Plugin with custom domain

Question

OK I will try to explane my problem better.

What I need to do is to create a kind o service for my WordPress.

From some external web sites I like to send some data to my main WordPress web site through the URL.

Lets say I have the web site http://www.mysite.ext/

On mysite.ext I will install my plugin. Some external web sites can send my ocasionaly some data by a URL (ie: http://www.mysite.ext/getdata/data_are_here/) In that case my plugin must recognize a query variable from the URL and analize the data

Note: It is not important to return any information to the external web sites. Just I have to collect the inrormation and process it.