batman-adv: Snoop DHCPACKs for DAT

In a 1000 nodes mesh network (Freifunk Hamburg) we can still see
30KBit/s of ARP traffic (equalling about 25% of all layer two
specific overhead, remaining after some filtering) flooded through
the mesh. These 30KBit/s are mainly ARP Requests from the
gateways / DHCP servers.

By snooping DHCPACKs we can learn about MAC/IP address pairs
in the DHCP range without relying on ARP. This patch is in preparation
to eliminate the need for mesh wide message flooding for IPv4 address
resolution.

Also this allows to quickly update a MAC/IP pair at least in the DHT when
DHCP reassigns an IP address to a new host.
Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>

net/batman-adv/distributed-arp-table.h

@@ -46,6 +46,12 @@ void batadv_dat_snoop_outgoing_arp_reply(struct batadv_priv *bat_priv,
 					 struct sk_buff *skb);
 bool batadv_dat_snoop_incoming_arp_reply(struct batadv_priv *bat_priv,
 					 struct sk_buff *skb, int hdr_size);
+void batadv_dat_snoop_outgoing_dhcp_ack(struct batadv_priv *bat_priv,
+					struct sk_buff *skb,
+					__be16 proto,
+					unsigned short vid);
+void batadv_dat_snoop_incoming_dhcp_ack(struct batadv_priv *bat_priv,
+					struct sk_buff *skb, int hdr_size);
 bool batadv_dat_drop_broadcast_packet(struct batadv_priv *bat_priv,
 				      struct batadv_forw_packet *forw_packet);
 
@@ -140,6 +146,19 @@ batadv_dat_snoop_incoming_arp_reply(struct batadv_priv *bat_priv,
 	return false;
 }
 
+static inline void
+batadv_dat_snoop_outgoing_dhcp_ack(struct batadv_priv *bat_priv,
+				   struct sk_buff *skb, __be16 proto,
+				   unsigned short vid)
+{
+}
+
+static inline void
+batadv_dat_snoop_incoming_dhcp_ack(struct batadv_priv *bat_priv,
+				   struct sk_buff *skb, int hdr_size)
+{
+}
+
 static inline bool
 batadv_dat_drop_broadcast_packet(struct batadv_priv *bat_priv,
 				 struct batadv_forw_packet *forw_packet)

net/batman-adv/routing.c

@@ -1043,6 +1043,8 @@ int batadv_recv_unicast_packet(struct sk_buff *skb,
 							hdr_size))
 			goto rx_success;
 
+		batadv_dat_snoop_incoming_dhcp_ack(bat_priv, skb, hdr_size);
+
 		batadv_interface_rx(recv_if->soft_iface, skb, hdr_size,
 				    orig_node);
 
@@ -1278,6 +1280,8 @@ int batadv_recv_bcast_packet(struct sk_buff *skb,
 	if (batadv_dat_snoop_incoming_arp_reply(bat_priv, skb, hdr_size))
 		goto rx_success;
 
+	batadv_dat_snoop_incoming_dhcp_ack(bat_priv, skb, hdr_size);
+
 	/* broadcast for me */
 	batadv_interface_rx(recv_if->soft_iface, skb, hdr_size, orig_node);
 

net/batman-adv/soft-interface.c

@@ -212,6 +212,7 @@ static netdev_tx_t batadv_interface_tx(struct sk_buff *skb,
 	enum batadv_forw_mode forw_mode;
 	struct batadv_orig_node *mcast_single_orig = NULL;
 	int network_offset = ETH_HLEN;
+	__be16 proto;
 
 	if (atomic_read(&bat_priv->mesh_state) != BATADV_MESH_ACTIVE)
 		goto dropped;
@@ -223,12 +224,15 @@ static netdev_tx_t batadv_interface_tx(struct sk_buff *skb,
 	vid = batadv_get_vid(skb, 0);
 	ethhdr = eth_hdr(skb);
 
-	switch (ntohs(ethhdr->h_proto)) {
+	proto = ethhdr->h_proto;
+
+	switch (ntohs(proto)) {
 	case ETH_P_8021Q:
 		vhdr = vlan_eth_hdr(skb);
+		proto = vhdr->h_vlan_encapsulated_proto;
 
 		/* drop batman-in-batman packets to prevent loops */
-		if (vhdr->h_vlan_encapsulated_proto != htons(ETH_P_BATMAN)) {
+		if (proto != htons(ETH_P_BATMAN)) {
 			network_offset += VLAN_HLEN;
 			break;
 		}
@@ -256,6 +260,9 @@ static netdev_tx_t batadv_interface_tx(struct sk_buff *skb,
 			goto dropped;
 	}
 
+	/* Snoop address candidates from DHCPACKs for early DAT filling */
+	batadv_dat_snoop_outgoing_dhcp_ack(bat_priv, skb, proto, vid);
+
 	/* don't accept stp packets. STP does not help in meshes.
 	 * better use the bridge loop avoidance ...
 	 *