Introduction to NetFilter

NetFilter (iptables) is the firewall, network address translation (NAT), and packet mangling code in 2.4.x kernels. NetFilter home: http://netfilter.samba.org/ , documentation  http://netfilter.samba.org/unreliable-guides/, iptables man page.

Path of packets through 2.4.x kernel

netfilter packet flow

Important difference from ipchains: forwarded packets do not traverse INPUT and OUTPUT chains.
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