How the Firewall works

"Is the hacker going to make my mind?" "This is the right thing to do, hackers want to drill eggs, like flies, see a trace from the system leaks out of the light will be stirring!" Okay, how do you protect your network? Computer experts may be a mouth to suggest you install the network firewall, then the first question comes: What is the firewall?

What is a firewall?

A firewall is a sort of filter plug (now you're not mistaken), you can make something you like go through the stopper, and everything else is filtered out. In the network world, to be filtered by the firewall is the communication packet that carries the communication data.

The world's firewalls will say at least two words: yes or No. To say is to accept or reject. The simplest firewall is an Ethernet bridge. But few people think the original firewall can be used for much. Most firewalls employ a wide variety of technologies and standards. These firewalls are of various forms: some replace the TCP/IP protocol stacks already installed on the system, some build their own software modules on the existing protocol stacks, and some simply have a stand-alone operating system. There are also application-type firewalls that provide protection only for certain types of network connections (such as SMTP or HTTP protocols, etc.). There are a number of hardware-based firewall products in fact should be grouped into a security router class. The above products can be called firewalls, because they work the same way: Analyze the data packets to and from the firewall, decide to release or throw them aside.

All firewalls have IP address filtering capabilities. This task examines the IP header and makes a release/discard decision based on its IP source address and destination address. Look at this picture below, two network segments between a firewall, one end of the firewall has a UNIX computer, the other side of the network segment is a PC client.

When a PC client initiates a Telnet request to a UNIX computer, the PC's Telnet client generates a TCP packet and passes it to the local protocol stack ready to be sent. Next, the protocol stack "plugs" the TCP packet into an IP packet and sends it to a UNIX computer via a path defined by the PC's TCP/IP stack. In this case, the IP packet must pass through a firewall across both the PC and the UNIX computer to reach the UNIX computer.