(Tue, Feb 11, 2014 at 07:00:41PM +0100) Rodrigo Rivas :
My guess here is that this "secure" network is full of non-upgraded (Windows?) machines, and security is attained exclusively by network isolation.
No they are all on linux. The problem here is that it creates a lot of problem for work: outdated browser (some websites unaccessible), outdated bash and programs (Hard to believe, but they have evolved a lot!!). We must do everything by ourselves, hence the "hidden" machine: to handle backup of our datas since he doesn't do it properly (last year his backup/nfs server failed and we stayed a few months with an inconsistently rebuilt home, and the "emergency solution" went to break also afterwards and...)
So my advice to the OP is to play safe and not to program any kind of inbound tunnel. That could end in disaster and you would be responsible! Just limit the access to your mole's handmade tunned, or play by the rules and not upgrade (ug!).
Hum hum... I'm still hesitating on what exactly I will do, but it won't stay like that... Anyway, thanks all for all the advices! -- Ismael