All packets in the same flow (L3/L4 depending on multipath hash policy) should be directed to the same target, but after [0]/[1] we see stray packets directed towards other targets. This, for instance, causes RST to be sent on TCP connections.
The first two patches solve the problem by ignoring route hints for destinations that are part of multipath group, by using new SKB flags for IPv4 and IPv6. The third patch is a selftest that tests the scenario.
Thanks to Ido, for reviewing and suggesting a way forward in [2].
[0]: commit 02b24941619f ("ipv4: use dst hint for ipv4 list receive") [1]: commit 197dbf24e360 ("ipv6: introduce and uses route look hints for list input.") [2]: https://lore.kernel.org/netdev/20230815201048.1796-1-sriram.yagnaraman@est.t...
Sriram Yagnaraman (3): ipv4: ignore dst hint for multipath routes ipv6: ignore dst hint for multipath routes selftests: forwarding: Add test for load-balancing between multiple servers
include/linux/ipv6.h | 1 + include/net/ip.h | 1 + net/ipv4/ip_input.c | 3 +- net/ipv4/route.c | 1 + net/ipv6/ip6_input.c | 3 +- net/ipv6/route.c | 2 + .../testing/selftests/net/forwarding/Makefile | 1 + .../net/forwarding/router_multipath_vip.sh | 403 ++++++++++++++++++ 8 files changed, 413 insertions(+), 2 deletions(-) create mode 100755 tools/testing/selftests/net/forwarding/router_multipath_vip.sh