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 ioctl 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 ioctl. 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 Examples
35 ========
36
37 Ioctls-
38 setsockopt(sockfd, SOL_SOCKET, SO_ATTACH_FILTER, &Filter, sizeof(Filter));
39 setsockopt(sockfd, SOL_SOCKET, SO_DETACH_FILTER, &value, sizeof(value));
40
41 See the BSD bpf.4 manpage and the BSD Packet Filter paper written by
42 Steven McCanne and Van Jacobson of Lawrence Berkeley Laboratory.
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