On Mon, Aug 19, 2024 at 03:01:00PM +0200, Stefano Brivio wrote:On Mon, 19 Aug 2024 19:52:49 +1000 David Gibson <david(a)gibson.dropbear.id.au> wrote:Huh. Not in the passt/VM case, though, which is where I actually encountered this.On Mon, Aug 19, 2024 at 11:27:49AM +0200, Stefano Brivio wrote:I guess it's because they're not IFA_F_PERMANENT, because addrconf_permanent_addr() has: case NETDEV_CHANGEMTU: /* if MTU under IPV6_MIN_MTU stop IPv6 on this interface. */ if (dev->mtu < IPV6_MIN_MTU) { addrconf_ifdown(dev, dev != net->loopback_dev); break; } but addrconf_ifdown() does: if (!keep_addr || !(ifa->flags & IFA_F_PERMANENT) || addr_is_local(&ifa->addr)) { hlist_del_init_rcu(&ifa->addr_lst); goto restart; } I'm not sure about the logic behind that. We could actually set those addresses as permanent once the DHCPv6 client configures them, if it's cleaner.On Mon, 19 Aug 2024 18:46:31 +1000 David Gibson <david(a)gibson.dropbear.id.au> wrote:Sounds like it. I wasn't aware of that one. /me tests.. actually, no it doesn't work.. # sysctl -a | grep keep_addr_on_down net.ipv6.conf.all.keep_addr_on_down = 1 net.ipv6.conf.default.keep_addr_on_down = 1 net.ipv6.conf.dummy0.keep_addr_on_down = 1 net.ipv6.conf.lo.keep_addr_on_down = 0 # ip addr add 2001:db8::1 dev dummy0 # ip a 1: lo: <LOOPBACK> mtu 65536 qdisc noop state DOWN group default qlen 1000 link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 2: dummy0: <BROADCAST,NOARP> mtu 1500 qdisc noop state DOWN group default qlen 1000 link/ether c2:02:f2:79:f9:94 brd ff:ff:ff:ff:ff:ff inet6 2001:db8::1/128 scope global valid_lft forever preferred_lft forever # ip link set dummy0 mtu 1200 # ip a 1: lo: <LOOPBACK> mtu 65536 qdisc noop state DOWN group default qlen 1000 link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 2: dummy0: <BROADCAST,NOARP> mtu 1200 qdisc noop state DOWN group default qlen 1000 link/ether c2:02:f2:79:f9:94 brd ff:ff:ff:ff:ff:ff # ip link set dummy0 mtu 1500 # ip a 1: lo: <LOOPBACK> mtu 65536 qdisc noop state DOWN group default qlen 1000 link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 2: dummy0: <BROADCAST,NOARP> mtu 1500 qdisc noop state DOWN group default qlen 1000 link/ether c2:02:f2:79:f9:94 brd ff:ff:ff:ff:ff:ff My guess is that IPv6 being deconfigured because of an unsuitable MTU is considered a different event from a mere "down".On Fri, Aug 16, 2024 at 03:39:41PM +1000, David Gibson wrote: > Based on Stefano's recent patch for faster tests. > > Allow the user to specify which addresses are translated when used by > the guest, rather than always being the gateway address or nothing. > We also allow this remapping to go to the host's global address (more > precisely the address assigned to the guest) rather than just host > loopback. > > Suggestions for better names for the new options in patches 20 & 22 > are most welcome. > > Along the way to implementing that make many changes to clarify what > various addresses we track mean, fixing a number of small bugs as > well. > > NOTE: there is a bug in 21/22 which breaks some of the passt_tcp perf > tests. I haven't managed to figure out why it's causing the problem, > or even what the exact triggering conditions are (running the single > stalling iperf alone doesn't do it). Have to wrap up for today, so I > thought I'd get this out for review anyway. I've identified the bug here. IMO, it's a pre-existing problem that only works by accident at the moment. The immediate fix is pretty obvious, but it raises some broader questions The problem arises because of the MTU changes we make in order to test throughput with different packet sizes. Specifically we change the MTU to values < 1280, which implicitly disables IPv6 since it requires an MTU >= 1280. When we change the MTU back to a larger value IPv6 is re-enabled, but some configuration has been lost in the meantime. After the MTU is restored the guest reconfigures with NDP, but does not re-DHCPv6. That means the guest gets a SLAAC address in the right prefix but not the exact /128 address we've tried to assign to it. However, at least with the sequence of things we have in the tests, the guest never sends any packets with the new address, so passt doesn't update addr_seen. When the inbound connection comes we send it to the assigned address instead of the guest's actual address and the guest rejects it.I still have to take a closer look, but I'm fairly sure I hit a similar issue while I was writing these tests originally. I pondered reconfiguring the address via DHCPv6, or using the keep_addr_on_down sysctl (net.ipv6.conf.<interface>.keep_addr_on_down), which was added around that time. Then:This "worked" previously, because before this patch, passt would translate the inbound connection to have source/dest as link-local addresses....I realised that this worked and forgot about the whole issue.We *do* have a current addr_ll_seen because (a) it won't change if the guest doesn't change MAC and (b) when IPv6 is re-enabled the NDP traffic the guest generates will have link-local addresses that update addr_ll_seen. With this patch, and a global address for --map-host-loopback, we now need to send to addr_seen instead of addr_ll_seen, hence exposing the bug. In the short term, the obvious fix would be to re-run dhclient -6 in the guest after we twiddle MTU but before running IPv6 tests.I guess setting keep_addr_on_down (even for "all" interfaces) should work as well.Oh, I see. Assuming that at some point the DHCP client will re-run.In this case, it's not true that the guest doesn't configure itself in the way we requested -- it's just a temporary diversion from that configuration.I'm not really sure what you're getting at here.This kind of opens a question about how hard we should try to accomodate guests which don't configure themselves how we told them.There's a notable distinction between guests temporarily diverging (in different ways) and guests we don't configure at all.Those are different cases that we can handle in different ways, I think. If it's a glitch that will only happen during testing, let's work around that. But if the guest really ignores DHCPv6 information, I think we should keep that working.Ok.Global unicast instead of link-local.It's probably more important to ensure we use the right type of address"type" in what sense here?Eh, maybe. Unless us trying to make sense of a nonsense situation causes some unpredictable behaviour that breaks something else.True, but if we make correctness as optional as possible, we'll be more compatible (less time spent by users fixing situations that don't necessarily need fixing, less time spent by developers to look into reports, no matter who's at fault).(security) rather than ensuring we somehow manage to deliver packets at any time (minor glitch otherwise), also because the one you describe is something we're unlikely to hit outside of tests.Well, you still wouldn't *need* DHCPv6 or NDP, but you'd have to manually configure the interface in the guest to match the address you've configured with -a. Just like you'd expect to have to correctly configure your address on a real network.Personally I'd be ok with saying that nothing works if the guest doesn't configure itself properly, thereby removing addr_seen and addr_ll_seen entirely. But I think, Stefano, you've been against that idea in the past.Yes, I still think we should support guests that don't use DHCPv6 or NDP at all,If we have separate interfaces for each guest, yes. But not if we have multiple guests behind a single tap because the initial guest sets up a bridge or routing. Then we have nothing but the address.Why? We will need to hash the interface/guest index anyway, for outbound flows.or where related exchanges fail for any reason. It improves reliability and compatibility at a small cost. In this case, I think it's a nice feature that we would resume communicating as soon as the guest shows its global unicast address.Hm, maybe. I'm not entirely convinced the cost is so small long term. It's pretty badly incompatible with having multiple guests behind the same passt instance: such as the initial guest bridging or routing to nested guests.And for inbound flows, if a guest steals the address of another guest, we'll give priority to the normal 'addr' versions instead of the '_seen' ones, to decide how to direct traffic.I don't see how we'd know we're in this situation, so when to prioritise which address over the other.Because predictability is good, and working _most_ of the time is a failure of predictability.I'm actually not sure if encountering this bug makes me more or less in favour of addr_seen. On the one hand I think it highlights the flakiness of this approach; there are situations where we just won't know the right address.I don't understand this argument: indeed, there are such situations, and they are annoying. Why should we make them more common?Oh, good point. Hrm... then I'm unsure why the guest wasn't re-DADing its new address.On the other hand if shows a relatively plausible case where the guest won't get exactly the address we want it to (it uses NDP but not DHCPv6) Hrm... actually this also shows a potential danger in the recent patches to disable DAD in the guest. With DAD enabled, when the guest grabs a new address, we'd expect it to emit DAD messages, which would have the side effect of updating our addr_seen (although I'm pretty sure I hit this patch before the nodad patches were applied, so that doesn't seem to be foolproof).Well, but we do that for containers with --config-net only. In that case, the addresses we configure have infinite lifetime anyway.Besides, I don't think we need to have addr_seen updated as quickly and correctly as possible just for the sake of it, we can also update it when we get any other neighbour solicitation because the guest is actually using the network. It's not meant to be perfect.If the guest is a pure server (a common case for containers AFAICT), then I don't know that we can expect NS messages for anything other than the default gateway, which is (typically) link-local and so won't help us to learn the new global address.A case just like the one in the tests: the interface bounces, and we get NDP traffic on the link-local address, but nothing on the global address before an inbound connection. -- David Gibson (he or they) | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you, not the other way | around. http://www.ozlabs.org/~dgibsonWe could maybe update addr_seen when we send RA messages to the guest - assuming that it will use the same host part (low 64-bits) for both link-local and global addresses. Not sure if that's a widely safe assumption or not.I don't understand: what case are you trying to cover with this?