[NETFILTER]: ebtables: add --snap-arp option

The attached patch adds --snat-arp support, which makes it possible to
change the source mac address in both the mac header and the arp header
with one rule.
Signed-off-by: Bart De Schuymer <bdschuym@pandora.be>
Signed-off-by: Patrick McHardy <kaber@trash.net>

include/linux/netfilter_bridge/ebt_nat.h

 #ifndef __LINUX_BRIDGE_EBT_NAT_H
 #define __LINUX_BRIDGE_EBT_NAT_H
 
+#define NAT_ARP_BIT  (0x00000010)
 struct ebt_nat_info
 {
 	unsigned char mac[ETH_ALEN];

include/linux/netfilter_bridge/ebtables.h

@@ -26,6 +26,10 @@
 #define EBT_CONTINUE -3
 #define EBT_RETURN   -4
 #define NUM_STANDARD_TARGETS   4
+/* ebtables target modules store the verdict inside an int. We can
+ * reclaim a part of this int for backwards compatible extensions.
+ * The 4 lsb are more than enough to store the verdict. */
+#define EBT_VERDICT_BITS 0x0000000F
 
 struct ebt_counter
 {

net/bridge/netfilter/ebt_mark.c

@@ -33,7 +33,7 @@ static int ebt_target_mark(struct sk_buff **pskb, unsigned int hooknr,
 	else
 		(*pskb)->mark ^= info->mark;
 
-	return info->target | -16;
+	return info->target | ~EBT_VERDICT_BITS;
 }
 
 static int ebt_target_mark_check(const char *tablename, unsigned int hookmask,
@@ -44,13 +44,13 @@ static int ebt_target_mark_check(const char *tablename, unsigned int hookmask,
 
 	if (datalen != EBT_ALIGN(sizeof(struct ebt_mark_t_info)))
 		return -EINVAL;
-	tmp = info->target | -16;
+	tmp = info->target | ~EBT_VERDICT_BITS;
 	if (BASE_CHAIN && tmp == EBT_RETURN)
 		return -EINVAL;
 	CLEAR_BASE_CHAIN_BIT;
 	if (tmp < -NUM_STANDARD_TARGETS || tmp >= 0)
 		return -EINVAL;
-	tmp = info->target & -16;
+	tmp = info->target & ~EBT_VERDICT_BITS;
 	if (tmp != MARK_SET_VALUE && tmp != MARK_OR_VALUE &&
 	    tmp != MARK_AND_VALUE && tmp != MARK_XOR_VALUE)
 		return -EINVAL;

net/bridge/netfilter/ebt_snat.c

@@ -12,6 +12,8 @@
 #include <linux/netfilter_bridge/ebt_nat.h>
 #include <linux/module.h>
 #include <net/sock.h>
+#include <linux/if_arp.h>
+#include <net/arp.h>
 
 static int ebt_target_snat(struct sk_buff **pskb, unsigned int hooknr,
    const struct net_device *in, const struct net_device *out,
@@ -31,24 +33,43 @@ static int ebt_target_snat(struct sk_buff **pskb, unsigned int hooknr,
 		*pskb = nskb;
 	}
 	memcpy(eth_hdr(*pskb)->h_source, info->mac, ETH_ALEN);
-	return info->target;
+	if (!(info->target & NAT_ARP_BIT) &&
+	    eth_hdr(*pskb)->h_proto == htons(ETH_P_ARP)) {
+		struct arphdr _ah, *ap;
+
+		ap = skb_header_pointer(*pskb, 0, sizeof(_ah), &_ah);
+		if (ap == NULL)
+			return EBT_DROP;
+		if (ap->ar_hln != ETH_ALEN)
+			goto out;
+		if (skb_store_bits(*pskb, sizeof(_ah), info->mac,ETH_ALEN))
+			return EBT_DROP;
+	}
+out:
+	return info->target | ~EBT_VERDICT_BITS;
 }
 
 static int ebt_target_snat_check(const char *tablename, unsigned int hookmask,
    const struct ebt_entry *e, void *data, unsigned int datalen)
 {
 	struct ebt_nat_info *info = (struct ebt_nat_info *) data;
+	int tmp;
 
 	if (datalen != EBT_ALIGN(sizeof(struct ebt_nat_info)))
 		return -EINVAL;
-	if (BASE_CHAIN && info->target == EBT_RETURN)
+	tmp = info->target | ~EBT_VERDICT_BITS;
+	if (BASE_CHAIN && tmp == EBT_RETURN)
 		return -EINVAL;
 	CLEAR_BASE_CHAIN_BIT;
 	if (strcmp(tablename, "nat"))
 		return -EINVAL;
 	if (hookmask & ~(1 << NF_BR_POST_ROUTING))
 		return -EINVAL;
-	if (INVALID_TARGET)
+
+	if (tmp < -NUM_STANDARD_TARGETS || tmp >= 0)
+		return -EINVAL;
+	tmp = info->target | EBT_VERDICT_BITS;
+	if ((tmp & ~NAT_ARP_BIT) != ~NAT_ARP_BIT)
 		return -EINVAL;
 	return 0;
 }