luci-app-acme: improve UI for inexperienced users

applications/luci-app-acme/htdocs/luci-static/resources/view/acme.js

@@ -17,12 +17,15 @@ return view.extend({
 				return certs;
 			}),
 			L.resolveDefault(fs.stat('/usr/lib/acme/client/dnsapi'), null),
+			fs.lines('/proc/sys/kernel/hostname'),
 		]);
 	},
 
 	render: function (data) {
 		let certs = data[0];
 		let hasDnsApi = data[1] != null;
+		let hostname = data[2];
+		let systemDomain = _guessDomain(hostname);
 		let wikiUrl = 'https://github.com/acmesh-official/acme.sh/wiki/';
 		var wikiInstructionUrl = wikiUrl + 'dnsapi';
 		var m, s, o;
@@ -96,6 +99,9 @@ return view.extend({
 				"The first name will be the subject name, subsequent names will be alt names. " +
 				"Note that all domain names must point at the router in the global DNS."));
 		o.datatype = "list(string)";
+		if (systemDomain) {
+			o.default = [systemDomain];
+		}
 		o.validate = function (section_id, value) {
 			if (!/^[*a-z0-9.-]*$/.test(value)) {
 				return _('Invalid domain');
@@ -579,6 +585,15 @@ return view.extend({
 	}
 })
 
+function _isFqdn(domain) {
+	// Is not an IP i.e. starts from alphanumeric and has least one dot
+	return /[a-z0-9-]\..*$/.test(domain);
+}
+
+function _guessDomain(hostname) {
+	return _isFqdn(hostname) ? hostname : (_isFqdn(window.location.hostname) ? window.location.hostname : '');
+}
+
 
 function _addDnsProviderField(s, provider, env, title, desc) {
 	let o = s.taboption('challenge_dns', form.Value, '_' + env, _(title),