Reading a packet capture file with Wireshark and tshark
Needle in Haystack
Wireshark doesn’t just work in real time. If you save a history of network activity in a pcap file using a tool such as tcpdump, you can filter the data with Wireshark to search for evidence.
Intrusion detection tools that use the libpcap C/ C++ library for network traffic capture (such as Snort and Tcpdump) can output packet capture information to a file for later reference. The format of this capture file is known as pcap. By capturing packet data to a file, an investigator can return later to study the history of an intrusion attempt – or to turn up other important clues about clandestine activity on the network.
Of course, the traffic history data stored in a pcap file is much too vast to study by just viewing the file manually. Security experts use specialized filtering tools to search through the file for pertinent information. One way to look for clues in a pcap file is to use the Wireshark protocol analysis tool  and its accompanying command-line utility tshark.
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