Our Answer Desk tip Sniffers aren't illegal, though they can be used unscrupulously.
Sniffers are programs that monitor traffic on the Internet.
Ideally, they help system administrators detect problems or bottlenecks on the network, so they can either be quickly solved or avoided altogether. The problem arises when sniffers are used to capture Internet data. As packets of data travel across the Internet, there are points where the data is analyzed so it can be directed to the proper destination on the Net. If a sniffer is used at one of these points, which are called routers, the data can conceivably be read.