services: add Wireguard

This allows connecting devices in a mesh as if they were all on the same
private local network.

services/default.nix

@@ -20,5 +20,6 @@
     ./sabnzbd.nix
     ./ssh-server.nix
     ./transmission.nix
+    ./wireguard.nix
   ];
 }

services/wireguard.nix

@@ -0,0 +1,118 @@
+# A simple, in-kernel VPN service
+#
+# Strongly inspired by [1].
+# [1]: https://github.com/delroth/infra.delroth.net/blob/master/roles/wireguard-peer.nix
+{ config, lib, pkgs, ... }:
+let
+  cfg = config.my.services.wireguard;
+  hostName = config.networking.hostName;
+
+  peers = config.my.secrets.wireguard.peers;
+  thisPeer = peers."${hostName}";
+  otherPeers = lib.filterAttrs (name: _: name != hostName) peers;
+in
+{
+  options.my.services.wireguard = with lib; {
+    enable = mkEnableOption "Wireguard VPN service";
+
+    iface = mkOption {
+      type = types.str;
+      default = "wg";
+      example = "wg0";
+      description = "Name of the interface to configure";
+    };
+
+    port = mkOption {
+      type = types.port;
+      default = 51820;
+      example = 55555;
+      description = "Port to configure for Wireguard";
+    };
+
+    net = {
+      v4 = {
+        subnet = mkOption {
+          type = types.str;
+          default = "10.0.0";
+          example = "10.100.0";
+          description = "Which prefix to use for internal IPs";
+        };
+        mask = mkOption {
+          type = types.int;
+          default = 24;
+          example = 28;
+          description = "The CIDR mask to use on internal IPs";
+        };
+      };
+      v6 = {
+        subnet = mkOption {
+          type = types.str;
+          default = "fd42:42:42";
+          example = "fdc9:281f:04d7:9ee9";
+          description = "Which prefix to use for internal IPs";
+        };
+        mask = mkOption {
+          type = types.int;
+          default = 64;
+          example = 68;
+          description = "The CIDR mask to use on internal IPs";
+        };
+      };
+    };
+  };
+
+
+  config.networking = lib.mkIf cfg.enable {
+    wireguard.interfaces."${cfg.iface}" = {
+      listenPort = cfg.port;
+      ips = with cfg.net; with lib; [
+        "${v4.subnet}.${toString thisPeer.clientNum}/${toString v4.mask}"
+        "${v6.subnet}::${toString thisPeer.clientNum}/${toHexString v6.mask}"
+      ];
+      # Insecure, I don't care
+      privateKey = thisPeer.privateKey;
+
+      peers = lib.mapAttrsToList
+        (_: peer: {
+          inherit (peer) publicKey;
+        } // lib.optionalAttrs (thisPeer ? externalIp) {
+          # Only forward from server to clients
+          allowedIPs = with cfg.net; [
+            "${v4.subnet}.${toString peer.clientNum}/32"
+            "${v6.subnet}::${toString peer.clientNum}/128"
+          ];
+        } // lib.optionalAttrs (peer ? externalIp) {
+          # Known addresses
+          endpoint = "${peer.externalIp}:${toString cfg.port}";
+        } // lib.optionalAttrs (!(thisPeer ? externalIp)) {
+          # Forward all traffic to server
+          allowedIPs = with cfg.net; [
+            "0.0.0.0/0"
+            "::/0"
+          ];
+          # Roaming clients need to keep NAT-ing active
+          persistentKeepalive = 10;
+        })
+        otherPeers;
+    } // lib.optionalAttrs (thisPeer ? externalIp) {
+      # Setup forwarding on server
+      # FIXME: 'eth0' should not hard-coded
+      postSetup = with cfg.net; ''
+        ${pkgs.iptables}/bin/iptables -t nat -A POSTROUTING -s ${v4.subnet}.0/${toString v4.mask} -o eth0 -j MASQUERADE
+        ${pkgs.iptables}/bin/ip6tables -t nat -A POSTROUTING -s ${v6.subnet}::0/${toString v6.mask} -o eth0 -j MASQUERADE
+      '';
+      postShutdown = with cfg.net; ''
+        ${pkgs.iptables}/bin/iptables -t nat -D POSTROUTING -s ${v4.subnet}.0/${toString v4.mask} -o eth0 -j MASQUERADE
+        ${pkgs.iptables}/bin/ip6tables -t nat -D POSTROUTING -s ${v6.subnet}::0/${toString v6.mask} -o eth0 -j MASQUERADE
+      '';
+    };
+
+    nat = lib.optionalAttrs (thisPeer ? externalIp) {
+      enable = true;
+      externalInterface = "eth0"; # FIXME: should not hard-coded
+      internalInterfaces = [ cfg.iface ];
+    };
+
+    firewall.allowedUDPPorts = lib.optional (thisPeer ? externalIp) cfg.port;
+  };
+}