Based on kernel version 3.9. Page generated on 2013-05-02 23:11 EST.
1 filter.txt: Linux Socket Filtering 2 Written by: Jay Schulist <email@example.com> 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.