Without getting into the technical aspects of IP spoofing. Let me suggest the following solution(s) to your problem.

First, if you are confident that this file did not come from your machine, simply state this in your response to the complaint. Secondly, it appears that you are actively interested in resolving this matter. I assume that the person who sent you the letter is also interested in a resolution. Therefore they should have no problems in providing your with detailed logs indicated such activities I suggest that you request this information as it will aid you in your investigation. The reason I suggest is because it is really easy for people complain that they are being hacked or attacked by this person or that person. (In your case, copyright infringment is suggested.) Thirdly, I assume that you have some way of logging access to your system this is important so that you can check your records against the information they provide you with (assuming that they will do this.) Finally, if you have checked your system and you are sure that you did not share this file and they don't provide you with any information. In your response indicate the following: (use your own words.)

Verfied that your system is currently not hosting the file(s) in question.

Restate your position of not ever consciously hosting the files.

Indicate your willingness to resolve this problem but your inability to do so without more detailed information.

Finally state that if they feel this information is in error. Welcome them to contact you directly.


Hope this helps good luck.