I have root access to a server (S), but have no control over the firewalls over the network. I set up a virtual machine (VM) inside the server (S) and wanted some users to access VM directly, i.e., without providing them access to anything else inside S.
A straight-forward solution was to set up a tunnel that listen to another port, like S:22000. However, users are on a different network, that filters all ports other than 22, 80, 113, and 443.
There is no service listening on 113. Can it be used as entry point of the tunnel?