Some friends with the help of various sites check and know when i'm invisible on yahoo messenger and keep bragging about this.
Being curious about this I've tested lots of sites that check if a user is invisible on yahoo messenger and all of them sent me a C1 packet type.
From what i've tested I'm(my ymsgr client) not sending anything back. So i only receive 1 packet from the bot that performs the check and that's it, they know if i'm invisible or not.
Next i thought that if i'm not sending anything back then maybe the yahoo server sends something back to i tested on a friend of mine and i sent him a c1 packet but i did not received anything back from nobody (neither the server neither my friend).
So how do they do that? I'm just looking for some hints, not expecting for code or someone else to solve it for me. I just like the thrill in learning and discovering by myself just that now i'm stuck with no idea :)
Thanks.
This is a new answer because it's too long to be a comment.
I looked around a bit and it would seem that the older versions of yahoo had a deal where if you were invisible and someone tried to start a voice chat with you, it would give you away. The ping packet could be trying that.
One thing I noticed on gtalk was that often a user appears online even after they have disconnected until someone sends them a ping packet. Also, if you are invisible and then you come back, the length of time that you've been online gives away the fact that you were invisible.
Could it be, then, that when you are pinged, your status, online time, etc all update on the yahoo server if you are online and what these services are doing then are checking to see if yahoo takes any action? I would presume that all yahoo is doing is not broadcasting your info if you're online, but you can still see that the server does something.
Instead of monitoring for an incoming/outgoing packet, why don't you check your registry on the yahoo server? I believe there's a url for that.
hope this helps,
Mechko