Check TLS connections to virtual servers are enabled (#2762)

Author: jeanbisutti

agent/agent-tooling/src/main/java/com/microsoft/applicationinsights/agent/internal/init/FirstEntryPoint.java

@@ -101,6 +101,8 @@ public void init() {
         JvmCompiler.disableJvmCompilerDirectives();
       }
 
+      checkTlsConnectionsToVirtualServersEnabled();
+
       if (startupLogger.isDebugEnabled()) {
         startupLogger.debug(
             "Input arguments: " + ManagementFactory.getRuntimeMXBean().getInputArguments());
@@ -128,6 +130,15 @@ public void init() {
     }
   }
 
+  private static void checkTlsConnectionsToVirtualServersEnabled() {
+    String tlsConnectionsToVirtualServersProp = "jsse.enableSNIExtension";
+    String propValue = System.getProperty(tlsConnectionsToVirtualServersProp);
+    if ("false".equals(propValue)) {
+      startupLogger.warn(
+          "System property -Djsse.enableSNIExtension=false is detected. If you have connection issues with Application Insights, please remove this.");
+    }
+  }
+
   private static String findSystemProperties() {
     Properties properties = System.getProperties();
     StringBuilder propsBuilder = new StringBuilder();