Merge branch 'ipv6-fix-ecmp-route-failover-on-carrier-loss'

Sagarika Sharma says:

====================
ipv6: fix ECMP route failover on carrier loss

This patchset resolves an issue where established IPv6 connections are
unable to transition to alternative ECMP nexthops upon carrier loss.

Unlike IPv4, the IPv6 routing subsystem does not actively invalidate
cached destinations during a NETDEV_CHANGE event. Sockets persist
with dead routes, leading to stalled traffic or connection drops.

This series introduces a fix to trigger route invalidation by
updating the route serial number on link carrier loss and provides
a corresponding selftest to validate the failover behavior for IPv4
and IPv6.
====================

Link: https://patch.msgid.link/20260430200909.527827-1-sharmasagarika@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

3 files changed, 218 insertions, 0 deletions

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 19eb6b702227..0dc0316530ca 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -4995,6 +4995,7 @@ static int fib6_ifdown(struct fib6_info *rt, void *p_arg)
 		    rt->fib6_flags & (RTF_LOCAL | RTF_ANYCAST))
 			break;
 		rt->fib6_nh->fib_nh_flags |= RTNH_F_LINKDOWN;
+		fib6_update_sernum(net, rt);
 		rt6_multipath_rebalance(rt);
 		break;
 	}
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..5768aa8bff6a
--- /dev/null
+++ b/tools/testing/selftests/net/tcp_ecmp_failover.sh
@@ -0,0 +1,216 @@
+#!/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 > /dev/null 2>&1
+}
+
+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 "$client_pid" > /dev/null 2>&1
+	kill -9 "$server_pid" > /dev/null 2>&1
+	wait 2> /dev/null
+
+	pkts=$(tcpdump_show $veth_up | wc -l)
+
+	tcpdump_cleanup "$veth_up"
+
+	if [ "$pkts" -lt 1000 ]; then
+		RET=$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]" "[$CLIENT_IP6]"
+	log_test "TCP IPv6 failover"
+	cleanup
+}
+
+require_command socat
+require_command tcpdump
+
+trap cleanup EXIT
+
+test_ipv4
+test_ipv6
+
+exit "$EXIT_STATUS"