netdev - [PATCH net-next] net: icmp: use input address in traceroute

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Message-Id: <20191101004414.2B2F995C102D@us180.sjc.aristanetworks.com>
Date:   Thu, 31 Oct 2019 17:44:13 -0700
From:   fruggeri@...sta.com (Francesco Ruggeri)
To:     kuznet@....inr.ac.ru, yoshfuji@...ux-ipv6.org, davem@...emloft.net,
        netdev@...r.kernel.org
Cc:     fruggeri@...sta.com
Subject: [PATCH net-next] net: icmp: use input address in traceroute

Even with icmp_errors_use_inbound_ifaddr set, traceroute returns the
primary address of the interface the packet was received on, even if
the path goes through a secondary address. In the example:

                    1.0.3.1/24
 ---- 1.0.1.3/24    1.0.1.1/24 ---- 1.0.2.1/24    1.0.2.4/24 ----
 |H1|--------------------------|R1|--------------------------|H2|
 ----            N1            ----            N2            ----

where 1.0.3.1/24 is R1's primary address on N1, traceroute from
H1 to H2 returns:

traceroute to 1.0.2.4 (1.0.2.4), 30 hops max, 60 byte packets
 1  1.0.3.1 (1.0.3.1)  0.018 ms  0.006 ms  0.006 ms
 2  1.0.2.4 (1.0.2.4)  0.021 ms  0.007 ms  0.007 ms

After applying this patch, it returns:

traceroute to 1.0.2.4 (1.0.2.4), 30 hops max, 60 byte packets
 1  1.0.1.1 (1.0.1.1)  0.033 ms  0.007 ms  0.006 ms
 2  1.0.2.4 (1.0.2.4)  0.011 ms  0.007 ms  0.007 ms

Original-patch-by: Bill Fenner <fenner@...sta.com>
Signed-off-by: Francesco Ruggeri <fruggeri@...sta.com>

---
 net/ipv4/icmp.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/icmp.c b/net/ipv4/icmp.c
index 4298aae74e0e..a72fbdf1fb85 100644
--- a/net/ipv4/icmp.c
+++ b/net/ipv4/icmp.c
@@ -682,7 +682,8 @@ void __icmp_send(struct sk_buff *skb_in, int type, int code, __be32 info,
 			dev = dev_get_by_index_rcu(net, inet_iif(skb_in));
 
 		if (dev)
-			saddr = inet_select_addr(dev, 0, RT_SCOPE_LINK);
+			saddr = inet_select_addr(dev, iph->saddr,
+						 RT_SCOPE_LINK);
 		else
 			saddr = 0;
 		rcu_read_unlock();
-- 
2.19.1