Network Diagnosis Tools with industrial-grade quality are not widely available for common users such as researchers and students. This kind of tool enables users to develop Distributed Embedded Systems using low-cost and reliable setups. In the context of RISC-V Extensions and Domain-Specific Architecture, this paper proposes a Real-Time RISC-V-based CAN-FD Bus Diagnosis Tool, named RiscDiag CanFd, as an open-source alternative. The RISC-V Core extension is a CAN-FD Communication Unit controlled by a dedicated ISA Extension. Besides the extended RISC-V core, the proposed SoC provides UDP Communication via Ethernet for connecting the proposed solution to a PC. Additionally, a GUI application was developed for accessing and using the hardware solution deployed in an FPGA. The proposed solution is evaluated by measuring the lost frame rate, the precision of captured frames timestamps and the latency of preparing data for Ethernet communication. Measurements revealed a 0% frame loss rate, a timestamp error under 0.001% and an acquisition cycle jitter under 10 ns.