Tcpdump is a command-line packet sniffing and protocol analyzer tool writtin in C/C++. It captures live RX/TX packets on a network interface in promiscuous mode, and displays the content of the packets in a human-readable format in real time. Tcpdump can write captured raw packets to a file, which then can be loaded later by Tcpdump for offline analysis. The underlying packet capture library (libpcap for Linux/Unix, and WinPcap for Windows) can selectively capture packets based on filtering rules expressed in BPF (berkely packet filter). The BPF-based filter can capture packets with specific source/destination IP addresses/prefixes, protocols (IP/ICMP/UDP/TCP), port numbers, protocol fields (TCP SYN/FIN/ACK), or even raw bit values at an arbitrary byte offset.

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