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