On Mon, Mar 08, 2021 at 09:25:28PM -0700, Marc Aurèle La France wrote: > On Tue, 9 Mar 2021, Pablo Neira Ayuso wrote: > > 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. > > ... which sets an incorrect destination MAC. Also, in this case, netfilter > doesn't reject any such thing. It doesn't even "see" the RST packet > dev_queue_xmit() sends out. That's OK as there is no further need to > process such a packet. dev_queue_xmit() skips the policy in the local out path for the generated RST packet. If you want to plain reject using dev_queue_xmit() then you have to use the ingress hook. > At least, the device whose connection request is being denied > doesn't hang anymore... The neighbour cache selects the destination MAC from the destination IP address of the RST packet. Your patch also refers to non-bridge scenario (no br_netfilter in place). Could you describe what you're trying to achieve in plain layman terms?
