notfound: better error message for IP

dashboard/src/notfound.html

@@ -51,9 +51,19 @@
 
     <script type="text/javascript">
         window.addEventListener('load', (event) => {
-            document.getElementById('message').innerHTML =
-                'You are seeing this page because the DNS record of <b>' + window.location.hostname + '</b> is set to this server\'s IP'
+            // https://stackoverflow.com/questions/37437890/check-if-url-has-domain-name-and-not-an-ip
+            const containsLetter = /[a-zA-z]/.test(window.location.hostname); // ignore technicality that IP can contain letters ! http://192.168.0x1.0x1 or http://0xc0.0xa8.1.1
+            const isIPv6 = location.hostname.startsWith('[') && location.hostname.endsWith(']');
+
+            let message;
+            if (!containsLetter || isIPv6) { // ipv4 or ipv6
+                message = 'You cannot view Cloudron dashboard by IP address. Instead, navigate to the domain you configured during setup i.e <b>https://my.domain.example</b> .'
+                    + '<br>If you do not remember your domain, SSH into your server and run <code>cloudron-support --owner-login</code> .'
+            } else { // hostname
+                message = 'You are seeing this page because the DNS record of <b>' + window.location.hostname + '</b> is set to this server\'s IP'
                 + ' but Cloudron has no app configured for this domain.';
+            }
+            document.getElementById('message').innerHTML = message;
         });
     </script>
 </head>