[PATCH net-next v2 7/8] selftests: traceroute: Test traceroute with different source IPs

Ido Schimmel
2025-09-08T07:35:25Z

When generating ICMP error messages, the kernel will prefer a source IP
that is on the same subnet as the destination IP (see
inet_select_addr()). Test this behavior by invoking traceroute with
different source IPs and checking that the ICMP error message is
generated with a source IP in the same subnet.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
---
tools/testing/selftests/net/traceroute.sh | 13 +++++++++----
1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/net/traceroute.sh b/tools/testing/selftests/net/traceroute.sh
index 8dc4e5d03e43..0ab9eccf1499 100755
--- a/tools/testing/selftests/net/traceroute.sh
+++ b/tools/testing/selftests/net/traceroute.sh
@@ -196,9 +196,10 @@ run_traceroute6()
################################################################################
# traceroute test
#
-# Verify that traceroute from H1 to H2 shows 1.0.1.1 in this scenario
+# Verify that traceroute from H1 to H2 shows 1.0.3.1 and 1.0.1.1 when
+# traceroute uses 1.0.3.3 and 1.0.1.3 as the source IP, respectively.
#
-# 1.0.3.1/24
+# 1.0.3.3/24 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 ----
@@ -226,6 +227,7 @@ setup_traceroute()

connect_ns $h1 eth0 1.0.1.3/24 - \
$router eth1 1.0.3.1/24 -
+ ip -n "$h1" addr add 1.0.3.3/24 dev eth0
ip netns exec $h1 ip route add default via 1.0.1.1

ip netns exec $router ip addr add 1.0.1.1/24 dev eth1
@@ -248,9 +250,12 @@ run_traceroute()

RET=0

- # traceroute host-2 from host-1 (expects 1.0.1.1). Takes a while.
- run_cmd $h1 "traceroute 1.0.2.4 | grep -q 1.0.1.1"
+ # traceroute host-2 from host-1. Expect a source IP that is on the same
+ # subnet as destination IP of the ICMP error message.
+ run_cmd "$h1" "traceroute -s 1.0.1.3 1.0.2.4 | grep -q 1.0.1.1"
check_err $? "traceroute did not return 1.0.1.1"
+ run_cmd "$h1" "traceroute -s 1.0.3.3 1.0.2.4 | grep -q 1.0.3.1"
+ check_err $? "traceroute did not return 1.0.3.1"
log_test "IPv4 traceroute"

cleanup_traceroute
--
2.51.0