[PATCH net v1 2/2] selftest: net: Add test for TCP flow failover with ECMP routes.

[Sagarika Sharma <sharmasagarika@google.com>]

From: Kuniyuki Iwashima <kuniyu@google.com>

Without the previous commit, TCP failed to switch to alternative
IPv6 routes immediately upon carrier loss.

It would persist with the dead route until reaching the threshold
net.ipv4.tcp_retries1, leading to unnecessary delays in failover.

Let's add a selftest for this scenario to ensure TCP fails over
immediately upon a carrier loss event.

Before:
  TEST: TCP IPv4 failover                                             [ OK ]
  TEST: TCP IPv6 failover                                             [FAIL]

After:
  TEST: TCP IPv4 failover                                             [ OK ]
  TEST: TCP IPv6 failover                                             [ OK ]

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Signed-off-by: Sagarika Sharma <sharmasagarika@google.com>
---
 tools/testing/selftests/net/Makefile          |   1 +
 .../selftests/net/tcp_ecmp_failover.sh        | 209 ++++++++++++++++++
 2 files changed, 210 insertions(+)
 create mode 100755 tools/testing/selftests/net/tcp_ecmp_failover.sh

diff --git a/tools/testing/selftests/net/Makefile b/tools/testing/selftests/net/Makefile
index a275ed584026..f3da38c54d27 100644
--- a/tools/testing/selftests/net/Makefile
+++ b/tools/testing/selftests/net/Makefile
@@ -96,6 +96,7 @@ TEST_PROGS := \
 	srv6_hl2encap_red_l2vpn_test.sh \
 	srv6_iptunnel_cache.sh \
 	stress_reuseport_listen.sh \
+	tcp_ecmp_failover.sh \
 	tcp_fastopen_backup_key.sh \
 	test_bpf.sh \
 	test_bridge_backup_port.sh \
diff --git a/tools/testing/selftests/net/tcp_ecmp_failover.sh b/tools/testing/selftests/net/tcp_ecmp_failover.sh
new file mode 100755
index 000000000000..f857d5db84d8
--- /dev/null
+++ b/tools/testing/selftests/net/tcp_ecmp_failover.sh
@@ -0,0 +1,209 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+#
+# Copyright 2026 Google LLC.
+#
+# This test verifies TCP flow failover between ECMP routes
+# upon carrier loss on the active device.
+#
+#   socat  ----------------------------->  socat
+#                        |
+#           .-- veth-c1 -|- veth-s1 --.
+#   dummy0 -|            |            |-- dummy0
+#           '-- veth-c2 -|- veth-s2 --'
+#                        |
+#
+
+REQUIRE_JQ=no
+REQUIRE_MZ=no
+NUM_NETIFS=0
+
+source forwarding/lib.sh
+
+CLIENT_IP="10.0.59.1"
+SERVER_IP="10.0.92.1"
+CLIENT_IP6="2001:db8:5a9a::1"
+SERVER_IP6="2001:db8:9292::1"
+
+setup_server()
+{
+	IP="ip -n $server"
+	NS_EXEC="ip netns exec $server"
+
+	$IP link add dummy0 type dummy
+	$IP link set dummy0 up
+
+	$IP -4 addr add $SERVER_IP/32 dev dummy0
+	$IP -6 addr add $SERVER_IP6/128 dev dummy0 nodad
+
+	$IP link set veth-s1 up
+	$IP link set veth-s2 up
+
+	$IP -4 addr add 192.168.1.2/24 dev veth-s1
+	$IP -4 addr add 192.168.2.2/24 dev veth-s2
+
+	$IP -4 route add $CLIENT_IP/32 \
+		nexthop via 192.168.1.1 dev veth-s1 weight 1 \
+		nexthop via 192.168.2.1 dev veth-s2 weight 1
+
+	$IP -6 addr add 2001:db8:1::2/64 dev veth-s1 nodad
+	$IP -6 addr add 2001:db8:2::2/64 dev veth-s2 nodad
+
+	$IP -6 route add $CLIENT_IP6/128 \
+		nexthop via 2001:db8:1::1 dev veth-s1 weight 1 \
+		nexthop via 2001:db8:2::1 dev veth-s2 weight 1
+}
+
+setup_client()
+{
+	IP="ip -n $client"
+	NS_EXEC="ip netns exec $client"
+
+	$IP link add dummy0 type dummy
+	$IP link set dummy0 up
+
+	$IP -4 addr add $CLIENT_IP/32 dev dummy0
+	$IP -6 addr add $CLIENT_IP6/128 dev dummy0 nodad
+
+	$IP link set veth-c1 up
+	$IP link set veth-c2 up
+
+	$IP -4 addr add 192.168.1.1/24 dev veth-c1
+	$IP -4 addr add 192.168.2.1/24 dev veth-c2
+
+	$IP -4 route add $SERVER_IP/32 \
+		nexthop via 192.168.1.2 dev veth-c1 weight 1 \
+		nexthop via 192.168.2.2 dev veth-c2 weight 1
+
+	$IP -6 addr add 2001:db8:1::1/64 dev veth-c1 nodad
+	$IP -6 addr add 2001:db8:2::1/64 dev veth-c2 nodad
+
+	$IP -6 route add $SERVER_IP6/128 \
+		nexthop via 2001:db8:1::2 dev veth-c1 weight 1 \
+		nexthop via 2001:db8:2::2 dev veth-c2 weight 1
+
+	# By default, tcp_retries1=3 triggers a route refresh
+	# after 3 retransmits (~5s).  Ensure this never occurs
+	# for test stability.
+	$NS_EXEC sysctl -qw net.ipv4.tcp_retries1=100
+
+	# When NETDEV_CHANGE is issued for a dev tied to an ECMP
+	# route, RTNH_F_LINKDOWN is flagged and the sernum is
+	# bumped to invalidate the route via sk_dst_check().
+	#
+	# Without ignore_routes_with_linkdown=1, subsequent
+	# lookups may still select the same RTNH_F_LINKDOWN route.
+	$NS_EXEC sysctl -qw net.ipv4.conf.veth-c1.ignore_routes_with_linkdown=1
+	$NS_EXEC sysctl -qw net.ipv4.conf.veth-c2.ignore_routes_with_linkdown=1
+
+	$NS_EXEC sysctl -qw net.ipv6.conf.veth-c1.ignore_routes_with_linkdown=1
+	$NS_EXEC sysctl -qw net.ipv6.conf.veth-c2.ignore_routes_with_linkdown=1
+}
+
+setup()
+{
+	setup_ns client server
+
+	ip -n $client link add veth-c1 type veth peer veth-s1 netns $server
+	ip -n $client link add veth-c2 type veth peer veth-s2 netns $server
+
+	setup_server
+	setup_client
+}
+
+cleanup()
+{
+	cleanup_all_ns
+}
+
+tcp_ecmp_failover()
+{
+	local pf=$1; shift
+	local server_ip=$1; shift
+	local client_ip=$1; shift
+
+	RET=0
+
+	tcpdump_start veth-s1 $server
+	tcpdump_start veth-s2 $server
+
+	ip netns exec $server \
+		socat -u TCP-LISTEN:8080,pf=$pf,bind=$server_ip,reuseaddr /dev/null &
+	server_pid=$!
+
+	# Wait for server to start listening.
+	# Sometimes client fails without this sleep.
+	sleep 1
+
+	ip netns exec $client \
+		socat -u /dev/zero TCP:$server_ip:8080,pf=$pf,bind=$client_ip &
+	client_pid=$!
+
+	# To capture enough packets.
+	sleep 3
+
+	tcpdump_stop veth-s1
+	tcpdump_stop veth-s2
+
+	pkts_s1=$(tcpdump_show veth-s1 | wc -l)
+	pkts_s2=$(tcpdump_show veth-s2 | wc -l)
+
+	tcpdump_cleanup veth-s1
+	tcpdump_cleanup veth-s2
+
+	# Detect the device chosen by the client
+	if [ $pkts_s1 -gt $pkts_s2 ]; then
+		veth_down=veth-s1
+		veth_up=veth-s2
+	else
+		veth_down=veth-s2
+		veth_up=veth-s1
+	fi
+
+	# Taking down $veth_down causes its peer to lose carrier,
+	# triggering NETDEV_CHANGE.  This flags RTNH_F_LINKDOWN
+	# and bumps the sernum for the route associated with that
+	# peer, invalidating the cached dst in the TCP socket.
+	#
+	# Consequently, sk_dst_check() fails, forcing the subsequent
+	# lookup to select the remaining healthy route via $veth_up.
+	ip -n $server link set $veth_down down
+
+	tcpdump_start $veth_up $server
+
+	# To capture enough packets.
+	sleep  3
+
+	tcpdump_stop $veth_up
+
+	kill -9 $client_pid 2>&1 > /dev/null
+	kill -9 $server_pid 2>&1 > /dev/null
+	wait 2> /dev/null
+
+	pkts=$(tcpdump_show $veth_up | wc -l)
+
+	tcpdump_cleanup $veth_up
+
+	if [ $pkts -lt 10000 ]; then
+		RET=$ksft_fail
+	fi
+}
+
+test_ipv4()
+{
+	setup
+	tcp_ecmp_failover IPv4 $SERVER_IP $CLIENT_IP
+	log_test "TCP IPv4 failover"
+	cleanup
+}
+
+test_ipv6()
+{
+	setup
+	tcp_ecmp_failover IPv6 "[$SERVER_IP6]" "[$CLIENT_IP6]"
+	log_test "TCP IPv6 failover"
+	cleanup
+}
+
+test_ipv4
+test_ipv6
-- 
2.54.0.545.g6539524ca2-goog