Can I uniquely identify a client machine in an ASP.NET application - asp.net-mvc

Question

Is there a way to uniquely identify a user that has logged on my application based on his device/machine ?
In a lot of cases, the IP is enough, but in case when the client has multiple machines behind a NAT, then the same IP is exposed, so I can't tell them apart. it should have same id irrespective of browser.
for e.g. If the user logs in on his account with computer A, then log in on the same account with computer B that share the same router, I need to get id apart those two devices.
I don't know if this is possible, but it would be life saving if it is.

Answer 1

I was faced with this problem where I wanted to ask for Google Authenticator on sign in but only once for each device used by a user. I used a function to give me a device id based on the hostname, the MAC address, and the IP address. I know the MAC address isn't always reliable so I thought combining the data into one string might be a way round it. Our application is limited to <100 users and most of them access from the office or home so I feel it should be good enough.
I put the IP address function (which gets the IPV4) into a separate function as I check whether they are in office (on 192.168..) or external before checking the device ID. The list of device ID's associated with a user is stored in a SQL table and checked after username/password entry but before log in is completed to decide whether to request a 2FA code.
Here's the code:
dim thisDeviceId as String=GetClientDeviceId()
public Function GetClientDeviceId() As string
Dim mac As String = String.Empty
For Each netInterface In NetworkInterface.GetAllNetworkInterfaces()
If _
netInterface.NetworkInterfaceType = NetworkInterfaceType.Wireless80211 OrElse
netInterface.NetworkInterfaceType = NetworkInterfaceType.Ethernet Then
Dim address = netInterface.GetPhysicalAddress()
mac = BitConverter.ToString(address.GetAddressBytes())
End If
Next
return string.Format("{0}-{1}-{2}",dns.GetHostEntry(HttpContext.current.Request.ServerVariables("REMOTE_ADDR")).HostName, mac,GetClientDeviceIpAddress())
End Function
public Function GetClientDeviceIpAddress() As string
Dim ipv4Address As String = String.Empty
For Each currentIpAddress As IPAddress In Dns.GetHostAddresses(Dns.GetHostName())
If currentIpAddress.AddressFamily.ToString() = System.Net.Sockets.AddressFamily.InterNetwork.ToString() Then
ipv4Address = currentIpAddress.ToString()
Exit For
End If
Next
return ipv4Address
End Function
Even though it's not bulletproof and could be improved upon it might help someone.