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-filter.txt: Linux Socket Filtering
-Written by: Jay Schulist <>
- Linux Socket Filtering is derived from the Berkeley
-Packet Filter. There are some distinct differences between
-the BSD and Linux Kernel Filtering.
-Linux Socket Filtering (LSF) allows a user-space program to
-attach a filter onto any socket and allow or disallow certain
-types of data to come through the socket. LSF follows exactly
-the same filter code structure as the BSD Berkeley Packet Filter
-(BPF), so referring to the BSD bpf.4 manpage is very helpful in
-creating filters.
-LSF is much simpler than BPF. One does not have to worry about
-devices or anything like that. You simply create your filter
-code, send it to the kernel via the SO_ATTACH_FILTER ioctl and
-if your filter code passes the kernel check on it, you then
-immediately begin filtering data on that socket.
-You can also detach filters from your socket via the
-SO_DETACH_FILTER ioctl. This will probably not be used much
-since when you close a socket that has a filter on it the
-filter is automagically removed. The other less common case
-may be adding a different filter on the same socket where you had another
-filter that is still running: the kernel takes care of removing
-the old one and placing your new one in its place, assuming your
-filter has passed the checks, otherwise if it fails the old filter
-will remain on that socket.
-setsockopt(sockfd, SOL_SOCKET, SO_ATTACH_FILTER, &Filter, sizeof(Filter));
-setsockopt(sockfd, SOL_SOCKET, SO_DETACH_FILTER, &value, sizeof(value));
-See the BSD bpf.4 manpage and the BSD Packet Filter paper written by
-Steven McCanne and Van Jacobson of Lawrence Berkeley Laboratory.