This paper describes the design and implementation of NetVCR, a high-performance packet replay engine for use in evaluating the performance of networking devices. NetVCR is implemented on commodity hardware using widely available open-source software. To achieve high throughput and accuracy, NetVCR employs novel mechanisms for managing trace files and accurate low-overhead timers. In addition, through the use of low-latency kernel patches and priority scheduling, NetVCR can be made highly resilient to background system load. Using these mechanisms, NetVCR is able to support packet replay at rates well above OC-12 on an x86-based server.