I was wondering if there's a way to block high definition videos such as youtube hd or other websites that offer hd streaming videos.
any proxy/firewall rule?

Even though I don't know, could you explain why you want to block it?

I don't want users to watch any hd video!

It's almost impossible. You would need to make custom L7 rules for each video site. (That's even IF it's possible to track in L7)

You will also need serious hardware to do this. (CCR is the only option for MT hardware)

If you're trying to do it to save bandwidth... Good luck. Spend your time trying to fill the hole instead of plugging it. You will spend less money in the end.

I was wondering if there's a way to block high definition videos such as youtube hd or other websites that offer hd streaming videos.
any proxy/firewall rule?

No problem regarding youtube-videos when using squid, as yt identifies HD-videos within URL. Only need to set up proper ACL & regex for blocking.
However, it is a user-friendlier solution to downgrade yt-HD-video to simple resolution, without real block.

I do not see any "hd" tag in youtube video url
It seems really impossible

I do not see any "hd" tag in youtube video url
It seems really impossible

Then I give you a hint to do the impossible: Examine "id"

this is a youtube hd video url:

YES, but only to be started with.
This URL will be redirected according to your location/requested resolution into something much more complicated. And this will contain "...id=..." specifying the resolution.
You should use firebug in firefox to observe.

As this is a MT-forum, no more info on this topic from me.