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filter.txt: Linux Socket Filtering

Written by: Jay Schulist 

Introduction

============

        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.

Examples

========

Ioctls-

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.