Most QoS (Quality of Service) adaptation systems in a heterogeneous network environment require source-code modifications in order to implement adaptation mechanisms, and network QoS parameters (e.g. bandwidth, latency, and jitter) are major sources for adaptation. This paper proposes a three-tier QoS proxy architecture where legacy applications can be transparently integrated without source-code modifications. The other salient feature of the architecture is its comprehensive coverage of QoS parameters for adaptation, including network QoS parameters such as bandwidth, latency and jitter, system QoS parameters such as CPU load and battery power status, and user profiles and QoS preferences. An end-user may assign different priorities to different applications through graphical user interfaces to best fit their QoS requirements. Packets from a client application are differentiated at the proxy, which integrates the inter-application and inter-client priority policies using priority fair queues.