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vSphere 5.5 and onwards introduces a packet-capture tool called pktcap-uw that can capture traffic on all points of an ESXi host and export the result to a .pcap format. Information on how to use pktcap-uw tools is provided in VMware KB 2051814—Using the pktcap-uw tool in ESXi 5.5 and later: https://kb.vmware.com/s/article/2051814.

In NSX 6.4, the Packet Capture tool is integrated in the UI for easier access:

There is a demo on how to use the new NSX 6.4 packet capture at https://www.youtube.com/watch?v=iRlknzU0BFM.