Scenario 6 : Not able to RDP to only ‘HealthMonitor’ worker role instance and getting an error : This computer can’t connect to the remote computer


Referring to my blog on Azure Cloud Service Troubleshooting Series, this is the 6th scenario of the lab. Please make sure you have followed the lab setup instructions for Super Convertor application as per this, to recreate the problem.

 

Symptom

As per the lab instructions I have enabled the RDP for all the cloud service roles, but for some reason Remote Desktop connection is failing only for 'HealthMonitor' worker role instance throwing the below error. Whereas RDP works as expected for the other cloud service role instances.

RDP Error

 

Troubleshooting

Remote Desktop Protocol is encapsulated and encrypted within TCP. By default, server listens on TCP port 3389 for RDP connection. Hence I will begin my troubleshooting by first checking whether TCP port 3389 is open or not. I am going to use one of my favorite tool PsPing for checking the connectivity to the port 3389 for my cloud service hostname, you can use telnet as well.

 

psping.exe cloudservicelabs.cloudapp.net:3389
PsPing v2.01 - PsPing - ping, latency, bandwidth measurement utility
Copyright (C) 2012-2014 Mark Russinovich
Sysinternals - www.sysinternals.com
TCP connect to 40.124.28.4:3389:
5 iterations (warmup 1) connecting test:
Connecting to 40.124.28.4:3389 (warmup): 281.25ms
Connecting to 40.124.28.4:3389: 279.08ms
Connecting to 40.124.28.4:3389: 283.01ms
Connecting to 40.124.28.4:3389: 269.29ms
Connecting to 40.124.28.4:3389: 270.12ms
TCP connect statistics for 40.124.28.4:3389:
Sent = 4, Received = 4, Lost = 0 (0% loss),
Minimum = 269.29ms, Maximum = 283.01ms, Average = 275.37ms

 

As per the above statistics, definitely RDP port is open and server is responding as expected to TCP ping over port 3389. That's strange ! Few questions came to my mind...

Checked the ServiceDefinition.csdef file but couldn't find any ACL configured and there were no startup tasks related to firewall configuration for this cloud service solution as well. The only thing that was left was to capture a Network Monitor trace and analyze the RDP traffic. Without any second thought I started collecting network trace from my client machine while replicating the issue and this is what I can see.

RDP-Trace

 

Let's split the above network trace screenshot into two halves for better understanding. Every TCP session is always established with the three way handshake, which depicted by the first half of the screenshot.

RDP-TCP-handshake

 

The first three frames (90, 97 and 98) with the Syn , Syn/Ack and Ack flags depicted as S, A..S, and A respectively is what is collectively referred to as the TCP three way handshake, which is successful in our case. Moving on to the later half of the network trace I can see that client has initiated a RDP connection request depicted by X224: Connection Request but didn't receive any confirmation from the server side as per RDP connection sequence. The X.224 Connection Request PDU is an RDP Connection Sequence PDU sent from client to server during the Connection Initiation phase of the RDP Connection Sequence (section 1.3.1.1 for an overview of the RDP Connection Sequence phases).

Instead server tore down the TCP session with a Fin flag (depicted as A…F in frame number 491). The client in turn acknowledges the server’s packet and then proceeds to abruptly terminate the session with the Reset flag (depicted as A.R..) in the screenshot below, also acknowledging the receipt of the previous pact from the server (thus the A in A.R..).

RDP-failure

 

This looks like a firewall issue as the server is gracefully terminating the RDP connection request after TCP handshake is complete. But where or how exactly firewall rules are configured ?

Upon reviewing the WorkerRole.cs of 'HealthMonitor' role, I found that firewall rules were programmatically added using INetFwRules interface 🙂 So point to be kept in mind that firewall rules can be added not only through startup tasks or ACL but also through application code.

 

Type Policy2 = Type.GetTypeFromProgID("HNetCfg.FwPolicy2", false);
INetFwPolicy2 FwPolicy = (INetFwPolicy2)Activator.CreateInstance(Policy2);
INetFwRules rules = FwPolicy.Rules;
rules.Remove("Magic Rule");
Type RuleType = Type.GetTypeFromProgID("HNetCfg.FWRule");
INetFwRule rule = (INetFwRule)Activator.CreateInstance(RuleType);
rule.Name = "Magic Rule";
rule.Protocol = 6;
rule.LocalPorts = "3389";
rule.Action = NET_FW_ACTION_.NET_FW_ACTION_BLOCK;
rule.Direction = NET_FW_RULE_DIRECTION_.NET_FW_RULE_DIR_IN;
rule.Enabled = true;
rules.Add(rule);

 

I hope you have got an idea how to troubleshoot RDP issues in Azure Cloud Service.

Happy Learning !

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