bridge: multicast to unicast

Implements an optional, per bridge port flag and feature to deliver
multicast packets to any host on the according port via unicast
individually. This is done by copying the packet per host and
changing the multicast destination MAC to a unicast one accordingly.

multicast-to-unicast works on top of the multicast snooping feature of
the bridge. Which means unicast copies are only delivered to hosts which
are interested in it and signalized this via IGMP/MLD reports
previously.

This feature is intended for interface types which have a more reliable
and/or efficient way to deliver unicast packets than broadcast ones
(e.g. wifi).

However, it should only be enabled on interfaces where no IGMPv2/MLDv1
report suppression takes place. This feature is disabled by default.

The initial patch and idea is from Felix Fietkau.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
[linus.luessing@c0d3.blue: various bug + style fixes, commit message]
Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Reviewed-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 37 insertions, 2 deletions

diff --git a/net/bridge/br_forward.c b/net/bridge/br_forward.c
index 7cb41aee4c82..a0f9d0037d24 100644
--- a/net/bridge/br_forward.c
+++ b/net/bridge/br_forward.c
@@ -174,6 +174,31 @@ out:
 	return p;
 }
 
+static void maybe_deliver_addr(struct net_bridge_port *p, struct sk_buff *skb,
+			       const unsigned char *addr, bool local_orig)
+{
+	struct net_device *dev = BR_INPUT_SKB_CB(skb)->brdev;
+	const unsigned char *src = eth_hdr(skb)->h_source;
+
+	if (!should_deliver(p, skb))
+		return;
+
+	/* Even with hairpin, no soliloquies - prevent breaking IPv6 DAD */
+	if (skb->dev == p->dev && ether_addr_equal(src, addr))
+		return;
+
+	skb = skb_copy(skb, GFP_ATOMIC);
+	if (!skb) {
+		dev->stats.tx_dropped++;
+		return;
+	}
+
+	if (!is_broadcast_ether_addr(addr))
+		memcpy(eth_hdr(skb)->h_dest, addr, ETH_ALEN);
+
+	__br_forward(p, skb, local_orig);
+}
+
 /* called under rcu_read_lock */
 void br_flood(struct net_bridge *br, struct sk_buff *skb,
 	      enum br_pkt_type pkt_type, bool local_rcv, bool local_orig)
@@ -241,10 +266,20 @@ void br_multicast_flood(struct net_bridge_mdb_entry *mdst,
 		rport = rp ? hlist_entry(rp, struct net_bridge_port, rlist) :
 			     NULL;
 
-		port = (unsigned long)lport > (unsigned long)rport ?
-		       lport : rport;
+		if ((unsigned long)lport > (unsigned long)rport) {
+			port = lport;
+
+			if (port->flags & BR_MULTICAST_TO_UNICAST) {
+				maybe_deliver_addr(lport, skb, p->eth_addr,
+						   local_orig);
+				goto delivered;
+			}
+		} else {
+			port = rport;
+		}
 
 		prev = maybe_deliver(prev, port, skb, local_orig);
+delivered:
 		if (IS_ERR(prev))
 			goto out;
 		if (prev == port)