How can i capture traffic with a fritz box?

Most of the FRITZ!Boxes have a non-documentated page which enables a traffic capture in 'wireshark' format for different ports. Open the page in a browser with following URL: fritz.box/html/capture.html, or replace fritz.box with your routers IP address (usually either 192.168.178.1 or 192.168.178.254). There you can choose the ports you want to capture from. Just follow the on-screen instructions and after successful download, you can open the file directly in Wireshark.