[IPV4]: Sysctl configurable icmp error source address.

This patch alows you to change the source address of icmp error
messages. It applies cleanly to 2.6.11.11 and retains the default
behaviour.

In the old (default) behaviour icmp error messages are sent with the ip
of the exiting interface.

The new behaviour (when the sysctl variable is toggled on), it will send
the message with the ip of the interface that received the packet that
caused the icmp error. This is the behaviour network administrators will
expect from a router. It makes debugging complicated network layouts
much easier. Also, all 'vendor routers' I know of have the later
behaviour.
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/sysctl.h

@@ -346,6 +346,7 @@ enum
 	NET_TCP_MODERATE_RCVBUF=106,
 	NET_TCP_TSO_WIN_DIVISOR=107,
 	NET_TCP_BIC_BETA=108,
+	NET_IPV4_ICMP_ERRORS_USE_INBOUND_IFADDR=109,
 };
 
 enum {

net/ipv4/icmp.c

@@ -207,6 +207,7 @@ int sysctl_icmp_ignore_bogus_error_responses;
 
 int sysctl_icmp_ratelimit = 1 * HZ;
 int sysctl_icmp_ratemask = 0x1818;
+int sysctl_icmp_errors_use_inbound_ifaddr;
 
 /*
  *	ICMP control array. This specifies what to do with each ICMP.
@@ -511,8 +512,12 @@ void icmp_send(struct sk_buff *skb_in, int type, int code, u32 info)
 	 */
 
 	saddr = iph->daddr;
-	if (!(rt->rt_flags & RTCF_LOCAL))
+	if (!(rt->rt_flags & RTCF_LOCAL)) {
+		if (sysctl_icmp_errors_use_inbound_ifaddr)
+			saddr = inet_select_addr(skb_in->dev, 0, RT_SCOPE_LINK);
+		else
 			saddr = 0;
+	}
 
 	tos = icmp_pointers[type].error ? ((iph->tos & IPTOS_TOS_MASK) |
 					   IPTOS_PREC_INTERNETCONTROL) :

net/ipv4/sysctl_net_ipv4.c

@@ -23,6 +23,7 @@ extern int sysctl_ip_nonlocal_bind;
 extern int sysctl_icmp_echo_ignore_all;
 extern int sysctl_icmp_echo_ignore_broadcasts;
 extern int sysctl_icmp_ignore_bogus_error_responses;
+extern int sysctl_icmp_errors_use_inbound_ifaddr;
 
 /* From ip_fragment.c */
 extern int sysctl_ipfrag_low_thresh;
@@ -395,6 +396,14 @@ ctl_table ipv4_table[] = {
 		.mode		= 0644,
 		.proc_handler	= &proc_dointvec
 	},
+	{
+		.ctl_name	= NET_IPV4_ICMP_ERRORS_USE_INBOUND_IFADDR,
+		.procname	= "icmp_errors_use_inbound_ifaddr",
+		.data		= &sysctl_icmp_errors_use_inbound_ifaddr,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= &proc_dointvec
+	},
 	{
 		.ctl_name	= NET_IPV4_ROUTE,
 		.procname	= "route",