What is netfilter/iptables?

netfilter and iptables are building blocks of a framework inside the Linux 2.4.x and 2.6.x kernel. This framework enables packet filtering, network address [and port] translation (NA[P]T) and other packet mangling. It is the re-designed and heavily improved successor of the previous Linux 2.2.x ipchains and Linux 2.0.x ipfwadm systems.

What is netfilter/iptables?

netfilter and iptables are building blocks of a framework inside the Linux 2.4.x and 2.6.x kernel. This framework enables packet filtering, network address [and port] translation (NA[P]T) and other packet mangling. It is the re-designed and heavily improved successor of the previous Linux 2.2.x ipchains and Linux 2.0.x ipfwadm systems.

netfilter is a set of hooks inside the Linux kernel that allows kernel modules to register callback functions with the network stack. A registered callback function is then called back for every packet that traverses the respective hook within the network stack.

iptables is a generic table structure for the definition of rulesets. Each rule within an IP table consists of a number of classifiers (iptables matches) and one connected action (iptables target).

netfilter, iptables and the connection tracking as well as the NAT subsystem together build the whole framework.

Main Features

• stateless packet filtering (IPv4 and IPv6)
• stateful packet filtering (IPv4)
• all kinds of network address and port translation (NAT/NAPT)
• flexible and extensible infrastructure
• multiple layers of API's for 3rd party extensions
• large number of plugins/modules kept in 'patch-o-matic' repository

What can I do with netfilter/iptables?

• build internet firewalls based on stateless and stateful packet filtering
• use NAT and masquerading for sharing internet access if you don't have enough public IP addresses
• use NAT to implement transparent proxies
• aid the tc and iproute2 systems used to build sophisticated QoS and policy routers
• do further packet manipulation (mangling) like altering the TOS/DSCP/ECN bits of the IP header