rdesktop seems to be deprecated, or at least lacks some of the security features required to connect to modern RDP sessions. Luckily, its replacement, FreeRDP seems to work well and is available on most popular Linux distributions. Unfortunately, the command line syntax for FreeRDP is a bit more unwieldy than rdesktop's, but Remmina makes setting it up a breeze.

Firstly, if you're on RHEL7 and you don't have Remmina and FreeRDP installed, run the following in the terminal.

sudo yum install remmina* freerdp*

That will install Remmina, FreeRDP, and all the plugins. If you're on a non-RedHat distro, just search for the equivalent and install that.

Once you have everything installed, connect to the VPN and then run Remmina from the menu. Click the "New Connection" button and fill out the following:

• Name: Any name you like
• Protocol: RDP
• Server: The hostname of the computer you want to connect to
• Username: Your soton username
• Password: Either fill it in with your soton password or leave it blank to be prompted each time you connect.
• Domain: SOTON

Save that, and you should be able to connect to your RDP session when you double click on it in the list. If it asks you to accept a certificate, just click OK.