Question

I’m investigating a project at the moment to create an application which will listen to “localhost” within a Windows terminal services environment.

I don’t have access to a test environment at present, but I wanted to check this design, especially what localhost/loopback 127.0.0.1 means within a multiuser machine.

If my application binds a TCP socket listening on 127.0.0.1:40000 then what clients would be able to access this?
– would it be open to clients within all sessions for all users on the machine/server?
– or would it just be each individual user/session?

I’m hoping/guessing the latter. If this is the case, then can each user in each session open their own app running a listener on 127.0.0.1:40000?

Thanks for any help on this design issue.

Answer 1

I will disappoint you, it’s the former.

TCP/IP sockets have no concept of “users” or “ownership”: There are 65535 available ports on a given network interface, and there can only be one process listening at any given one. What user owns the process is irrelevant – if you have User1’s process listening on 127.0.0.1:40000, then User2’s process’ attempt to listen on the same port will fail.

Likewise, there is no intrinsic access control: if there’s a listening port at a given port, anything that can reach the computer at that port can access the port (in other words, the listening and connecting processes – server and client – don’t need to belong to the same user; they might even be on different hosts).