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How to: Capture traffic on your ByteBlower Port
Posted by Wouter Debie, Last modified by Pieter Vandercammen on 11 May 2020 05:18 PM


When you are testing your device with ByteBlower, you sometimes like to have a pcap-capture of the data for debugging purpose. With the ByteBlower API we can easly capture that network traffic and present it to you as a pcap-file. This feature will help you to debug quickly the problem with your device.

Let me explain how to do this.

Using the ByteBlower GUI

This is the simplest way of capturing traffic. Available since GUI v2.11, and server v2.9. Here you can see how it works :

Using the Remote Capture Tool

This used to be the simplest way of capturing traffic on your port, until we brought the capture functionality to the GUI. You just need to download and execute the tool. It can be used on Windows, Mac and Linux. Follow this link ( ) to download the tool.


This tool only works on ByteBlower Server running 2.1 and higher

Using the ByteBlower lower-layer API

When you are using our TCL API to transmit your traffic you can use the Rx.Capture of a ByteBlower Port to create a capture. Using our API allows you to automate when to create a capture. Let your script determine when you need to create a capture.

All you need is the Rx.Capture.Add call on your ByteBlower Port.


Just like you add a Trigger to a ByteBlower port you can add a Capture. On this capture-object you can set a capture filter and thus define which frames you would like to see captured. After that just start the capture and you are all set. Now lets put these simple words into a working script.

For this post, we assume we have created a back-to-back scenario with:

  • Two configured ByteBlower ports srcPort and dstPort
  • a stream Stream configured to flow between srcPort and dstPort

 Create a capture on the dstPort and configure it

set dstPortCapture [ $dstPort Rx.Capture.Add ]

Now you have a capture Object. Using the Tk command you can visualize it to see what you can do with this object.

Tk screenshot of Rx.Capture object

It is important to set a capture filter on this capture. This will allow you to capture only the packets you are interested in.

$capture Filter.Set "dst port 513"

The filter must be a BPF filter. On you can find more info on the syntax of these filters and some day-to-day examples.

Start the capture

You can start the capture now.

$capture Start

Now start your traffic and every frame that matches your filter will be captured. You can see how many frames have been captured with the in the result capture object

set captureResult [ $capture Result.Get ]
$captureResult Refresh
$captureResult PacketCount.Get

Stop the capture and get the PCAP-file

Like the start-method there is a stop method the capturing.

$capture Stop

To retrieve your pcap-file use the Pcap.Save method.

$captureResult Refresh
$captureResult Pcap.Save "C:/Users/Excentis/Sniffs/DeviceX.pcap"

On your disk you will find DeviceX.pcap containing the packets that matched your filter that arrived on your ByteBlower destination port (destPort). If you want, you can use Frames.Get to retrieve a TCL-list containing the packets represented in hex-encoding. This way you could use TCL to parse your retrieved packets...


You can find the api documentation of the RxCapture here:

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