[IPV4]: Document icmp_errors_use_inbound_ifaddr sysctl

Taken largely from the commit of the patch that added this feature:

1c2fb7f93cb20621772bf304f3dba0849942e5db

I'm not sure about the ordering of the options in sysctl.txt,
so I took a wild guess about where it fits.
    
Signed-Off-By: Horms <horms@verge.net.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 17 insertions, 0 deletions

diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index 2b7cf19a06ad..26364d06ae92 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -427,6 +427,23 @@ icmp_ignore_bogus_error_responses - BOOLEAN
         will avoid log file clutter.
         Default: FALSE
 
+icmp_errors_use_inbound_ifaddr - BOOLEAN
+
+        If zero, icmp error messages are sent with the primary address of
+        the exiting interface.
+ 
+        If non-zero, the message will be sent with the primary address of
+        the interface that received the packet that caused the icmp error.
+        This is the behaviour network many administrators will expect from
+        a router. And it can make debugging complicated network layouts
+        much easier. 
+
+        Note that if no primary address exists for the interface selected,
+        then the primary address of the first non-loopback interface that
+        has one will be used regarldess of this setting.
+
+        Default: 0
+
 igmp_max_memberships - INTEGER
         Change the maximum number of multicast groups we can subscribe to.
         Default: 20