Pin the MAC-first zone arm with a host-route nexthop scenario

cmd/scenariotest/scenarios/host_route_nexthop.go

@@ -0,0 +1,74 @@
+package scenarios
+
+import (
+	"github.com/bigstack-oss/lachesis/internal/scenariotest"
+	"github.com/bigstack-oss/lachesis/internal/scenariotest/steps"
+	"github.com/bigstack-oss/lachesis/internal/testenv/scenario"
+)
+
+const (
+	// hostRouteDriveBytes is the transfer this scenario pushes.
+	hostRouteDriveBytes = 1 << 20
+	// hostRouteNoiseBudget bounds the external zone, which is where the
+	// asserted bytes land if the MAC-first arm does NOT win.
+	hostRouteNoiseBudget = 256 << 10
+)
+
+// hostRouteNexthop pins the MAC-first arm of the hybrid zone lookup
+// (docs/architecture/packet-classification.md#the-hybrid-lookup-explained).
+// A host route steers vm-a at an L2-adjacent VM, so the peer MAC and
+// the destination IP disagree: the MAC arm answers same_tenant, while
+// the trie would answer external for a prefix absent from the topology
+// (docs/architecture/trie-construction.md#the-five-step-algorithm step 1).
+// Asserting that one grew while the other stayed flat is what makes
+// this shape discriminating where twovms-same-tenant is not.
+//
+//	vm-a (T1, .5) ──host route 10.0.82.0/24 via .50──> vm-nh (T1, .50)
+//
+// vm-nh only has to own a MAC and answer ARP for it: the drive is a
+// sized-ping push at a dead address, so nothing needs to forward. The
+// in-guest route stands in for a Neutron host_routes entry, and the rx
+// side is deliberately unasserted — rationale in lachesis#302.
+func hostRouteNexthop() *scenariotest.Scenario {
+	const (
+		dstCIDR    = "10.0.82.0/24"
+		dstProbeIP = "10.0.82.5"
+		nexthopIP  = "10.0.80.50"
+	)
+	b := scenario.New()
+	b.Network("net-T1", "T1").
+		Subnet("sub-T1", "10.0.80.0/24", "10.0.80.1").
+		VM("vm-a", "T1", "10.0.80.5").
+		VM("vm-nh", "T1", nexthopIP)
+	// Router + external gateway exist only so both VMs can hold an SSH
+	// floating IP; the asserted hop never reaches the router.
+	b.ExternalNetwork("net-ext", "admin")
+	b.Router("r-T1", "T1").
+		Attach("sub-T1", "10.0.80.1").
+		ExternalGateway("net-ext")
+
+	return &scenariotest.Scenario{
+		Name:    "host-route-nexthop",
+		Desc:    "A host route at an L2-adjacent VM bills that peer's tenant, not the destination prefix's catchall.",
+		Builder: b,
+		Steps: []scenariotest.Step{
+			steps.AddRouteStep{VM: "vm-a", CIDR: dstCIDR, Via: nexthopIP},
+
+			steps.CaptureStep{},
+			steps.DriveStep{Flows: []scenariotest.Flow{
+				{From: "vm-a", To: scenariotest.ExternalTarget(dstProbeIP), Bytes: hostRouteDriveBytes, Proto: scenariotest.TCP},
+			}},
+
+			steps.AssertStep{Note: "the peer MAC decided the zone, not the destination prefix",
+				Expect: []scenariotest.Expect{
+					{TenantID: "T1", Zone: "same_tenant", Direction: "tx", MinBytes: hostRouteDriveBytes},
+				}},
+			// The counterfactual. This also catches a silently-failed
+			// AddRouteStep: without the route the pings go to r-T1's MAC,
+			// the trie resolves 10.0.82.5 to the catchall, and the bytes
+			// land here instead.
+			steps.MaxGrowthStep{Tenant: "T1", Zone: "external",
+				Budget: hostRouteNoiseBudget, Note: "MAC-first arm won — not the trie catchall"},
+		},
+	}
+}

cmd/scenariotest/scenarios/registry.go

@@ -55,6 +55,7 @@ func All() []*scenariotest.Scenario {
 		ghostGraceWindow(),
 		unresolvedLatebind(),
 		vmApplianceNexthop(),
+		hostRouteNexthop(),
 		gcPressureRelief(),
 		octaviaLBAttribution(),
 	}