linux-kernel - [PATCH v2] bridge: ebtables: fix reception of frames DNAT-ed to bridge device

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Message-Id: <20170321222845.8373-1-linus.luessing@c0d3.blue>
Date:   Tue, 21 Mar 2017 23:28:45 +0100
From:   Linus Lüssing <linus.luessing@...3.blue>
To:     netdev@...r.kernel.org
Cc:     "David S . Miller" <davem@...emloft.net>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>,
        bridge@...ts.linux-foundation.org, netfilter-devel@...r.kernel.org,
        coreteam@...filter.org, linux-kernel@...r.kernel.org,
        Linus Lüssing <linus.luessing@...3.blue>
Subject: [PATCH v2] bridge: ebtables: fix reception of frames DNAT-ed to bridge device

When trying to redirect bridged frames to the bridge device itself
via the ebtables nat-prerouting chain and the dnat target then this
currently fails:

The ethernet destination of the frame is dnat'ed to the MAC address of
the bridge itself just fine and the correctly altered frame can even
be captured via a tcpdump on br0 (with or without promisc mode).

However, the IP code drops it in the beginning of ip_input.c/ip_rcv()
as the dnat target did not update the skb->pkt_type. If after
dnat'ing the packet is now destined to us then the skb->pkt_type
needs to be updated from PACKET_OTHERHOST to PACKET_HOST, too.

Signed-off-by: Linus Lüssing <linus.luessing@...3.blue>

---
Changelog v2:
* refrain from altering pkt_type for multicast packets
  with a unicast destination MAC
---
 net/bridge/br_input.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/net/bridge/br_input.c b/net/bridge/br_input.c
index 013f2290b..fd7bc4c 100644
--- a/net/bridge/br_input.c
+++ b/net/bridge/br_input.c
@@ -198,8 +198,13 @@ int br_handle_frame_finish(struct net *net, struct sock *sk, struct sk_buff *skb
 	if (dst) {
 		unsigned long now = jiffies;
 
-		if (dst->is_local)
+		if (dst->is_local) {
+			/* fix up potential DNAT inconsistencies */
+			if (skb->pkt_type == PACKET_OTHERHOST)
+				skb->pkt_type = PACKET_HOST;
+
 			return br_pass_frame_up(skb);
+		}
 
 		if (now != dst->used)
 			dst->used = now;
-- 
2.1.4