The explosive growth of Internet-connected consumer devices in the digital home has made home networks one of the important emerging topics in networking research. A rich body of research efforts have been made to study broadband performance and home network management, little is known about network traffic that are exchanged within home networks or between Internet-connected devices in home networks and end hosts on the Internet. In this paper we design and implement a built-in traffic monitoring system on programmable home routers to collect and analyze incoming, outgoing and internal traffic for residential home networks. To illustrate the applications of the proposed built-in traffic monitoring system, we deploy the system in two real home networks, and demonstrate its capabilities of detecting unwanted traffic towards home networks as well as those suspicious traffic originating from compromised devices in home networks. In addition, our correlation analysis on unwanted traffic towards distributed home networks reveals aggressive scanners on the Internet and sheds lights on traffic characteristics of these scanners.