diff mbox

[RFC,2/2] batman-adv: Snoop DHCPACKs for DAT

Message ID 1467355678-32269-3-git-send-email-linus.luessing@c0d3.blue
State RFC, archived
Headers show

Commit Message

Linus L├╝ssing July 1, 2016, 6:47 a.m. UTC
In a typical mesh network, when a new client connects then it will
usually first try to grab an IPv4 address via DHCP. Afterwards in
public mesh networks a client will try to contact the internet over
the server.

While the IPv4 address of the DHCP-Server is usually well propagated
in the DHT, the IPv4 address a newly joining client is not.

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) flooded through the mesh. These 30KBit/s are mainly ARP
Requests from the gateways / DHCP servers.

Through snooping DHCPACKs we can actually learn about MAC/IP address
pairs without the need of any flooded ARP messages in advance. This
allows servers to fill their local DAT cache with according entries
before any communciation with a client can possibly have taken place.

TODOs:
* Actually only snoop DHCPACKs, not all BOOTPREPLY messages
  (e.g. ommit DHCPOFFERs, non-ACK'd OFFERs could cause trouble once
   reoffered)
  -> iterate over DHCP options, find & check option 53
* More sanity checks for the IP header
* Kerneldoc
* Test not only in VMs, but also check in a larger, public mesh
  network for the desired effect
---
 net/batman-adv/distributed-arp-table.c | 167 +++++++++++++++++++++++++++++++++
 net/batman-adv/distributed-arp-table.h |   4 +
 net/batman-adv/packet.h                |  38 ++++++++
 net/batman-adv/soft-interface.c        |  11 ++-
 4 files changed, 218 insertions(+), 2 deletions(-)
diff mbox

Patch

diff --git a/net/batman-adv/distributed-arp-table.c b/net/batman-adv/distributed-arp-table.c
index fa36196..1e3d28a 100644
--- a/net/batman-adv/distributed-arp-table.c
+++ b/net/batman-adv/distributed-arp-table.c
@@ -28,6 +28,7 @@ 
 #include <linux/if_ether.h>
 #include <linux/if_vlan.h>
 #include <linux/in.h>
+#include <linux/ip.h>
 #include <linux/jiffies.h>
 #include <linux/kernel.h>
 #include <linux/kref.h>
@@ -40,6 +41,7 @@ 
 #include <linux/spinlock.h>
 #include <linux/stddef.h>
 #include <linux/string.h>
+#include <linux/udp.h>
 #include <linux/workqueue.h>
 #include <net/arp.h>
 
@@ -959,6 +961,8 @@  batadv_dat_arp_create_reply(struct batadv_priv *bat_priv, __be32 ip_src,
 	if (!skb)
 		return NULL;
 
+	skb_set_network_header(skb, ETH_HLEN);
+
 	if (vid & BATADV_VLAN_HAS_TAG)
 		skb = vlan_insert_tag(skb, htons(ETH_P_8021Q),
 				      vid & VLAN_VID_MASK);
@@ -1222,6 +1226,169 @@  out:
 	return dropped;
 }
 
+static bool batadv_dat_check_dhcp_ipudp(struct sk_buff *skb, __be16 proto)
+{
+	struct iphdr *iphdr, _iphdr;
+	struct udphdr *udphdr, _udphdr;
+	unsigned int offset = skb_network_offset(skb);
+
+	if (proto != htons(ETH_P_IP))
+		return false;
+
+	iphdr = skb_header_pointer(skb, offset, sizeof(_iphdr), &_iphdr);
+	if (!iphdr || iphdr->protocol != IPPROTO_UDP)
+		return false;
+
+	/* TODO: Some more IP header sanity checks */
+
+	offset += sizeof(_iphdr);
+	skb_set_transport_header(skb, offset);
+
+	udphdr = skb_header_pointer(skb, offset, sizeof(_udphdr), &_udphdr);
+	if (!udphdr || udphdr->source != htons(67))
+		return false;
+
+	return true;
+}
+
+static int batadv_dat_check_dhcp(struct sk_buff *skb, __be16 proto)
+{
+	u8 *op, tmp_op;
+	u8 *htype, tmp_htype;
+	u8 *hlen, tmp_hlen;
+	unsigned int dhcp_offset;
+	unsigned int offset;
+
+	if (!batadv_dat_check_dhcp_ipudp(skb, proto))
+		return -EINVAL;
+
+	dhcp_offset = skb_transport_offset(skb) + sizeof(struct udphdr);
+	if (skb->len < dhcp_offset + sizeof(struct batadv_dhcp_packet))
+		return -EINVAL;
+
+	offset = dhcp_offset + offsetof(struct batadv_dhcp_packet, op);
+
+	op = skb_header_pointer(skb, offset, sizeof(tmp_op), &tmp_op);
+	if (!op)
+		return -EINVAL;
+
+	offset = dhcp_offset + offsetof(struct batadv_dhcp_packet, htype);
+
+	htype = skb_header_pointer(skb, offset, sizeof(tmp_htype), &tmp_htype);
+	if (!htype || *htype != BATADV_HTYPE_ETHERNET)
+		return -EINVAL;
+
+	offset = dhcp_offset + offsetof(struct batadv_dhcp_packet, hlen);
+
+	hlen = skb_header_pointer(skb, offset, sizeof(tmp_hlen), &tmp_hlen);
+	if (!hlen || *hlen != ETH_ALEN)
+		return -EINVAL;
+
+	return *op;
+}
+
+static int batadv_dat_get_dhcp_message_type(struct sk_buff *skb)
+{
+	/* TODO: Search for DHCP Option 53, return its value */
+	return BATADV_DHCPACK;
+}
+
+static __be32 *batadv_dat_dhcp_get_yiaddr(struct sk_buff *skb, __be32 *buffer,
+					  int buf_len)
+{
+	unsigned int offset = skb_transport_offset(skb) + sizeof(struct udphdr);
+
+	offset += offsetof(struct batadv_dhcp_packet, yiaddr);
+
+	return skb_header_pointer(skb, offset, buf_len, buffer);
+}
+
+static u8 *batadv_dat_get_dhcp_chaddr(struct sk_buff *skb, u8 *buffer,
+				      int buf_len)
+{
+	unsigned int offset = skb_transport_offset(skb) + sizeof(struct udphdr);
+
+	offset += offsetof(struct batadv_dhcp_packet, chaddr);
+
+	return skb_header_pointer(skb, offset, buf_len, buffer);
+}
+
+static bool batadv_dat_put(struct batadv_priv *bat_priv, u8 *hw_src,
+			   __be32 ip_src, u8 *hw_dst, __be32 ip_dst,
+			   unsigned short vid)
+{
+	struct sk_buff *skb;
+	int hdr_size;
+	u16 type;
+	int ret = false;
+
+	skb = batadv_dat_arp_create_reply(bat_priv, ip_src, ip_dst, hw_src,
+					  hw_dst, vid);
+	if (!skb)
+		return false;
+
+	/* Check for validity of provided addresses */
+	hdr_size = skb_network_offset(skb) - ETH_HLEN;
+	type = batadv_arp_get_type(bat_priv, skb, hdr_size);
+	if (type != ARPOP_REPLY)
+		goto err_skip_commit;
+
+	batadv_dat_entry_add(bat_priv, ip_src, hw_src, vid);
+	batadv_dat_entry_add(bat_priv, ip_dst, hw_dst, vid);
+
+	batadv_dat_send_data(bat_priv, skb, ip_src, vid, BATADV_P_DAT_DHT_PUT);
+	batadv_dat_send_data(bat_priv, skb, ip_dst, vid, BATADV_P_DAT_DHT_PUT);
+
+	ret = true;
+
+err_skip_commit:
+	dev_kfree_skb(skb);
+	return ret;
+}
+
+void batadv_dat_snoop_outgoing_dhcp_ack(struct batadv_priv *bat_priv,
+					struct sk_buff *skb,
+					__be16 proto,
+					unsigned short vid)
+{
+	int type;
+	u8 *chaddr, _chaddr[ETH_ALEN];
+	__be32 *yiaddr, _yiaddr;
+
+	if (!atomic_read(&bat_priv->distributed_arp_table))
+		return;
+
+	if (batadv_dat_check_dhcp(skb, proto) != BATADV_BOOTREPLY)
+		return;
+
+	type = batadv_dat_get_dhcp_message_type(skb);
+	if (type != BATADV_DHCPACK)
+		return;
+
+	yiaddr = batadv_dat_dhcp_get_yiaddr(skb, &_yiaddr, sizeof(_yiaddr));
+	if (!yiaddr)
+		return;
+
+	chaddr = batadv_dat_get_dhcp_chaddr(skb, _chaddr, sizeof(_chaddr));
+	if (!chaddr)
+		return;
+
+	/* ARP sender MAC + IP -> DHCP Client (chaddr+yiaddr),
+	 * ARP target MAC + IP -> DHCP Server (ethhdr/iphdr sources)
+	 */
+	if (!batadv_dat_put(bat_priv, chaddr, *yiaddr, eth_hdr(skb)->h_source,
+			    ip_hdr(skb)->saddr, vid))
+		return;
+
+	batadv_dbg(BATADV_DBG_DAT, bat_priv,
+		   "Snooped from DHCPACK (server-side): %pI4, %pM (vid: %i)\n",
+		   &ip_hdr(skb)->saddr, eth_hdr(skb)->h_source,
+		   BATADV_PRINT_VID(vid));
+	batadv_dbg(BATADV_DBG_DAT, bat_priv,
+		   "Snooped from DHCPACK (client-side): %pI4, %pM (vid: %i)\n",
+		   yiaddr, chaddr, BATADV_PRINT_VID(vid));
+}
+
 /**
  * batadv_dat_drop_broadcast_packet - check if an ARP request has to be dropped
  * (because the node has already obtained the reply via DAT) or not
diff --git a/net/batman-adv/distributed-arp-table.h b/net/batman-adv/distributed-arp-table.h
index 813ecea..c81234c 100644
--- a/net/batman-adv/distributed-arp-table.h
+++ b/net/batman-adv/distributed-arp-table.h
@@ -44,6 +44,10 @@  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);
 bool batadv_dat_drop_broadcast_packet(struct batadv_priv *bat_priv,
 				      struct batadv_forw_packet *forw_packet);
 
diff --git a/net/batman-adv/packet.h b/net/batman-adv/packet.h
index 6b011ff..1531a99 100644
--- a/net/batman-adv/packet.h
+++ b/net/batman-adv/packet.h
@@ -664,4 +664,42 @@  struct batadv_tvlv_mcast_data {
 	u8 reserved[3];
 };
 
+enum batadv_bootpop {
+	BATADV_BOOTREQUEST	= 1,
+	BATADV_BOOTREPLY	= 2,
+};
+
+enum batadv_boothtype {
+	BATADV_HTYPE_ETHERNET	= 1,
+};
+
+enum batadv_dhcptype {
+	BATADV_DHCPDISCOVER	= 1,
+	BATADV_DHCPOFFER	= 2,
+	BATADV_DHCPREQUEST	= 3,
+	BATADV_DHCPDECLINE	= 4,
+	BATADV_DHCPACK		= 5,
+	BATADV_DHCPNAK		= 6,
+	BATADV_DHCPRELEASE	= 7,
+	BATADV_DHCPINFORM	= 8,
+};
+
+struct batadv_dhcp_packet {
+	u8 op;
+	u8 htype;
+	u8 hlen;
+	u8 hops;
+	__be32 xid;
+	__be16 secs;
+	__be16 flags;
+	__be32 ciaddr;
+	__be32 yiaddr;
+	__be32 siaddr;
+	__be32 giaddr;
+	u8 chaddr[16];
+	u8 sname[64];
+	u8 file[128];
+	u8 options[0];
+};
+
 #endif /* _NET_BATMAN_ADV_PACKET_H_ */
diff --git a/net/batman-adv/soft-interface.c b/net/batman-adv/soft-interface.c
index 216ac03..12bc41b 100644
--- a/net/batman-adv/soft-interface.c
+++ b/net/batman-adv/soft-interface.c
@@ -204,6 +204,7 @@  static int 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;
@@ -212,12 +213,15 @@  static int 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;
 		}
@@ -244,6 +248,9 @@  static int 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 ...
 	 *