Active networking is a powerful technology to insert new functionality into the networking. In this paper we look at how active networking technology can be used to customize network services. We observe that users often want slightly different versions of network services such as multicast and network quality of service. We propose to implement these services as a base service that provides the basic service functionality and a customization code modules that allows users to customize the service. The customization module uses a service-specific API to modify service behavior. We compare this architecture with the traditional active networking architecture based on execution environments and active applications. We also present several examples of customizable network services.