diff -ur v2.5.73/linux/Documentation/filesystems/proc.txt linux/Documentation/filesystems/proc.txt
--- v2.5.73/linux/Documentation/filesystems/proc.txt	Sun Jun 15 10:23:55 2003
+++ linux/Documentation/filesystems/proc.txt	Tue Jul  1 23:29:02 2003
@@ -1502,6 +1502,15 @@
 
 Log packets with source addresses with no known route to kernel log.
 
+loop 
+----
+
+By default (loop=0) the traffic between local IP addresses
+is routed via interface "lo". Setting this flag for two
+interfaces allows traffic between their IP addresses to
+be looped externally. This is useful for setups where the
+interfaces are attached to same broadcast medium.
+
 mc_forwarding
 -------------
 
diff -ur v2.5.73/linux/Documentation/networking/ip-sysctl.txt linux/Documentation/networking/ip-sysctl.txt
--- v2.5.73/linux/Documentation/networking/ip-sysctl.txt	Sun Jun 15 10:23:56 2003
+++ linux/Documentation/networking/ip-sysctl.txt	Tue Jul  1 23:27:02 2003
@@ -390,6 +390,13 @@
 forwarding - BOOLEAN
 	Enable IP forwarding on this interface.
 
+loop - BOOLEAN
+	By default (loop=0) the traffic between local IP addresses
+	is routed via interface "lo". Setting this flag for two
+	interfaces allows traffic between their IP addresses to
+	be looped externally. This is useful for setups where the
+	interfaces are attached to same broadcast medium.
+
 mc_forwarding - BOOLEAN
 	Do multicast routing. The kernel needs to be compiled with CONFIG_MROUTE
 	and a multicast routing daemon is required.
diff -ur v2.5.73/linux/include/linux/inetdevice.h linux/include/linux/inetdevice.h
--- v2.5.73/linux/include/linux/inetdevice.h	Sun Apr 20 11:02:16 2003
+++ linux/include/linux/inetdevice.h	Mon Jun 30 23:46:20 2003
@@ -21,6 +21,7 @@
 	int	medium_id;
 	int	no_xfrm;
 	int	no_policy;
+	int	loop;
 	void	*sysctl;
 };
 
@@ -70,6 +71,7 @@
 	  (ipv4_devconf.accept_redirects || (in_dev)->cnf.accept_redirects)))
 
 #define IN_DEV_ARPFILTER(in_dev)	(ipv4_devconf.arp_filter || (in_dev)->cnf.arp_filter)
+#define IN_DEV_LOOP(in_dev)		((in_dev)->cnf.loop)
 
 struct in_ifaddr
 {
diff -ur v2.5.73/linux/include/linux/sysctl.h linux/include/linux/sysctl.h
--- v2.5.73/linux/include/linux/sysctl.h	Sun Jun 15 10:25:03 2003
+++ linux/include/linux/sysctl.h	Mon Jun 30 23:47:14 2003
@@ -359,6 +359,7 @@
 	NET_IPV4_CONF_MEDIUM_ID=14,
 	NET_IPV4_CONF_NOXFRM=15,
 	NET_IPV4_CONF_NOPOLICY=16,
+	NET_IPV4_CONF_LOOP=17,
 };
 
 /* /proc/sys/net/ipv6 */
diff -ur v2.5.73/linux/net/ipv4/devinet.c linux/net/ipv4/devinet.c
--- v2.5.73/linux/net/ipv4/devinet.c	Sun Jun 15 10:25:07 2003
+++ linux/net/ipv4/devinet.c	Mon Jun 30 23:49:26 2003
@@ -1122,7 +1122,7 @@
 
 static struct devinet_sysctl_table {
 	struct ctl_table_header *sysctl_header;
-	ctl_table		devinet_vars[17];
+	ctl_table		devinet_vars[18];
 	ctl_table		devinet_dev[2];
 	ctl_table		devinet_conf_dir[2];
 	ctl_table		devinet_proto_dir[2];
@@ -1258,6 +1258,14 @@
 			.mode		= 0644,
 			.proc_handler	= &ipv4_doint_and_flush,
 			.strategy	= &ipv4_doint_and_flush_strategy,
+		},
+		{
+			.ctl_name	= NET_IPV4_CONF_LOOP,
+			.procname	= "loop",
+			.data		= &ipv4_devconf.loop,
+			.maxlen		= sizeof(int),
+			.mode		= 0644,
+			.proc_handler	= &proc_dointvec,
 		},
 	},
 	.devinet_dev = {
diff -ur v2.5.73/linux/net/ipv4/fib_frontend.c linux/net/ipv4/fib_frontend.c
--- v2.5.73/linux/net/ipv4/fib_frontend.c	Wed May 28 00:02:50 2003
+++ linux/net/ipv4/fib_frontend.c	Mon Jun 30 23:52:38 2003
@@ -167,15 +167,16 @@
 					.tos = tos } },
 			    .iif = oif };
 	struct fib_result res;
-	int no_addr, rpf;
+	int no_addr, rpf, loop;
 	int ret;
 
-	no_addr = rpf = 0;
+	no_addr = rpf = loop = 0;
 	read_lock(&inetdev_lock);
 	in_dev = __in_dev_get(dev);
 	if (in_dev) {
 		no_addr = in_dev->ifa_list == NULL;
 		rpf = IN_DEV_RPFILTER(in_dev);
+		loop = IN_DEV_LOOP(in_dev);
 	}
 	read_unlock(&inetdev_lock);
 
@@ -184,6 +185,11 @@
 
 	if (fib_lookup(&fl, &res))
 		goto last_resort;
+	if (loop && res.type == RTN_LOCAL) {
+		*spec_dst = FIB_RES_PREFSRC(res);
+		fib_res_put(&res);
+		return 0;
+	}
 	if (res.type != RTN_UNICAST)
 		goto e_inval_res;
 	*spec_dst = FIB_RES_PREFSRC(res);
diff -ur v2.5.73/linux/net/ipv4/route.c linux/net/ipv4/route.c
--- v2.5.73/linux/net/ipv4/route.c	Tue Jun 17 21:39:39 2003
+++ linux/net/ipv4/route.c	Tue Jul  1 00:29:05 2003
@@ -1967,6 +1967,11 @@
 			dev_put(dev_out);
 			goto out;	/* Wrong error code */
 		}
+		err = -ENETDOWN;
+		if (!(dev_out->flags&IFF_UP)) {
+			dev_put(dev_out);
+			goto out;
+		}
 
 		if (LOCAL_MCAST(oldflp->fl4_dst) || oldflp->fl4_dst == 0xFFFFFFFF) {
 			if (!fl.fl4_src)
@@ -2036,10 +2041,41 @@
 		goto e_inval;
 
 	if (res.type == RTN_LOCAL) {
-		if (!fl.fl4_src)
-			fl.fl4_src = fl.fl4_dst;
+		struct in_device *in_dev;
+		u32 src;
+
 		if (dev_out)
 			dev_put(dev_out);
+		dev_out = FIB_RES_DEV(res);
+		in_dev = in_dev_get(dev_out);
+		src = fl.fl4_src? : FIB_RES_PREFSRC(res);
+		if (in_dev && IN_DEV_LOOP(in_dev) && src) {
+			struct net_device *dev_src;
+
+			in_dev_put(in_dev);
+			in_dev = NULL;
+			dev_src = ip_dev_find(src);
+			if (dev_src && dev_src != dev_out &&
+			    (in_dev = in_dev_get(dev_src)) &&
+			    IN_DEV_LOOP(in_dev)) {
+				in_dev_put(in_dev);
+				dev_out = dev_src;
+				fl.fl4_src = src;
+				fl.oif = dev_out->ifindex;
+				res.type = RTN_UNICAST;
+				if (res.fi) {
+					fib_info_put(res.fi);
+					res.fi = NULL;
+				}
+				goto make_route;
+			}
+			if (dev_src)
+				dev_put(dev_src);
+		}
+		if (in_dev)
+			in_dev_put(in_dev);
+		if (!fl.fl4_src)
+			fl.fl4_src = fl.fl4_dst;
 		dev_out = &loopback_dev;
 		dev_hold(dev_out);
 		fl.oif = dev_out->ifindex;
