How to detect whether a protocol is supported through web browser?

Similar to iTunes' 'Link Maker' (http://www.apple.com/itunes/linkmaker/faq/). Through web browser, iTunes webpages is capable of finding out if itms:// has been registered on client workstation, hence开发者_JAVA百科 displaying correct message ('View in iTunes' button or 'Download iTunes' promotion banner).

How is this conducted?

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Unfortunately, there is really no easy way to do this. iTunes actually uses the following script to do the detection. From the code you can see they use a browser plug-in within each web browser to accomplish this.

In Internet Explorer, there is a feature via the protocolLong property, which can be used, but obviously it's browser specific.

There may be other hacks available, but I think some sort on browser plug-in would be the surest solution.