Wireshark

To troubleshoot low-level network issues, you need to see packet by packet what is sent and received. Wireshark is the benchmark: the world's foremost network protocol analyzer, with deep capture, per-packet analysis, thousands of protocols and powerful filtering and statistics, a staple for network engineers and security researchers. Open source and free.

Features

Protocol analysis: The foremost network protocol analyzer.

Deep capture: Analyze traffic packet by packet.

Many protocols: Supports thousands of protocols.

Open and free: An open-source project, free to use.