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 option 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 option. 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 34SO_LOCK_FILTER option allows to lock the filter attached to a 35socket. Once set, a filter cannot be removed or changed. This allows 36one process to setup a socket, attach a filter, lock it then drop 37privileges and be assured that the filter will be kept until the 38socket is closed. 39 40Examples 41======== 42 43Ioctls- 44setsockopt(sockfd, SOL_SOCKET, SO_ATTACH_FILTER, &Filter, sizeof(Filter)); 45setsockopt(sockfd, SOL_SOCKET, SO_DETACH_FILTER, &value, sizeof(value)); 46setsockopt(sockfd, SOL_SOCKET, SO_LOCK_FILTER, &value, sizeof(value)); 47 48See the BSD bpf.4 manpage and the BSD Packet Filter paper written by 49Steven McCanne and Van Jacobson of Lawrence Berkeley Laboratory. 50