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For **nexthop object** multipath, patch > > 1/3 is also needed so `fib6_table_lookup()` picks the best-scoring > > nexthop before path selection is skipped. Patch 1 not in tree; patch 2 > > alone does not worsen nexthop-object behavior. > [...] > > **Recommendation for v6.18.y:** Backport this commit. Ideally backport > > patch 1/3 from the same series together for complete nexthop-object > > multipath coverage; patch 2 alone is still valuable and safe for classic > > multipath routes. > > Hi Sasha, > > we are the nexthop-object case from your dependency note, hit in > production on 6.12.y, so here is a real-world data point and a request > to carry patch 1/3 along. Just to be clear, this report is from a kernel that lacks both patches, right? I fetched linux-6.12.y and I can't find my patches there. Note that I targeted both patches at net-next since they don't fix regressions. This use case never worked correctly. > > (Disclaimer: Analysis below done by AI, watched and confirmed by human) > > Setup: Debian 13 host, kernel 6.12.101, dual-homed to two ToR switches > over unnumbered eBGP (FRR). FRR installs the default route through a > nexthop group, which is what every FRR or Cilium dual-homed host looks > like: > > $ ip -6 route show default > default nhid 20 proto bgp metric 20 pref medium > nexthop via fe80:: dev ens31f0np0 weight 1 > nexthop via fe80:: dev ens30f0np0 weight 1 > nexthop via fe80:: dev ens31f1np1 weight 1 > nexthop via fe80:: dev ens30f1np1 weight 1 > > The kernel's own lookup ignores the oif for this route, with and > without a source address: > > $ ip -6 route get 2606:4700:4700::1111 oif ens31f1np1 > 2606:4700:4700::1111 from :: via fe80:: dev ens30f1np1 \ > proto bgp src fd00:2:1::11 metric 20 pref medium > $ ip -6 route get 2606:4700:4700::1111 oif ens31f1np1 from 2001:db8:a::1 > 2606:4700:4700::1111 from 2001:db8:a::1 via fe80:: dev ens30f1np1 \ > proto bgp src fd00:2:1::11 metric 20 pref medium fib6_select_path() overwrites whatever fib6_table_lookup() returned and selects a nexthop based on the calculated hash. In this case, both hashes ended up picking the same nexthop. > > Same for a socket. A test tool sends UDP on an AF_INET6/SOCK_RAW/ > IPPROTO_UDP socket with IPV6_HDRINCL and SO_BINDTODEVICE=ens31f1np1 > (root; the source address is a service address of the host, not the > interface address). The option is accepted and readable back: > > setsockopt(SO_BINDTODEVICE, "ens31f1np1") = 0 > getsockopt(SO_BINDTODEVICE) -> "ens31f1np1" > > but the frames leave the sibling interface. tcpdump on both NICs while > 3 packets are sent: > > ens31f1np1 (the bound device): 0 packets captured > ens30f1np1: 3 packets captured > IP6 2001:db8:a::1.41022 > 2606:4700:4700::1111.53: ... > > Impact for us: anything that pins traffic to one uplink of a dual-homed > BGP host (per-link probes, validation of switch-side policy routing) > silently takes another link. We lost a day to it before suspecting the > kernel. I don't think that it matters if you bind to a device or not. AFAICT, with nexthop groups, 6.12.y completely ignores the oif and simply chooses a nexthop based on the calculated hash. > > From reading 6.12 route.c: with nexthop objects, fib6_table_lookup() > returns the first nexthop of the group unless the lookup is strict, so > the have_oif_match test that this patch adds to ip6_pol_route() only > becomes true for nexthop groups once 484bb9d164df ("ipv6: Select best > matching nexthop object in fib6_table_lookup()") makes the table lookup > prefer the oif-matching nexthop. Its commit message describes it as > preparation for this patch. For nexthop-object users this patch is > therefore incomplete on its own; please consider queueing both: > > 484bb9d164df ipv6: Select best matching nexthop object in fib6_table_lookup() > d25e7e9d8a6c ipv6: Honor oif when choosing nexthop for locally generated traffic > > https://patch.msgid.link/20260611154605.992528-2-idosch@nvidia.com > https://patch.msgid.link/20260611154605.992528-3-idosch@nvidia.com > > Happy to test a 6.12.y tree with one or both applied on these hosts and > report back. Can you test 7.2.y? It already has both patches.