netfilter: nf_nat: Handle routing changes in MASQUERADE target

When the route changes (backup default route, VPNs) which affect a
masqueraded target, the packets were sent out with the outdated source
address. The patch addresses the issue by comparing the outgoing interface
directly with the masqueraded interface in the nat table.

Events are inefficient in this case, because it'd require adding route
events to the network core and then scanning the whole conntrack table
and re-checking the route for all entry.

Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

1 files changed, 4 insertions, 0 deletions

diff --git a/net/ipv4/netfilter/iptable_nat.c b/net/ipv4/netfilter/iptable_nat.c
index ac635a7b4416..da2c8a368f68 100644
--- a/net/ipv4/netfilter/iptable_nat.c
+++ b/net/ipv4/netfilter/iptable_nat.c
@@ -134,6 +134,10 @@ nf_nat_ipv4_fn(unsigned int hooknum,
                 /* ESTABLISHED */
                 NF_CT_ASSERT(ctinfo == IP_CT_ESTABLISHED ||
                              ctinfo == IP_CT_ESTABLISHED_REPLY);
+                if (nf_nat_oif_changed(hooknum, ctinfo, nat, out)) {
+                        nf_ct_kill_acct(ct, ctinfo, skb);
+                        return NF_DROP;
+                }
         }
 
         return nf_nat_packet(ct, ctinfo, hooknum, skb);