ifstat is like some kind of vmstat for network interfaces. Every X seconds, it prints two columns per network interface monitoring the input and output bandwidth.

Quite useful when you need to diagnose a slow network, identify bottlenecks, monitor bandwidth consumption, etc. When invoked with the -t parameter it adds a timestamp on each line, very important if you plan to left it running for hours dumping its output to a file.

# ifstat -t

  Time            eth0               eth1
HH:MM:SS   KB/s in  KB/s out   KB/s in  KB/s out
12:40:25      0.05      0.19      0.00      0.00
12:40:26      0.26      0.24      0.00      0.00
12:40:27      6.87     30.14      0.00      0.00
12:40:28      0.17      0.26      0.00      0.00
12:40:29      0.05      0.22      0.00      0.00
12:40:30      0.05      0.14      0.00      0.00
12:40:31      0.05      0.14      0.00      0.00
12:40:32      0.29      0.38      0.00      0.00
12:40:33      0.29      0.38      0.00      0.00
12:40:34      0.13      0.22      0.00      0.00
12:40:35      2.85      8.70      0.00      0.00
12:40:36      9.01     38.84      0.00      0.00
12:40:37      0.55      0.24      0.00      0.00
12:40:38      0.05      0.14      0.00      0.00