and what's about to do a kind of

client                               some server of yours
                                   at some place
======                             ======================
send(petition to you server) .
                              \____ Annotate that date for 
                                    later use.
timeout(wait(response))             All is okay (they has 
                       |            pay you, and so):
                       |____________send(ACK to client)
[download]

i don't know how to call it, ^_^>, but it could be your solution:

1. You install a script that sends a request every week or day or hour or even every 60 or 30 seconds (whatever you want).
2. If you have a server (you must have a server for this scheme to work), then logs the client request (something like this: [Sat, 11 Feb 2006 10:29:55 +0100] ID:SOME_ID_YOU_ASSIGN_TO_THE_CLIENT received from the ip:x.x.x.x an acknowledge request
3. Your server sends a reply to your client.
4. If the client didn't receive the ack whitin X secs, then, it sums one to some counter ($counter++).
5. If the client, didn't receive any ack in X succeed times ($counter > X); then do something for preventing the client to use the program (this could be a thing a little bit nasty ... like to auto-rm, to introduce some extrange byte, to make the perl compiler to abort, or to make the binary not to execute ...)
6. Everytime the client have a response ($counter=0); not worries and continuing serving

So, when your are not agree with the client, you disconnect your application or filter the client ID you assigned. If the client, filters your ip, the application will autoremove (or so). If the clients, changes its IP, you can know it, and the application will work. If the ...

Uff, i feel bad, guilty and dirty ... i must pray at the oratorium for expiating my sins ... ugh

turo

update Sat, 11 Feb 2006 11:50:31 +0100
i feel really bad for, this ... the dark side is possessing me ...