Open-MX is a new message passing layer implemented on top of the generic Ethernet stack of the Linux kernel. It provides high-performance communication on top of any Ethernet hardware while exhibiting the Myrinet Express application interface. Open-MX also enables wire-interoperability with Myricom's MXoE hosts. This article presents the design of the Open-MX stack which reproduces the MX firmware in a Linux driver. MPICH-MX and PVFS2 layers are already able to work flawlessly on Open-MX. The first performance evaluation shows interesting latency and bandwidth results on 1 and 10~gigabit hardware.