Method for implementing private network penetration of monitoring business, network appliance and server

The invention discloses a method, a network device and a server used for realizing private network traversal in monitoring service. The method includes the following steps: receiving the link keep-alive information sent by a client through the network device in the private network at fixed time so as to keep the smoothness of the link between the client; acquiring the address and the port number carried by a link layer from the link keep-alive information, and saving the address and the port number as link information communicating with the client; according to the link information, encapsulating the information data to be sent to the client and sending the information data packet to the client through the network device. The method, the server and the network device are suitable for various networking conditions and can be modified for normal use in time in the case of upgrade of the network or change of the access way, without additionally increasing or changing the network device or the server.