Thursday, November 26, 2020

Can Azure WVD inbound traffic pass through my firewall?

11 min to read.

Abstract

Corona Virus is giving birth to new technologies and new way of working!

Remote work has been in the center of technology discussion in all organizations. Azure Windows Virtual Desktop has been on leading front to enable organization to enable remote working.

When we talk to security teams about remote working using Azure WVD; every organization wants remote working incoming traffic to flow through their perimeter firewall deployed on Azure. This is obvious because all incoming traffic will be coming from internet and getting into Azure WVD environment of customer.

This is fair ask and every organizations security team wants to monitor all incoming traffic from internet.

So obvious question that comes to Azure Architects is -

Can you please make sure that all incoming/ inbound/ ingress traffic from client devices to Azure WVD flows through our Firewall devices hosted on Azure?

The answer is Yes.

However in my opinion the question is incorrect!

How? Let’s talk about it.

Azure WVD and Reverse Connect

See, I am not blaming security team for asking incorrect question; it is more of unawareness of how Azure WVD connection work. It is more of responsibility of Azure Architects working within an organization to make Security teams aware of how Azure WVD traffic flow and connections work.

Reverse connect is a unique technology offered only on Azure WVD. Reverse connect technology means your WVD host VM doesn’t need any inbound ports to be opened. Even the default RDP port, TCP/3389, doesn’t have to be open. Instead, an agent creates an outbound connection using TCP/443 into the Windows Virtual Desktop management service.

Most of the time we misunderstand the Azure WVD Reverse Connect as shown in the Red color line in below diagram [click to get better view] - 

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As per general understanding I have observed below understanding of the traffic flow –

1.      Client device connects to WVD common URL either from browser or from app installed on client machine. This traffic flows over internet.

2.      The request from client machine is forwarded to WVD management control plane.

3.      The broker in WVD management service initiates connection to actual WVD host. This traffic also flows over internet.

In above traffic flow the traffic is reaching to WVD host VM directly without passing through the firewall device.

This is not correct understanding.

Reverse connect means Broker will not initiate the traffic to WVD host VM rather WVD host VM will initiate the connection to Broker of WVD management service.

This means the traffic to connect to WVD host is OUTBOUND/ EGRESS/ OUTGOING traffic from WVD host and not INCOMING/ INBOUND/ INGRESS traffic to WVD host VM.

Passing WVD host connection traffic through Firewall

Now that we understand about WVD host making outbound call for honoring connection requests; we just need to have this traffic going through Firewall device. The firewall device can be either NVA or Azure Firewall.

To pass this outbound traffic initiated from WVD host VM through firewall we will need to add UDR – User Defined Route on WVD Subnet. The routes will be as follows –

-        If destination is on-premises then next hop is Gateway

-        If destination is 0.0.0.0/0 then next hop is firewall device private IP

The connection traffic will be falling under the category of 0.0.0.0/0. This way connection traffic initiated from WVD host VM will also pass through Firewall device.

This way security teams requirements will also be satisfied as now they actually can inspect outgoing traffic in firewall from WVD Subnet.

Refer to below diagram – [click to get better view]

 



The correct network traffic flow will be as follows shown in green in diagram –

1.      Client device connects to WVD common URL either from browser or from app installed on client machine. This traffic flows over internet.

2.      The request from client machine is forwarded to WVD management control plane.

3.      The WVD host VM has an agent running on top of it. This agent then initiates the traffic to WVD management service as an OUTBOUND connection.

4.      WVD management service traffic falls under category of 0.0.0.0/0 and hence the traffic is forwarded to firewall device.

5.      From firewall device the traffic goes to WVD management service.

6.      After successful authentication from user the actual connectivity is established between client device and WVD host VM.

All of this communication happens over 443 and there is no requirement of opening 3389 default RDP port anywhere. This is the beauty of  WVD reverse connect.

Wrapping up

Remember, there is no Inbound traffic in case of connecting to WVD host VM.

Therefore correct question to security teams –

Can you allow WVD connection traffic to pass through my firewall device?

Answer - Yes using UDR on WVD subnet; we can pass WVD connection traffic to flow from firewall device.

Conclusion

Hope this article helped to design your WVD deployment architecture in right way and gave better understanding of WVD traffic flow.

Happy Remote working!!

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