Using tcpdump

The tcpdump tool is used to collect and analyze network packets. It is ideal for the analysis of individual packages and for their consistency. In the case of automatic processing, tcpdump has several advantages.

To read a previously recorded pcap file, you can use the following command:

tcpdump -nnr dump.pcap

The default tcpdump displays information about each packet, while the output format is protocol-dependent:

TCP:

[Timestamp] [Layer 3 Protocol] [Source IP].
    [Source Port]> [Destination IP]. [Destination Port]:
    [TCP Flags], [TCP Sequence Number],
    [TCP Acknowledgement Number], [TCP Windows Size] ,
    [Data Length]

UDP:

[Timestamp] [Layer 3 Protocol] [Source IP].
    [Source Port]> [Destination IP]. [Destination Port]:
    [Layer 4 Protocol], [Data Length]

In addition to make the output more informative, -v -vvv can be used, as in the following example:

tcpdump -nnvvvr dump.pcap

With the -x option, you can make tcpdump output packets in the heh...