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Linux Journal: Kernel Korner: Inside the Linux Packet Filter

Jan 17, 2002, 09:18 (0 Talkback[s])
(Other stories by Gianluca Insolvibile)
"Another data structure that will recur quite often is the sk_buff (short for socket buffer), which represents a packet inside the kernel. The structure is arranged in such a way that addition and removal of header and trailer information to the packet data can be done in a relatively inexpensive way: no data actually needs to be copied since everything is done by just shifting pointers.

Before going on, it may be useful to clear up possible ambiguities. Despite having a similar name, the Linux socket filter has a completely different purpose with respect to the Netfilter framework introduced into the kernel in early 2.3 versions. Even if Netfilter allows you to bring packets up to user space and feed them to your programs, the focus there is to handle network address translation (NAT), packet mangling, connection tracking, packet filtering for security purposes and so on. If you just need to sniff packets and filter them according to certain rules, the most straightforward tool is LSF.

Now we are going to follow the trip of a packet from its very ingress into the computer to its delivery to user land at the socket level. We first consider the general case of a plain (i.e., not PF_PACKET) socket. Our analysis at link layer level is based on Ethernet, since this is the most widespread and representative LAN technology. Cases of other link layer technologies do not present significant differences."

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