This is particularly true if you have a repetitive issue where network performance suffers, as it can help to identify the cause and narrow down the course of action to take to address by IT staff or a network administrator. There are many reasons and purposes for capturing packet traces, but perhaps the most common is for network troubleshooting purposes, either to identify a connectivity issue, or to better understand a particular networking issue. Why capture a packet trace, and what good does sniffing packets do? We’re not going to cover interpreting the results or what you can do with the information found in the capture file in this specific walkthrough. What you do with the capture file and its contents is up to you. The screen shot below shows what this looks like in the CPA app: If you want to, you can change the file extension from wcap to pcap and you’ll be able to open the output file in other apps too, including Cocoa Packet Analyzer (App Store link) as well.

Browsing the packet capture file through the command line will look like the following:

This file can be viewed from the command line with tcpdump, or with an app like WireShark. Opening the WCAP / PCAP Capture File in Mac OS X wcap extension and include the time of the packet capture, the name should look something like “2017.04.20_17-27-12-PDT.wcap”. The captured packet file will appear on the desktop with a.

When satisfied with the length of the packet capture, or when sufficient network traffic has been sniffed, click on “Stop” to end the packet trace and to save the captured packet file to the Desktop of OS X.

Select the Wi-Fi Channel and channel Width to sniff and capture packets for, using the wi-fi network stumbler tool can be helpful to identify which channels and widths to sniff network traffic for, then click “Start”.

Ignore the splash screen and pull down the “Window” menu, choose “Sniffer” from the list of options in the Wireless Diagnostics menu.

Choose “Open Wireless Diagnostics” from the list to open the wi-fi utility.

Option+Click on the Wi-Fi menu item in the OS X menu bar.

This process will automatically disconnect from any active wireless network and transmission on the Mac, instead dedicating the Macs wi-fi card to sniff wireless network traffic and to capture detected data into a packet transfer file. How to Sniff Packets with Wireless Diagnostics in OS X

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