What are the two main types of firewalls? What level of network do they work at? Briefly describe the principles and characteristics of these two firewall technologies.

Packet filtering firewall can be divided into "packet filtering type" and "application proxy type", which works in the network layer and transport layer of OSI network reference model. It determines whether to allow the packet to pass according to the source address, destination address, port number and protocol type of the packet header. Only packets that meet the filtering conditions will be forwarded to the corresponding destination, and the rest will be discarded from the data stream.

The application proxy firewall works at the highest level of OSI, that is, the application layer. Its characteristic is to completely "block" the network communication traffic, and realize the monitoring function of the application layer communication traffic by writing a special agent program for each application service.