![Mike Linksvayer](https://identi.ca/uploads/mlinksva/2013/7/24/K6hRzw_thumb.png)
I think only the client is. https://en.wikipedia.org/wiki/Prey_(software) also links to a minimal free software server, presumably independently implemented, not updated since 2013. In any case, it relies on a server. One of the interesting things about Adeona was its use of a distributed hash table.