diff --git a/Sources/Containerization/VirtualMachineAgent+Interface.swift b/Sources/Containerization/VirtualMachineAgent+Interface.swift
index e2fe7227..4bbae7dd 100644
--- a/Sources/Containerization/VirtualMachineAgent+Interface.swift
+++ b/Sources/Containerization/VirtualMachineAgent+Interface.swift
@@ -73,9 +73,14 @@ extension VirtualMachineAgent {
             )
         }
 
-        if ipv4Gateway == nil && ipv6Gateway == nil {
-            logger?.debug("no gateway for \(name)")
+        guard ipv4Gateway != nil || ipv6Gateway != nil else {
+            // `Interface` documents a nil gateway as "no default route", so
+            // installing one anyway would give the guest an on-link default
+            // route that makes every destination look directly reachable.
+            logger?.debug("no gateway for \(name), skipping the default route")
+            return
         }
+
         try await routeAddDefault(
             name: name,
             route: .init(ipv4Gateway: ipv4Gateway, ipv6Gateway: ipv6Gateway)
diff --git a/Tests/ContainerizationTests/InterfaceTests.swift b/Tests/ContainerizationTests/InterfaceTests.swift
index c00ecca1..491eba26 100644
--- a/Tests/ContainerizationTests/InterfaceTests.swift
+++ b/Tests/ContainerizationTests/InterfaceTests.swift
@@ -14,7 +14,10 @@
 // limitations under the License.
 //===----------------------------------------------------------------------===//
 
+import ContainerizationError
 import ContainerizationExtras
+import ContainerizationOCI
+import Foundation
 import Testing
 
 @testable import Containerization
@@ -56,4 +59,125 @@ struct InterfaceTests {
         #expect(nat.ipv6Address == nil)
         #expect(nat.ipv6Gateway == nil)
     }
+
+    /// Records the routing calls `setupInterface` makes so the routing decisions can be
+    /// asserted without booting a sandbox. Everything outside the networking surface is
+    /// unsupported, which is what the protocol asks unimplemented operations to report.
+    private actor RecordingAgent: VirtualMachineAgent {
+        private(set) var addresses: [InterfaceAddress] = []
+        private(set) var linkRoutes: [LinkRoute] = []
+        private(set) var defaultRoutes: [DefaultRoute] = []
+        private(set) var linksBroughtUp: [String] = []
+
+        func addressAdd(name: String, address: InterfaceAddress) async throws {
+            addresses.append(address)
+        }
+
+        func up(name: String, mtu: UInt32?) async throws {
+            linksBroughtUp.append(name)
+        }
+
+        func routeAddLink(name: String, route: LinkRoute) async throws {
+            linkRoutes.append(route)
+        }
+
+        func routeAddDefault(name: String, route: DefaultRoute) async throws {
+            defaultRoutes.append(route)
+        }
+
+        private func unsupported(_ operation: String) -> ContainerizationError {
+            ContainerizationError(.unsupported, message: operation)
+        }
+
+        func standardSetup() async throws { throw unsupported("standardSetup") }
+        func close() async throws { throw unsupported("close") }
+        func filesystemOperation(operation: FilesystemOperation, path: String) async throws { throw unsupported("filesystemOperation") }
+        func getenv(key: String) async throws -> String { throw unsupported("getenv") }
+        func setenv(key: String, value: String) async throws { throw unsupported("setenv") }
+        func mount(_ mount: ContainerizationOCI.Mount) async throws { throw unsupported("mount") }
+        func umount(path: String, flags: Int32) async throws { throw unsupported("umount") }
+        func mkdir(path: String, all: Bool, perms: UInt32) async throws { throw unsupported("mkdir") }
+        func kill(pid: Int32, signal: Int32) async throws -> Int32 { throw unsupported("kill") }
+        func down(name: String) async throws { throw unsupported("down") }
+        func configureDNS(config: DNS, location: String) async throws { throw unsupported("configureDNS") }
+
+        func createProcess(
+            id: String,
+            containerID: String?,
+            stdinPort: UInt32?,
+            stdoutPort: UInt32?,
+            stderrPort: UInt32?,
+            ociRuntimePath: String?,
+            configuration: ContainerizationOCI.Spec,
+            options: Data?
+        ) async throws { throw unsupported("createProcess") }
+        func startProcess(id: String, containerID: String?) async throws -> Int32 { throw unsupported("startProcess") }
+        func signalProcess(id: String, containerID: String?, signal: Int32) async throws { throw unsupported("signalProcess") }
+        func resizeProcess(id: String, containerID: String?, columns: UInt32, rows: UInt32) async throws { throw unsupported("resizeProcess") }
+        func waitProcess(id: String, containerID: String?, timeoutInSeconds: Int64?) async throws -> Containerization.ExitStatus { throw unsupported("waitProcess") }
+        func deleteProcess(id: String, containerID: String?) async throws { throw unsupported("deleteProcess") }
+    }
+
+    /// `Interface` documents a nil gateway as "no default route", so an interface with
+    /// neither a v4 nor a v6 gateway must not get one. Installing it anyway leaves the
+    /// guest with an on-link default route that makes every destination look local.
+    @Test func noDefaultRouteWhenBothGatewaysAreNil() async throws {
+        let agent = RecordingAgent()
+        let interface = V4OnlyInterface(
+            ipv4Address: try CIDRv4("172.16.0.3/24"),
+            ipv4Gateway: nil,
+            macAddress: nil)
+
+        try await agent.setupInterface(interface, name: "eth0", setDefaultRoute: true, logger: nil)
+
+        #expect(await agent.linksBroughtUp == ["eth0"])
+        #expect(await agent.addresses.count == 1)
+        #expect(await agent.defaultRoutes.isEmpty)
+        #expect(await agent.linkRoutes.isEmpty)
+    }
+
+    @Test func defaultRouteInstalledForIPv4Gateway() async throws {
+        let agent = RecordingAgent()
+        let interface = V4OnlyInterface(
+            ipv4Address: try CIDRv4("172.16.0.3/24"),
+            ipv4Gateway: try IPv4Address("172.16.0.1"),
+            macAddress: nil)
+
+        try await agent.setupInterface(interface, name: "eth0", setDefaultRoute: true, logger: nil)
+
+        let routes = await agent.defaultRoutes
+        #expect(routes.count == 1)
+        #expect(routes.first?.ipv4Gateway == (try IPv4Address("172.16.0.1")))
+        #expect(routes.first?.ipv6Gateway == nil)
+    }
+
+    /// A v6-only gateway still needs the default route, so the nil v4 gateway alone must
+    /// not suppress it.
+    @Test func defaultRouteInstalledForIPv6OnlyGateway() async throws {
+        let agent = RecordingAgent()
+        let interface = NATInterface(
+            ipv4Address: try CIDRv4("192.0.2.2/24"),
+            ipv4Gateway: nil,
+            ipv6Address: try CIDRv6("fd00::2/64"),
+            ipv6Gateway: try IPv6Address("fd00::1"))
+
+        try await agent.setupInterface(interface, name: "eth0", setDefaultRoute: true, logger: nil)
+
+        let routes = await agent.defaultRoutes
+        #expect(routes.count == 1)
+        #expect(routes.first?.ipv4Gateway == nil)
+        #expect(routes.first?.ipv6Gateway == (try IPv6Address("fd00::1")))
+    }
+
+    @Test func noDefaultRouteWhenNotRequested() async throws {
+        let agent = RecordingAgent()
+        let interface = V4OnlyInterface(
+            ipv4Address: try CIDRv4("172.16.0.3/24"),
+            ipv4Gateway: try IPv4Address("172.16.0.1"),
+            macAddress: nil)
+
+        try await agent.setupInterface(interface, name: "eth0", setDefaultRoute: false, logger: nil)
+
+        #expect(await agent.defaultRoutes.isEmpty)
+    }
 }