I am making an online videoconverter. I managed to make the youtube plugin to get the flv link but I am finding it to hard to make the other ones e.g dailymotion, so it possible to capture the streaming url using something like tcpdump because this will also mean that I dont have to update the plugin if they change the way they serve the video?
Each video sharing site is different, you'll need to do some research on the sites of interest to get the location of their mp4/flv files.
Use a tool like Firebug and search for files loading prior to the video stream starting.
For your reference (as per your question), Dailymotion stores a file containing their video locations using this address format:
http://cdn.dailymotion.com/cdn/{VIDEO_CODEC_DIMENSIONS}/video/{VIDEO_ID}.mp4?auth={AUTH_TOKEN}&redirect=0
Please remember to check any legal implications of using other sites' videos.