Methods and apparatus that allow clients to replace standard packet processing provided by network processing devices on host devices with client network instances that provide custom packet processing for packets going to and/or coming from the clients' resource instances in a provider network environment. Thus, packet processing functionality needed by a client that may not be supported or provided by the service provider's standard packet processing may be added to the clients' private network implementations. In addition, standard private network functionality provided by the service provider may be removed, modified, or replaced using the client network instances. Development tools, services, and interfaces may be provided for developing, integrating, and managing the client network instances.