If Python, if you are developing a system service that communicates with user applications through sockets, and you want to treat sockets connected by different users differently, how would you go about that?
If I know that all connecting sockets will be from localhost, is there a way to lookup through the OS (either on windows or linux) which user is making the connection request?
On Linux and other unixy system, you can use the ident service.
I'm not sure if Windows offers something similar.
On Linux you can get the source (i.e. client-side) port of the socket and parse the output of the lsof(8) utility searching for who is using that port.
Here's the manual page.
Unfortunately, at this point in time the python libraries don't support the usual SCM_CREDENTIALS method of passing credentials along a Unix socket.
You'll need to use an "ugly" method as described in another answer to find it.