tls: emit a warning when servername is an IP address

Refs: nodejs#18071

Author: Rodger Combs

lib/_tls_wrap.js

@@ -48,6 +48,8 @@ const kSNICallback = Symbol('snicallback');
 
 const noop = () => {};
 
+let ipServernameWarned = false;
+
 function onhandshakestart(now) {
   debug('onhandshakestart');
 
@@ -1152,8 +1154,17 @@ exports.connect = function(...args /* [port,] [host,] [options,] [cb] */) {
   if (options.session)
     socket.setSession(options.session);
 
-  if (options.servername)
+  if (options.servername) {
+    if (!ipServernameWarned && net.isIP(options.servername)) {
+      process.emitWarning(
+        'Setting the TLS ServerName to an IP address is not supported by ' +
+        'RFC6066. This will be ignored in a future version.',
+        'UnsupportedWarning'
+      );
+      ipServernameWarned = true;
+    }
     socket.setServername(options.servername);
+  }
 
   if (options.socket)
     socket._start();