netdev - Re: [Bridge] [PATCH net] bridge: ebtables: fix reception of frames DNAT-ed to bridge device

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Date:   Tue, 21 Mar 2017 11:11:21 +0100
From:   Pablo Neira Ayuso <pablo@...filter.org>
To:     Linus Lüssing <linus.luessing@...3.blue>
Cc:     netdev@...r.kernel.org, bridge@...ts.linux-foundation.org,
        Florian Westphal <fw@...len.de>, linux-kernel@...r.kernel.org,
        coreteam@...filter.org, netfilter-devel@...r.kernel.org,
        Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>,
        "David S . Miller" <davem@...emloft.net>
Subject: Re: [Bridge] [PATCH net] bridge: ebtables: fix reception of frames
 DNAT-ed to bridge device

On Tue, Mar 21, 2017 at 01:09:47AM +0100, Linus Lüssing wrote:
> On Sun, Mar 19, 2017 at 05:55:06PM +0100, Linus Lüssing wrote:
> > On Fri, Mar 17, 2017 at 02:10:44PM +0100, Pablo Neira Ayuso wrote:
> > > Wait.
> > > 
> > > May this break local multicast listener that are bound to the bridge
> > > interface? Assuming the bridge interface got an IP address, and that
> > > there is local multicast listener.
> > > 
> > > Missing anything here?
> > 
> > Hm, for multicast packets usually the code path a few lines
> > later in br_handle_frame_finish() should be taken instead.
> > 
> > But you might be right for IP multicast packets with a unicast MAC
> > destination (due to whatever reason, for instance via DNAT'ing
> > again).
> > 
> > Will check that - thanks!
> 
> Ok, I tested DNAT'ing an IP multicast packet to the unicast MAC address
> of the bridge interface.
> 
> Both ping-ing to an IPv4 and IPv6 multicast listener on br0 worked
> and was replied to fine, both with or without changing skb->pkt_type
> from PACKET_MULTICAST to PACKET_HOST.
> ("$ ping 224.1.0.123" and "$ ping6 ff02::1:ff40:707c%in0" from a
>  network namespace, tied into the bridge via veth)
> 
> Also, a DNAT'ed PACKET_BROADCAST worked, with or without changing
> it to PACKET_HOST.
> 
> I also checked via tcpdump that the destination MAC was changed
> successfully.

Thanks for looking into this more in depth.

> So, so far I wasn't able to find any bugs with the current
> patch. But I think I like the idea of leaving the skb->pkt_type
> unaltered for PACKET_MULTICAST and PACKET_BROADCAST, seems cleaner.

Yes, and matching on skb->pkt_type would not break from netfilter.

> I'd just add an "if (skb->pkt_type == PACKET_OTHERHOST)" check
> then and resend a PATCH v2.

Thanks.