Thank you for your answer.
I sniffed the network traffic between the set-top-box and the DHCP server. The DHCP server I'm running is dhcp3-server, provided by debian repositories.
I looked at the server logs just after my post and I saw that logs mentioned the aminocom MAC address and never the micropro MAC address... I made a grep with absolute addresses (00 00 ec d9 15 81 and 00 02 02 15 18 3b) on the whole logs. It's really confusing...
Well, it's ok for the telnet server, I think it's the same on the 125 model, even if I can't test it for the moment...
And amino is not the first company who does not publish any technical documents about its boxes... look at
netgem's netbox, it's the same nightmare... even if network configuration is a bit easier...
For amino, a NDA is necessary to get technical support, I think this is the reason why people do not publish information.