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+# 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.";
+    };
+  };
+in {
+  # Here we actually define the module's options.  They're what we said they
+  # were above, all bundled up into a "submodule" which is really just a set
+  # of options.
+  options =
+  { "100tb".config = lib.mkOption
+    { type = lib.types.submodule { inherit options; };
+      description = "Host-specific configuration relevant to a 100TB system.";
+    };
+  };
+
+  # Now compute the configuration that results from whatever values were
+  # supplied for our options.  A lot of this is currently very similar to
+  # what's in bootstrap-configuration.nix (which is well commented).  The
+  # similarity makes sense - both that configuration and this one need to get
+  # a 100TB machine to boot and let an admin SSH in.
+  #
+  # Values that go into `config` here are merged into values that go into
+  # `config` in any other active modules.  Basically, everything in this
+  # `config` is treated as if it were in the configuration set defined by
+  # `/etc/nixos/configuration.nix`.  The module just gives us a way to factor
+  # separate concerns separately and make reuse easier.
+  #
+  # Note that this is not where Tahoe-LAFS configuration goes.  It's just
+  # about getting base platform into good shape.
+  #
+  # Perhaps at some point this can be refactored to remove the duplication.
+  # It's slightly tricky because we don't want to introduce any external
+  # dependencies to bootstrap-configuration.nix because that would make it
+  # harder to deploy in the bootstrap environment.
+  config =
+  { boot.loader.grub.enable = true;
+    boot.loader.grub.version = 2;
+    boot.loader.grub.device = "/dev/disk/by-id/${cfg.grubDeviceID}";
+
+    boot.loader.timeout = 1;
+    networking.firewall.enable = false;
+    services.openssh.enable = true;
+
+    users.users.root.openssh.authorizedKeys.keys = [
+      cfg.rootPublicKey
+    ];
+
+    networking.dhcpcd.enable = false;
+    networking.interfaces = {
+      "${cfg.interface}".ipv4.addresses = [
+        { address = cfg.publicIPv4; inherit (cfg) prefixLength; }
+      ];
+    };
+    networking.defaultGateway = {
+      address = cfg.gateway;
+      interface = cfg.gatewayInterface;
+    };
+    networking.nameservers = [
+      "4.2.2.1"
+      "8.8.8.8"
+    ];
+  };
+}