Finding “live” NAT interface and broadcast address for a UDP LAN Chat Program

So I decided to do a simple Local Area Network based chat program that transmitted plaintext ASCII encoded text across UDP to all devices (listening on the port) on a network.

Little did I know it’s much harder than that when not all system Route Tables point 255.255.255.255 to the interface’s global broadcast IPv4… v6 is another story.

In Visual Studio VB.NET we take a look at NetworkInformation import declaration to discover all Adaptors on our system, then we need to find the IP of the active network interface.

When you have VMWare, OpenVPN, and a plethora of other interfaces, finding the active one (with network connection) programmatically is a nightmare.

 

Using this snippet you’ll be able to find the local address of the Adaptor that is part of the local system and has a gateway attached. This is an interim solution and most likely only supports networks with routers (for their NAT):

tec1

 

Next, to determine global broadcast address you’ll need the current Subnet Mask for that adaptor/interface. Simply utilise the same Import System.Net.NetworkInformation:

tec2

 

Finally we need to use a bitwise comparison between the Subnet Mask and the current internal IPv4 to find the broadcast address. For example 192.168.1.4 (local IP) with subnet mask (255.255.255.0) would have the broadcast address of 192.168.1.255 for that interface, and then with that information we can broadcast a LAN wide UDP datagram. Use this overly complicated bitwise comparison as an example of how to simply switch around the integers appropriately:

tec3

 

Then we can simply refer to the 3 functions to get our main broadcast address:

Dim broadcastAddress As String = FindBroadCastAddress(getIP(), getSubnetMask()).ToString()

 

Ok, now we make our program.

So using the broadcast address we can use Import System.Net to declare a listener and sender UdpClient.

Note: The following settings will be set as global vars/consts.

  • Private Const port As Integer = 13337
  • Private receivingClient As UdpClient
  • Private sendingClient As UdpClient
  • Private receivingThread As Thread
  • Private computerName As String = My.Computer.Name

 

tec4

 

Now for the listener we can utilise IPAddress(0) or IPAddress.Any to make sure our message is received on any active interface:

tec5

 

Add any functionality you see fit, like I’ve done with the Text Box hints!

Download my copy!

https://mcdonell.space/programs/LanChat.zip

 

Leave a Reply

Your email address will not be published.