ipv4/icmp: redirect messages can use the ingress daddr as source

This patch allows configuring how the source address of ICMP
redirect messages is selected; by default the old behaviour is
retained, while setting icmp_redirects_use_orig_daddr force the
usage of the destination address of the packet that caused the
redirect.

The new behaviour fits closely the RFC 5798 section 8.1.1, and fix the
following scenario:

Two machines are set up with VRRP to act as routers out of a subnet,
they have IPs x.x.x.1/24 and x.x.x.2/24, with VRRP holding on to
x.x.x.254/24.

If a host in said subnet needs to get an ICMP redirect from the VRRP
router, i.e. to reach a destination behind a different gateway, the
source IP in the ICMP redirect is chosen as the primary IP on the
interface that the packet arrived at, i.e. x.x.x.1 or x.x.x.2.

The host will then ignore said redirect, due to RFC 1122 section 3.2.2.2,
and will continue to use the wrong next-op.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 8 insertions, 1 deletions

diff --git a/net/ipv4/icmp.c b/net/ipv4/icmp.c
index 36e26977c908..f3c356b7c1f0 100644
--- a/net/ipv4/icmp.c
+++ b/net/ipv4/icmp.c
@@ -659,7 +659,9 @@ void icmp_send(struct sk_buff *skb_in, int type, int code, __be32 info)
          */
 
         saddr = iph->daddr;
-        if (!(rt->rt_flags & RTCF_LOCAL)) {
+        if (!((type == ICMP_REDIRECT) &&
+              net->ipv4.sysctl_icmp_redirects_use_orig_daddr) &&
+            !(rt->rt_flags & RTCF_LOCAL)) {
                 struct net_device *dev = NULL;
 
                 rcu_read_lock();
@@ -1222,6 +1224,11 @@ static int __net_init icmp_sk_init(struct net *net)
         net->ipv4.sysctl_icmp_ratemask = 0x1818;
         net->ipv4.sysctl_icmp_errors_use_inbound_ifaddr = 0;
 
+        /* Control paramerer - use the daddr of originating packets as saddr
+         * in redirect messages?
+         */
+        net->ipv4.sysctl_icmp_redirects_use_orig_daddr = 0;
+
         return 0;
 
 fail: