I want to test telnet for a certain port on a certain machine, but via a VIP which is in front of that machine. Basically I want to follow the same route as the application does. Is there any possiblility to force telnet to follow a certain route (via the VIP I mentioned) ?
I must mention that by default telnet from the local host to destination follows a direct route that doesn't include the VIP.
...Or any other way to check the connection between 2 machines on a certain port (and a certain route - via a VIP ) ?
You can try creating a static route from the localhost to the destination machine using the vip as a gateway...but there's no guarantee due to the dynamic nature of routing...as some intermediate router in the network cloud may send it another way as that path is shorter etc...
The network controls the route, not you, and that includes your application; so what makes your application special enough to have particular routing arranged for it? That is the question. Since it's probably not magic either, it likely uses either the destination or the port number to decide where it goes. It might also be something stranger like TCP type-of-service or other unusual TCP options.
I'd attempt a connection on the port your application uses, which may be enough.