en Copyright 2015 Java http://kernelsources.org:8080/source/history/linux/tools/testing/selftests/net/ecmp_rehash.sh#ae57a55332436ca58d1824d3375478afd51dca6a selftests: net: add local ECMP rehash testAdd ecmp_rehash.sh with nine scenarios verifying that TCP rehashselects a different local ECMP path for IPv6: - SYN retransmission (forward path blocked during setup) - SYN/ACK retransmission (reverse path blocked during setup) - Midstream RTO (forward path blocked on established connection) - Midstream ACK rehash (reverse path blocked on established connection) - PLB rehash (ECN-driven congestion on established connection) - Hash policy 1 negative test (rehash attempted but path unchanged) - No flowlabel leak (client mp_hash does not alter on-wire flowlabel) - Dst rebuild consistency (dst invalidation does not change path) - Syncookie server path consistency (SYN-ACK and post-cookie ACKs use the same ECMP path)The policy 1 test verifies that fib_multipath_hash_policy=1 computesa deterministic 5-tuple hash, so txhash re-rolls do not change theECMP path while TcpTimeoutRehash still increments.The flowlabel leak test sets auto_flowlabels=0 on the client andinstalls tc filters on client egress that drop TCP packets withnonzero flowlabel, confirming that the client's fl6->mp_hash doesnot leak into the on-wire IPv6 flow label.The PLB test needs DCTCP, a restricted congestion control. Ratherthan relax the host-global tcp_allowed_congestion_control (noper-netns equivalent), it pins dctcp on the test routes via thecongctl route attribute, confined to the test namespaces.The dst rebuild test streams data, invalidates the cached dst byadding and removing a dummy route (bumping the fib6_node sernum),and verifies that traffic stays on the same path. The sernum changecauses ip6_dst_check() to fail on the next transmit, triggering afresh route lookup via inet6_csk_route_socket().ECMP_REBUILD_ROUNDS=10 repeats the check to reduce the probabilityof a buggy kernel passing by chance with 2-way ECMP.The syncookie server path consistency test verifies that theserver's SYN-ACK and subsequent ACKs use the same ECMP path.With syncookies, the request socket is freed after the SYN-ACK,so cookie_tcp_reqsk_init() must derive the same txhash (from thecookie) that was used for the SYN-ACK's route lookup.The syncookie test forces tcp_syncookies=2; it skips whenCONFIG_SYN_COOKIES is not available. selftests/net/config selectsit (and CONFIG_TCP_CONG_DCTCP for the PLB test).Signed-off-by: Neil Spring <ntspring@meta.com>Link: https://patch.msgid.link/20260615042158.1600746-3-ntspring@meta.comSigned-off-by: Jakub Kicinski <kuba@kernel.org> List of files: /linux/tools/testing/selftests/net/ecmp_rehash.sh Mon, 15 Jun 2026 06:21:58 +0200 Neil Spring <ntspring@meta.com>