Iftop

Iftop is a free and open-source network monitoring tool that displays a real-time list of network connections on a system's network interface. It operates in a similar manner to the top utility used for monitoring processes, but instead focuses on network traffic.

Iftop listens to network traffic on a specified interface and aggregates the data by connection, showing the bandwidth usage between pairs of hosts. It presents this data in a constantly updating, sorted list, making it easy to identify which hosts are generating the most network traffic.

Iftop primarily works by capturing network packets and analyzing their source and destination IP addresses and port numbers. It then correlates this information to provide a live, visual representation of network traffic patterns. The information is displayed in a columnar format showing the source host, destination host, and the rate at which data is being transmitted and received.

Iftop can be used to diagnose network bottlenecks, identify excessive bandwidth consumption, and understand the communication patterns between different hosts on a network. It supports filtering traffic based on IP address or subnet, making it possible to focus on specific areas of the network.