = lib.types.int;
      example = lib.literalExample 24;
      description = "The statically configured network's prefix length.";
    };
    gateway = lib.mkOption
    { type = lib.types.str;
      example = lib.literalExample "192.0.2.1";
      description = "The statically configured address of the network gateway.";
    };
    gatewayInterface = lib.mkOption
    { type = lib.types.str;
      example = lib.literalExample "eno0";
      description = "The name of the network interface for the default route.";
      default = cfg.interface;
    };
    grubDeviceID = lib.mkOption
    { type = lib.types.str;
      example = lib.literalExample "wwn-0x5000c500936410b9";
      description = "The ID of the disk on which to install grub.";
    };
    rootPublicKey = lib.mkOption
    { type = lib.types.str;
      example = lib.literalExample "ssh-ed25519 AAAA... username@host";
      description = "The public key to install for the root user.";