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How to: Capture traffic on your ByteBlower Port
Posted by Wouter Debie, Last modified by Craig Godbold on 05 June 2023 11:28 AM


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 :

Options to keep the file size manageable

With the default settings all network traffic is captured from the selected interface. Often this results in very large PCAP files. Since version 2.13 of the ByteBlower GUI, two options are open to reduce filesize:

  • Configure a BPF filter.  This filter is applied by the ByteBlower server, only traffic matching the filter is forwarded to the ByteBlower GUI.
  • Truncate individual frames. Only the first number are kept in the PCAP and the remainder is dropped.

The default settings for both is capture all traffic.

These two options are available in the advanced config part of the capture dialog. They are configured before the capture starts. Did the capture already begin? The options become editable again in dialog after stopping the capture.

Using the Remote Capture Tool

This was the easiest way of capturing traffic on your port, until we brought the capture functionality to the GUI. It's a command-line tool that can be downloaded from the setup pages. It can be used on Windows, Mac and Linux.


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:


Why does my capture sometimes contain packets with invalid checksums?

When creating a packet capture (.pcap file) using the ByteBlower GUI, then it's possible that some packets will have an incorrect checksum in the capture. Wireshark typically marks those in red with the hint: "maybe caused by UDP checksum offload?".

That hint is actually correct. In some cases the ByteBlower makes use of checksum offloading. This means that the checksumming is done by the NIC hardware. Because the packets are stored in the capture before sending them to the NIC they still have the invalid checksum in the capture.

Currently checksum offloading is only used for Frame Blasting flows that have one of the following features enabled: latency, out-of-sequence, random size modifier, growing size modifier or field modifier. In all other cases the ByteBlower will perform the checksumming in software.

Remember that the actual packet sent on the network will have the correct checksum! You can verify this by capturing on the receiving interface. The incoming packets will should have a correct checksum. Unless, of course, something bad happened like packet corruption.

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