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Based on kernel version 3.13. Page generated on 2014-01-20 22:03 EST.

1	filter.txt: Linux Socket Filtering
2	Written by: Jay Schulist <jschlst@samba.org>
3	
4	Introduction
5	============
6	
7		Linux Socket Filtering is derived from the Berkeley
8	Packet Filter. There are some distinct differences between
9	the BSD and Linux Kernel Filtering.
10	
11	Linux Socket Filtering (LSF) allows a user-space program to
12	attach a filter onto any socket and allow or disallow certain
13	types of data to come through the socket. LSF follows exactly
14	the same filter code structure as the BSD Berkeley Packet Filter
15	(BPF), so referring to the BSD bpf.4 manpage is very helpful in
16	creating filters.
17	
18	LSF is much simpler than BPF. One does not have to worry about
19	devices or anything like that. You simply create your filter
20	code, send it to the kernel via the SO_ATTACH_FILTER option and
21	if your filter code passes the kernel check on it, you then
22	immediately begin filtering data on that socket.
23	
24	You can also detach filters from your socket via the
25	SO_DETACH_FILTER option. This will probably not be used much
26	since when you close a socket that has a filter on it the
27	filter is automagically removed. The other less common case
28	may be adding a different filter on the same socket where you had another
29	filter that is still running: the kernel takes care of removing
30	the old one and placing your new one in its place, assuming your
31	filter has passed the checks, otherwise if it fails the old filter
32	will remain on that socket.
33	
34	SO_LOCK_FILTER option allows to lock the filter attached to a
35	socket. Once set, a filter cannot be removed or changed. This allows
36	one process to setup a socket, attach a filter, lock it then drop
37	privileges and be assured that the filter will be kept until the
38	socket is closed.
39	
40	Examples
41	========
42	
43	Ioctls-
44	setsockopt(sockfd, SOL_SOCKET, SO_ATTACH_FILTER, &Filter, sizeof(Filter));
45	setsockopt(sockfd, SOL_SOCKET, SO_DETACH_FILTER, &value, sizeof(value));
46	setsockopt(sockfd, SOL_SOCKET, SO_LOCK_FILTER, &value, sizeof(value));
47	
48	See the BSD bpf.4 manpage and the BSD Packet Filter paper written by
49	Steven McCanne and Van Jacobson of Lawrence Berkeley Laboratory.
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