Regarding yahoo messenger, I have not looked at this for a while. As I understood, it used a single outgoing port and if blocked - end of yahoo.... Or has this changed since I last looked?
This must have changed as Yahoo will try to connect 4 different servers on 8 different ports for basic IM use. If you want to use other features like the webcam, file shareing, or voice chat there are different servers and different ports that your client will connect to. Take a look at http://help.yahoo.com/help/us/sbc/messenger/signin/signin-02.html if you want to see Yahoo's definition of what the client does. I wrote a RegEx of the various host names that Yahoo will connect to, "((scs(|a-z)|filetransfer).msg|v(0-99).(vc|vip).sc(a-z|0-99)|webcam).yahoo.com". If I could implement a match in DNS for this RegEx I would do so to prevent computers on my networks from finding things.
What fun it must be being a netadmin - what don't you want to block, by which I mean wouldn't it be easier to block everything and run squid/mail server.
Andy.
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