Remote desktop access over internet

Question

We have one desktop based application. Users of this application are primarily non-technical people. It will be great help for us to remotely access their desktop for number of reason(troubleshooting client side issues, guiding and training them for complex workflow).

I looking for solution where we can remotely access desktop of user, with following constraints

1. It needs to be opensource/freeware, so can't use TeamViewer or similar solution. (compulsory)
2. Whatever needs to be install on client's system, it should work for both Windows and Linux.(compulsory)
3. Most of the clients(users) of our application sits behind corporate proxy, they will not have public IP address, can't ask them for port forwarding either.(compulsory)
4. Whatever needs to be install on client's system, it should get run without administrator privilege.(optional)

I read about noVNC and websockify but can't figure out how to crack this.

Answer 1

I use Zoho Assist, Windows/Mac/Linux, the free version supports one tech and connection is via a url or code. There are some limitations in the free version such as no file transfers. https://www.zoho.com/assist/pricing.html