fix: enforce TCP isolation on --internal networks by dropping default gateway

Sources/ContainerResource/Network/Attachment.swift

@@ -25,8 +25,7 @@ public struct Attachment: Codable, Sendable {
     /// The CIDR address describing the interface IPv4 address, with the prefix length of the subnet.
     public let ipv4Address: CIDRv4
     /// The IPv4 gateway address.
-    public let ipv4Gateway: IPv4Address
-    /// The CIDR address describing the interface IPv6 address, with the prefix length of the subnet.
+    public let ipv4Gateway: IPv4Address?    /// The CIDR address describing the interface IPv6 address, with the prefix length of the subnet.
     /// The address is nil if the IPv6 subnet could not be determined at network creation time.
     public let ipv6Address: CIDRv6?
     /// The MAC address associated with the attachment (optional).
@@ -40,7 +39,7 @@ public struct Attachment: Codable, Sendable {
         network: String,
         hostname: String,
         ipv4Address: CIDRv4,
-        ipv4Gateway: IPv4Address,
+        ipv4Gateway: IPv4Address?,
         ipv6Address: CIDRv6?,
         macAddress: MACAddress?,
         mtu: UInt32? = nil,

Sources/ContainerResource/Network/NetworkStatus.swift

@@ -23,14 +23,14 @@ public struct NetworkStatus: Codable, Sendable {
     public let ipv4Subnet: CIDRv4
 
     /// The IPv4 gateway address.
-    public let ipv4Gateway: IPv4Address
+    public let ipv4Gateway: IPv4Address?
 
     /// The IPv6 subnet assigned to the network, if IPv6 is enabled.
     public let ipv6Subnet: CIDRv6?
 
     public init(
         ipv4Subnet: CIDRv4,
-        ipv4Gateway: IPv4Address,
+        ipv4Gateway: IPv4Address?,
         ipv6Subnet: CIDRv6?
     ) {
         self.ipv4Subnet = ipv4Subnet

Sources/Services/NetworkVmnet/Server/ReservedVmnetNetwork.swift

@@ -36,7 +36,7 @@ public final class ReservedVmnetNetwork: ContainerNetworkServer.Network {
     private struct NetworkInfo {
         let network: vmnet_network_ref
         let ipv4Subnet: CIDRv4
-        let ipv4Gateway: IPv4Address
+        let ipv4Gateway: IPv4Address?
         let ipv6Subnet: CIDRv6
     }
 
@@ -168,7 +168,7 @@ public final class ReservedVmnetNetwork: ContainerNetworkServer.Network {
         let lower = IPv4Address(subnetValue & maskValue)
         let upper = IPv4Address(lower.value + ~maskValue)
         let runningSubnet = try CIDRv4(lower: lower, upper: upper)
-        let runningGateway = IPv4Address(runningSubnet.lower.value + 1)
+        let runningGateway: IPv4Address? = configuration.mode == .hostOnly ? nil : IPv4Address(runningSubnet.lower.value + 1)
 
         var prefixAddr = in6_addr()
         var prefixLength = UInt8(0)

Sources/Services/RuntimeLinux/Server/RuntimeService.swift

@@ -1149,7 +1149,9 @@ public actor RuntimeService {
 
     private nonisolated func getDefaultNameservers(from attachments: [Attachment]) -> [String] {
         for attachment in attachments {
-            return [attachment.ipv4Gateway.description]
+            if let gateway = attachment.ipv4Gateway {
+                        return [gateway.description]
+                    }
         }
         return []
     }

Tests/IntegrationTests/Network/TestCLINetwork.swift

@@ -124,8 +124,7 @@ struct TestCLINetwork {
     }
 
     @available(macOS 26, *)
-    @Test func testIsolatedNetwork() async {
-        await withKnownIssue("curl error 7 despite retries", isIntermittent: true) {
+    @Test func testIsolatedNetwork() async throws {
             try await ContainerFixture.with { f in
                 let net = "\(f.testID)-net"
                 let server = "\(f.testID)-server"
@@ -163,15 +162,15 @@ struct TestCLINetwork {
                     return result.status == 0
                 }
 
-                // External connection should be blocked — the isolated network has no gateway.
+                // Reaching a literal IP proves egress is open regardless of DNS state.
                 let externalResult = try f.run([
                     "run", "--rm", "--network", net, curlImage,
-                    "curl", "--connect-timeout", "5", "http://google.com",
+                    "curl", "-sSk", "--connect-timeout", "5", "https://1.1.1.1/",
                 ])
-                let hostOnlyBlockedCodes: Set<Int32> = [6, 7, 28]
                 #expect(
-                    hostOnlyBlockedCodes.contains(externalResult.status),
-                    "external connection from isolated network should be blocked, got exit \(externalResult.status)")
+                    externalResult.status != 0,
+                    "hostOnly network must not reach external IPs. curl exited with \(externalResult.status)"
+                )
             }
         }
     }
@@ -210,4 +209,3 @@ struct TestCLINetwork {
             #expect(result.error.contains("network not found"))
         }
     }
-}