I saw on Twitter a discussion related to Azure VMs and RDP connection that are open by default. The main purpose of this topic is to present different use cases when the RDP connection (not) is available by default.
Use Case 1: Single VM (VM with Public IP inside a default VNET) – RDP Active by default for public access
In this context, we have a VM that is created from Azure Portal (or script) as a single entity. It is not part of any scale set or other type of custom configuration. It is just a simple Windows Server 2016 Datacenter machine, which is part of a default VNET with a Public IP allocated to it.
In this case, by default the RDP will be configured. The default Network Security Group (NGS) that is created together with our VM will allow RDP connection to the machine. The default VNET allows RDP connection to our VM, because there are no custom NGS rules to restrict it and we have a Public IP attached to our VM.
Use Case 2: Single VM (VM without Public IP inside a default VNET) – RDP Active but connection cannot be establish from Public Internet (outside VNET)
This case is similar with the previous one. The only difference is at IP level, where our VM doesn’t have a Public IP allocated to it.
In this case, we still have by default the NGS rule at VM levels configured to accept RDP connections, but it is impossible to connect to the VM or VNET from public internet because there is no Public IP available.
Use Case 3: VMs inside Scale Set (VMs inside a Scale Set) – RDP is active by default for public access
We have one or more VMs inside a Scale Set. By default, each VM instance at Scale Set level has the RDP active on standard port (3389). Additional to this, the Load Balancer that is created together with Scale Set has Inbound NAT rules configured in such a way to support RDP. Taking a closer look we will see ports that are higher than 5000 used for RDP connection to a specific instance of VM from Scale Set.
As we can see from the previous 3 use cases, by default the RDP connection is open, especially when you have a Public IP. This happens usually when you are using Azure Portal to create resources. When you are working with production environment, I expect that you will use Azure Resource Manager (ARM) scripts, where you will configure the environment in the way you want.