DynDNS: Store and compare to IP from last update

options/dyndns.nix

@@ -33,7 +33,11 @@ in
     };
 
     config = lib.mkIf cfg.enable {
-        systemd.services.dyndns = {
+        systemd.services.dyndns = let
+            stateDirectory = "dyndns";
+            homeDirectory = "/var/lib/${stateDirectory}";
+        in
+        {
             enable = true;
             after = [ "network.target" ];
             unitConfig = {
@@ -46,7 +50,14 @@ in
                 ProtectSystem = true;
                 ProtectHome = true;
                 NoNewPrivileges = true;
+                ReadWriteDirectories = [ homeDirectory ];
+                StateDirectory = stateDirectory;
             };
+            # preStart = ''
+            #     #!${pkgs.bash}/bin/bash
+
+            #     [ -d "${homeDirectory}" ] || mkdir -p "${homeDirectory}"
+            # '';
             script = ''
                 #!${pkgs.bash}/bin/bash
 
@@ -57,6 +68,7 @@ in
                 dyndns_server="${cfg.server}"
                 dyndns_user="${cfg.username}"
                 dyndns_password="$(cat "${cfg.passwordFile}")"
+                state_file="${homeDirectory}/current_ipv6"
 
                 new_ip=$(${pkgs.iproute}/bin/ip -6 a show scope global -temporary dev "$interface" | ${pkgs.gnused}/bin/sed -n -E 's/^\ *inet6\ (2001(:[0-9a-f]+)+).*$/\1/p' | head -1)
 
@@ -65,7 +77,12 @@ in
                     exit 1
                 fi
 
-                current_ip=$(${pkgs.dig}/bin/dig aaaa +short "$host")
+                if [ ! -f "$state_file" ] ; then
+                    echo "No state file found, determining currently set IP via DNS query."
+                    ${pkgs.dig}/bin/dig aaaa +short "$host" > "$state_file"
+                fi
+
+                current_ip=$(cat "$state_file")
 
                 if [ -z "$current_ip" ] ; then
                     echo "Could not determine current AAAA record."
@@ -79,6 +96,8 @@ in
 
                 echo "Updating IP to $new_ip."
                 ${pkgs.curl}/bin/curl "https://$dyndns_user:$dyndns_password@$dyndns_server/?hostname=$host&myip=$new_ip"
+
+                echo "$new_ip" > "$state_file"
             '';
         };