vpn: add a service to auto-refresh wireguard endpoints

modules/vpn.nix

@@ -1,14 +1,16 @@
 # debugging:
 # - `journalctl -u systemd-networkd`
+# - `networkctl --help`
 #
 # docs:
 # - wireguard (nixos): <https://nixos.wiki/wiki/WireGuard#Setting_up_WireGuard_with_systemd-networkd>
 # - wireguard (arch): <https://wiki.archlinux.org/title/WireGuard>
 #
-# to route all internet traffic through a VPN endpoint, run `systemctl start vpn-${vpnName}`
+# to route all internet traffic through a VPN endpoint, run `sane-vpn up ${vpnName}`
 # to route an application's traffic through a VPN: `sane-vpn do ${vpnName} ${command[@]}`
 # to show the routing table: `ip rule`
 # to show the NAT rules used for bridging: `sudo iptables -t nat --list-rules -v`
+# to force a peer address change (e.g. DNS change): `wg set "${interface}" peer "${publicKey}" endpoint "${endpoint}"`
 #
 # the rough idea here is:
 # 1. each VPN has an IP address: if we originate a packet, and the source address is the VPN's address, then it gets routed over the VPN trivially.
@@ -179,6 +181,23 @@ let
     # but i couldn't get that to work for netns with SNAT, so set rpfilter to "loose".
     networking.firewall.checkReversePath = "loose";
 
+    systemd.services."${name}-refresh" = {
+      # periodically re-apply peers, to ensure DNS mappings stay fresh
+      # borrowed from <repo:nixos/nixpkgs:nixos/modules/services/networking/wireguard.nix>
+      wantedBy = [ "network.target" ];
+      path = with pkgs; [ wireguard-tools ];
+      serviceConfig.Restart = "always";
+      serviceConfig.RestartSec = "60"; #< retry delay when we fail (because e.g. there's no network)
+      serviceConfig.Type = "simple";
+      unitConfig.StartLimitIntervalSec = 0;
+      script = ''
+        while wg set ${name} peer ${publicKey} endpoint ${endpoint}; do
+          # in the normal case that DNS resolves, and whatnot, sleep before the next attempt
+          sleep 180
+        done
+      '';
+    };
+
     # networking.firewall.extraCommands = with pkgs; ''
     #   # wireguard packet marking. without this, rpfilter drops responses from a wireguard VPN
     #   # because the "reverse path check" fails (i.e. it thinks a response to the packet would go out via a different interface than what the wireguard packet arrived at).