NAME

Linux::PacketFilter - Simple interface to Linux packet filtering

SYNOPSIS

    # Reject any packet that starts with a period:
    my $filter = Linux::PacketFilter->new(

        # Load the accumulator with the 1st byte in the packet:
        [ 'ld b abs', 0 ],

        # If the accumulator value is an ASCII period, continue;
        # otherwise, skip a line. (See below for what “k8” means.)
        [ 'jmp jeq k8', ord('.'), 0, 1 ],

        # If we continued, we’ll get here and thus reject the packet.
        [ ret => 0 ],

        # If we get here, we skipped a line above. That means
        # the packet’s first byte wasn’t an ASCII period,
        # so we'll return the full packet.
        [ ret => 0xffffffff ],
    );

    $filter->apply( $socket );

DESCRIPTION

This module is a simple, small, pure-Perl compiler for Linux’s “classic” Berkeley Packet Filter (BPF) implementation.

HOW TO USE THIS MODULE

If you’re familiar with BPF already, the SYNOPSIS above should mostly make sense “out-of-the-box”. If you’re new to BPF, though, take heart; it’s fairly straightforward.

The best source I have found for learning about BPF itself is bpf(4) in the BSD man pages; see the section entitled FILTER MACHINE.

Linux-specific implementation notes are available in the kernel source tree at /Documentation/networking/filter.txt. This contains a lot of detail about uses for BPF that don't pertain to packet filtering, though.

Here is another helpful guide. Take especial note of the need to convert between network and host byte order. (See below for a convenience that this module provides for this conversion.)

You might also take interest in the original BPF white paper.

NOTE: This module works with Linux’s “classic” BPF, not the much more powerful (and complex) “extended” BPF.

METHODS

$obj = CLASS->new( @filters )

Creates an object that represents an array of instructions for the BPF filter machine. Each @filters member is an array reference that represents a single instruction and has either 2 or 4 members, which correspond with the BPF_STMT and BPF_JUMP macros, respectively.

The first member of each array reference is, rather than a number, a space-separated string of options, lower-cased and without the leading BPF_. So where in C you would write:

    BPF_LD | BPF_W | BPF_ABS

... in this module you write:

    'ld w abs'

The full list of options for a single instruction is:

• b, h, w, x, k, k_n, k_N (See below for an explanation of the last two.)

• ld, ldx, st, stx, alu, jmp, ret, misc

• imm, abs, ind, mem, len, msh

• add, sub, mul, div, or, and, lsh, rsh, neg, mod, xor

• ja, jeq, jgt, jge, jset

Byte order conversion

Since it’s common to need to do byte order conversions with packet filtering, Linux::PacketFilter adds a convenience for this: the codes k_n and k_N indicate to encode the given constant value in 16-bit or 32-bit network byte order, respectively.

Note that Linux always consumes BPF instruction constants in network order. Thus, if you’re on a little-endian system, to match against numbers that are in host order (e.g., numbers in Netlink headers) you’ll need to do a byte-order conversion.

To add to the fun: when BPF compares a 16- or 8-bit number from “k”, it expects to do so from the first available register. This works fine on little-endian systems, but on big-endian systems that means It would be more natural for this module to encode the constants in network order; however, that would also put it at variance with C implementations, which would compromise the usefulness of existing documentation.

$ok = OBJ->attach( $SOCKET )

Attaches the filter instructions to the given $SOCKET.

Note that this class purposely omits public access to the value that is given to the underlying setsockopt(2) system call. This is because that value contains a pointer to a Perl string. That pointer is only valid during this object’s lifetime, and bad stuff (e.g., segmentation faults) can happen when you give the kernel pointers to strings that Perl has already garbage-collected.

The return is the same as the underlying call to Perl’s "setsockopt" in perlfunc built-in. $! is set as that function leaves it.

AUTHOR

Copyright 2019 Gasper Software Consulting (http://gaspersoftware.com)

SEE ALSO

Linux::SocketFilter::Assembler suits a similar purpose to this module’s but appears to be geared solely toward PF_PACKET sockets. It also defines its own language for specifying the filters, which I find less helpful than this module’s approach of “porting” the C macros to Perl, thus better capitalizing on existing documention.

cpanm Linux::PacketFilter

perl -MCPAN -e shell
install Linux::PacketFilter