(Though now I think about it, were they annoyed at the commercialisation of a piece of research? Being a Uni, probably. Either way, you've left an impression!)
It seems to me that people are donating because you've earned it. If you really feel a need to give back, perhaps the gift of education would be better. W. Richard Stephen's TCP/IP Illustrated is a great book, but looking long in the tooth these days. I'm sure you've forgotten more about TCP/IP than most of us have learned, but perhaps a book on TCP/IP by a guy that wrote a stack would be a good start?
Welcome!