Adding An ENI (Elastic Network Interface) To A Windows EC2 - powershell

Question

Okay AWS Windows Powershell guys... here's a question for you. How do you add an ENI (Elastic Network Interface) to your Windows EC2? Seems simple enough as many examples show:
Add-EC2NetworkInterface -NetworkInterfaceId <your-eni-id> `
-InstanceId <your-ec2-id> `
-DeviceIndex 1 `
-Force
but in my past experiences DeviceId=1 stopped working and I switched to DeviceIndex=2. I went along my way and suddenly DeviceIndex=2 doesn't work, generating the exception
Instance 'i-xxxxxxxxxxxxxxxxx' already has an interface attached at device index '2'
and the ENI shows 'attaching' forever (and must be forcibly detached). However, today using DeviceIndex=1 does attach the ENI again. Now I recognize that I should programmatically determine which DeviceIndex is available and use that, but the closest I've seen to this value is "InterfaceIndex" in this blog:
https://blogs.technet.microsoft.com/heyscriptingguy/2014/01/15/using-powershell-to-find-connected-network-adapters/
with this command:
get-wmiobject win32_networkadapter | select netconnectionid, name, InterfaceIndex, netconnectionstatus
but after trying those values, it clearly isn't the value I'm looking for. Adding DeviceIndex=1 will get a network adapter at InterfaceIndex=29, for instance.
It's odd that this parameter is required in api when attaching an ENI from the console does not even have a place to enter this value (and works perfectly).
So to summarize, how do I determine the DeviceIndex to use for adding a new ENI to a Windows EC2?

Answer 1

I'm playing with smaller EC2 instances that only allow a max of two network devices, but I'd be willing to bet that this is the answer. If you look at the network interfaces of your ec2:
$instances = Get-Instances -InstanceId <your-ec2-id>
$instances.Instances[0].NetworkInterfaces.Attachment
you'll see the network devices that are attached along with the DeviceIndex property I've been looking for:
AttachmentId : eni-attach-aaaaaaaa
AttachTime : 5/15/2017 12:47:31 PM
DeleteOnTermination : True
DeviceIndex : 0
Status : attached
AttachmentId : eni-attach-bbbbbbbb
AttachTime : 5/17/2017 5:28:21 PM
DeleteOnTermination : False
DeviceIndex : 1
Status : attached
My guess is that the console finds the current and generates the next value for you, so you don't have to provide it, but since the Add-EC2NetworkInterface requires it, find the greatest value for DeviceIndex and attach using the next highest number.
Keep in mind that even if your call fails you still must detach the ENI (with Dismount-EC2NetworkInterface or from the AWS Console) because it will most likely be stuck in the 'attaching' state.
Hope this helps someone!