I have a Linux virtual machine inside a customer's private network. For security, this VM is reachable only via VPN + Citrix + Windows + a Windows SSH client (eg PuTTY). I am tasked to ensure this Citrix design is secure, and users can not access their Linux VM's or other resources on the internal private network in any way outside of using Citrix.
The VM can access the internet. This task should be easy. The VM's internet gateway allows it to connect anywhere on the internet to TCP ports 80, 443, and 8090 only. Connecting to an internet bastion box on one of these ports works and I can send and receive clear text data using netcat. I plan to use good old SSH, listening on tcp/8090 on the bastion, with a reverse port forward configured to expose sshd on the VM to the public, to show their Citrix gateway can be circumvented.
I hit an immediate snag. The moment I try to establish an SSH or SSL connection over o