Despite network monitoring and testing being critical for computer networks, current solutions are both extremely expensive and inflexible. Into this lacuna we launch the Open Source Network Tester (OSNT), a fully open-source traffic generator and capture system. Our prototype implementation on the NetFPGA-10G supports 4x10Gbps traffic generation across all packet sizes and traffic capture is supported up to 2x10Gbps with naïve host software. Our system implementation provides methods for scaling and coordinating multiple generator/capture systems and supports 6.25ns timestamp resolution with clock drift and phase coordination maintained by a GPS input. Additionally, our approach has demonstrated lower-cost than comparable commercial systems while achieving comparable levels of precision and accuracy; all within an open-source framework extensible with new features to support new applications, while permitting validation and review of the implementation.