The tcpdump command is a packet analyzer that is able to capture and provide a description of the traffic being transmitted across a network interface. It is common to most flavors of Linux, and it provides access to a unique view of the network at the packet level that can prove vital when troubleshooting the network environment.

The basic syntax for using tcpdump is expressed in the following way:

# tcpdump -i <device_name>

You can also specify a protocol like this:

# tcpdump -i <device_name> tcp

While a port value can be used in the following way:

# tcpdump -i <device_name> port 22

Verbosity options can be issued by using -v or -vv, while DNS can be avoided with the -n option. However, because tcpdump will continue running until the request is cancelled, it is always preferable to use the -c option in order to capture a pre-determined number of events in the following way:

# tcpdump -c 10 -i <device_name>

Taking this one step further...