Add new sysctl, medium_id, to devinet.

It is used to differentiate the devices by the medium
they are attached to.  It is used to change proxy_arp behavior:
the proxy arp feature is enabled for packets forwarded between
two devices attached to different media.

Documentation/networking/ip-sysctl.txt

@@ -355,6 +355,17 @@ mc_forwarding - BOOLEAN
 	Do multicast routing. The kernel needs to be compiled with CONFIG_MROUTE
 	and a multicast routing daemon is required.
 
+medium_id - INTEGER
+	Integer value used to differentiate the devices by the medium they
+	are attached to. Two devices can have different id values when
+	the broadcast packets are received only on one of them.
+	The default value 0 means that the device is the only interface
+	to its medium, value of -1 means that medium is not known.
+	
+	Currently, it is used to change the proxy_arp behavior:
+	the proxy_arp feature is enabled for packets forwarded between
+	two devices attached to different media.
+
 proxy_arp - BOOLEAN
 	Do proxy arp.
 

include/linux/inetdevice.h

@@ -18,6 +18,7 @@ struct ipv4_devconf
 	int	mc_forwarding;
 	int	tag;
 	int     arp_filter;
+	int	medium_id;
 	void	*sysctl;
 };
 
@@ -48,6 +49,7 @@ struct in_device
 #define IN_DEV_TX_REDIRECTS(in_dev)	(ipv4_devconf.send_redirects || (in_dev)->cnf.send_redirects)
 #define IN_DEV_SEC_REDIRECTS(in_dev)	(ipv4_devconf.secure_redirects || (in_dev)->cnf.secure_redirects)
 #define IN_DEV_IDTAG(in_dev)		((in_dev)->cnf.tag)
+#define IN_DEV_MEDIUM_ID(in_dev)	((in_dev)->cnf.medium_id)
 
 #define IN_DEV_RX_REDIRECTS(in_dev) \
 	((IN_DEV_FORWARD(in_dev) && \

include/linux/sysctl.h

@@ -333,7 +333,8 @@ enum
 	NET_IPV4_CONF_BOOTP_RELAY=10,
 	NET_IPV4_CONF_LOG_MARTIANS=11,
 	NET_IPV4_CONF_TAG=12,
-	NET_IPV4_CONF_ARPFILTER=13
+	NET_IPV4_CONF_ARPFILTER=13,
+	NET_IPV4_CONF_MEDIUM_ID=14,
 };
 
 /* /proc/sys/net/ipv6 */

net/ipv4/arp.c

@@ -449,6 +449,32 @@ int arp_bind_neighbour(struct dst_entry *dst)
 	return 0;
 }
 
+/*
+ * Check if we can use proxy ARP for this path
+ */
+
+static inline int arp_fwd_proxy(struct in_device *in_dev, struct rtable *rt)
+{
+	struct in_device *out_dev;
+	int imi, omi = -1;
+
+	if (!IN_DEV_PROXY_ARP(in_dev))
+		return 0;
+
+	if ((imi = IN_DEV_MEDIUM_ID(in_dev)) == 0)
+		return 1;
+	if (imi == -1)
+		return 0;
+
+	/* place to check for proxy_arp for routes */
+
+	if ((out_dev = in_dev_get(rt->u.dst.dev)) != NULL) {
+		omi = IN_DEV_MEDIUM_ID(out_dev);
+		in_dev_put(out_dev);
+	}
+	return (omi != imi && omi != -1);
+}
+
 /*
  *	Interface to link layer: send routine and receive handler.
  */
@@ -755,7 +781,7 @@ int arp_process(struct sk_buff *skb)
 		} else if (IN_DEV_FORWARD(in_dev)) {
 			if ((rt->rt_flags&RTCF_DNAT) ||
 			    (addr_type == RTN_UNICAST  && rt->u.dst.dev != dev &&
-			     (IN_DEV_PROXY_ARP(in_dev) || pneigh_lookup(&arp_tbl, &tip, dev, 0)))) {
+			     (arp_fwd_proxy(in_dev, rt) || pneigh_lookup(&arp_tbl, &tip, dev, 0)))) {
 				n = neigh_event_ns(&arp_tbl, sha, &sip, dev);
 				if (n)
 					neigh_release(n);

net/ipv4/devinet.c

@@ -1032,7 +1032,7 @@ int devinet_sysctl_forward(ctl_table *ctl, int write, struct file * filp,
 static struct devinet_sysctl_table
 {
 	struct ctl_table_header *sysctl_header;
-	ctl_table devinet_vars[14];
+	ctl_table devinet_vars[15];
 	ctl_table devinet_dev[2];
 	ctl_table devinet_conf_dir[2];
 	ctl_table devinet_proto_dir[2];
@@ -1066,6 +1066,9 @@ static struct devinet_sysctl_table
 	{NET_IPV4_CONF_PROXY_ARP, "proxy_arp",
          &ipv4_devconf.proxy_arp, sizeof(int), 0644, NULL,
          &proc_dointvec},
+	{NET_IPV4_CONF_MEDIUM_ID, "medium_id",
+         &ipv4_devconf.medium_id, sizeof(int), 0644, NULL,
+         &proc_dointvec},
 	{NET_IPV4_CONF_BOOTP_RELAY, "bootp_relay",
          &ipv4_devconf.bootp_relay, sizeof(int), 0644, NULL,
          &proc_dointvec},