add supportability update

Author: mrbullwinkle

articles/azure-monitor/app/troubleshoot-availability.md

@@ -4,7 +4,7 @@ description: Troubleshoot web tests in Azure Application Insights. Get alerts if
 ms.topic: conceptual
 author: lgayhardt
 ms.author: lagayhar
-ms.date: 09/19/2019
+ms.date: 04/28/2020
 
 ms.reviewer: sdash
 ---
@@ -64,6 +64,10 @@ Check the classic alerts configuration to confirm your email is directly listed,
 
 Check to ensure the application receiving the webhook notification is available, and successfully processes the webhook requests. See [this](https://docs.microsoft.com/azure/monitoring-and-diagnostics/monitor-alerts-unified-log-webhook) for more information.
 
+### I am getting  403 Forbidden errors, what does this mean?
+
+This error indicates that you need to add firewall exceptions to allow the availability agents to test your target url. For a full list of agent IP addresses to allow, consult the [IP exception article](https://docs.microsoft.com/azure/azure-monitor/app/ip-addresses#availability-tests).
+
 ### Intermittent test failure with a protocol violation error?
 
 The error ("protocol violation..CR must be followed by LF") indicates an issue with the server (or dependencies). This happens when malformed headers are set in the response. It can be caused by load balancers or CDNs. Specifically, some headers might not be using CRLF to indicate end of line, which violates the HTTP specification and therefore fail validation at the .NET WebRequest level. Inspect the response to spot headers, which might be in violation.