NF_IP_FORWARD: This hook is triggered after an incoming packet has been routed if the packet is to be forwarded to another host.NF_IP_LOCAL_IN: This hook is triggered after an incoming packet has been routed if the packet is destined for the local system.This hook is processed before any routing decisions have been made regarding where to send the packet. NF_IP_PRE_ROUTING: This hook will be triggered by any incoming traffic very soon after entering the network stack.The following hooks represent these well-defined points in the networking stack: The hooks that a packet will trigger depends on whether the packet is incoming or outgoing, the packet’s destination, and whether the packet was dropped or rejected at a previous point. As packets progress through the stack, they will trigger the kernel modules that have registered with these hooks. There are five netfilter hooks that programs can register with. The kernel modules associated with iptables register with these hooks in order to ensure that the traffic conforms to the conditions laid out by the firewall rules. These kernel hooks are known as the netfilter framework.Įvery packet that passes through the networking layer (incoming or outgoing) will trigger these hooks, allowing programs to interact with the traffic at key points. The iptables firewall works by interacting with the packet filtering hooks in the Linux kernel’s networking stack. In some distributions, it has been replaced by a new tool called nftables, but iptables syntax is still commonly used as a baseline. What Are IPTables and Netfilter?įor many years, the firewall software most commonly used in Linux was called iptables. We will discuss how iptables interacts with netfilter and how the various components fit together to provide a comprehensive filtering system. In this guide, we will dive into the iptables architecture with the aim of making it more comprehensible for users who need to build their own firewall policies. Creating reliable firewall policies can be daunting, due to complex syntax and the number of interrelated parts involved. In the Linux ecosystem, iptables is a widely used firewall tool that works with the kernel’s netfilter packet filtering framework. Firewalls are an important tool that can be configured to protect your servers and infrastructure.
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