What is Linux firewall?

Linux provides us with an excellent firewall tool, netfilter/iptables (http: // www.netfilter.org /). It is completely free and can run well on a low-configuration old machine. Netfilter/iptables is powerful and flexible, and can be used to precisely control inbound and outbound information.
In fact, each major Linux version has different firewall software suites. The Iptabels (netfilter) application is considered to be the fourth-generation application in Linux that implements the packet forwarding function. The first generation is the ipfw transplanted from BSD Unix by Alan Cox, used in Linux kernel 1.1.
In the kernel of Version 2.0, Jos Vos and some other programmers have extended ipfw and added ipfwadm user tools. In the 2.2 kernel, Russell and Michael Neuling made some important improvements, that is, in this kernel, Russell added ipchains to help users control excessive rules. Then Russell completed its kernel framework named netfilter (http://www.netfilter.org. These firewall software suites are generally improved from their predecessors and are becoming more and more outstanding.
Netfilter/iptables is included in the kernel after 2.4. It can implement functions such as firewall, NAT (Network Address Translation), and data packet splitting. Netfilter works inside the kernel, while iptables allows you to define the table structure of the rule set. Netfilter/iptables evolved from ipchains and ipwadfm (IP firewall management). For the sake of simplicity, I will name it iptables.
Other good uses of iptables are to create a firewall for Unix, Linux, and BSD workstations. Of course, you can also create a firewall for a subnet to protect other system platforms. Iptables only reads the packet header, so it does not burden the information flow and does not require verification. For better security, you can combine it with a proxy server (such as squid.