Re: [PATCH nf] netfilter REJECT: Fix destination MAC in RST packets

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Mon, Mar 08, 2021 at 09:21:20AM -0700, Marc Aurèle La France wrote:
> On Mon, 8 Mar 2021, Pablo Neira Ayuso wrote:
> > On Sun, Mar 07, 2021 at 06:16:34PM -0700, Marc Aurèle La France wrote:
> > > In the non-bridge case, the REJECT target code assumes the REJECTed
> > > packets were originally emitted by the local host, but that's not
> > > necessarily true when the local host is the default route of a subnet
> > > it is on, resulting in RST packets being sent out with an incorrect
> > > destination MAC.  Address this by refactoring the handling of bridged
> > > packets which deals with a similar issue.  Modulo patch fuzz, the
> > > following applies to v5 and later kernels.
> 
> > The code this patch updates is related to BRIDGE_NETFILTER. Your patch
> > description refers to the non-bridge case. What are you trying to
> > achieve?
> 
> Via DHCP, my subnet's default route is a Linux system so that it can monitor
> all outbound traffic.  By doing so, for example, I have determined that my
> Android phone connects to Facebook despite the fact that I have no such app
> installed.  I want to know, and control, what other behind-the-scenes
> (under-handed) traffic devices on my subnet generate.
>
> > dev_queue_xmit() path should not be exercised from the prerouting
> > chain, packets generated from the IP later must follow the
> > ip_local_out() path.
> 
> Well, I can tell you dev_queue_xmit() does in fact work in prerouting
> chains, as it must for the bridging case.  The only potential problem I've
> found so far is that the RST packet doesn't go through any netfilter hooks.

That's the issue, Netfilter rejects code from the IP layer, so the
packets follows the ip_local_out() path.

You could use ingress to reject through dev_queue_xmit() / neigh_xmit().