On Sun, 24 Dec 2023 at 10:13, Simon Horman <horms@xxxxxxxxxx> wrote: > Thanks Brad, > > I agree with your analysis and that the problem appears to > have been introduced by the cited commit. Thanks for the review Simon. > I am curious to know what use case triggers this / > why it when unnoticed for a year. We encountered this issue while upgrading some routers from linux 5.15 to 6.2. The dataplane on these routers is provided by an openvswitch bridge which is controlled via openflow by faucet. These routers are also performing SNAT on all traffic to/from the wan interface via openvswitch conntrack openflow rules. We noticed that after upgrading the linux kernel, traceroute/mtr no longer worked when run from clients behind the router. We eventually discovered the reason for this is that the ICMP time exceeded messages elicited by traceroute were matching openflow rules with the incorrect destination ip, despite there being an openflow rule to undo the nat. Other packets in the established or new state matched the expected openflow rules. A git bisect between 5.15 and 6.2 showed that this change in behaviour was introduced by commit ebddb1404900. After the above patch is applied our routers perform nat correctly again for traceroute/mtr.
