Software inside 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, ip_tables, connection tracking (ip_conntrack, nf_conntrack) and the NAT subsystem together build the major parts of the framework.
- stateless packet filtering (IPv4 and IPv6)
- stateful packet filtering (IPv4 and IPv6)
- all kinds of network address and port translation, e.g. NAT/NAPT (IPv4 and IPv6)
- flexible and extensible infrastructure
- multiple layers of API's for 3rd party extensions
- build internet firewalls based on stateless and stateful packet filtering
- deploy highly available stateless and stateful firewall clusters
- 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.