Makes interface gateway configuration optional. (#156)

Here we implement the mechanism for ensuring that `container` doesn't
set up two default routes on containers that connect to multiple
networks. We'll implement the policy in apple/container#243.

Sources/Containerization/Interface.swift

@@ -16,7 +16,13 @@
 
 /// A network interface.
 public protocol Interface: Sendable {
+    /// The interface IPv4 address and subnet prefix length, as a CIDR address.
+    /// Example: `192.168.64.3/24`
     var address: String { get }
-    var gateway: String { get }
+
+    /// The IP address for the default route, or nil for no default route.
+    var gateway: String? { get }
+
+    /// The interface MAC address, or nil to auto-configure the address.
     var macAddress: String? { get }
 }

Sources/Containerization/LinuxContainer.swift

@@ -500,12 +500,14 @@ extension LinuxContainer {
                 // For every interface asked for:
                 // 1. Add the address requested
                 // 2. Online the adapter
-                // 3. Add the gateway address
+                // 3. If a gateway IP address is present, add the default route.
                 for (index, i) in self.interfaces.enumerated() {
                     let name = "eth\(index)"
                     try await agent.addressAdd(name: name, address: i.address)
                     try await agent.up(name: name)
-                    try await agent.routeAddDefault(name: name, gateway: i.gateway)
+                    if let gateway = i.gateway {
+                        try await agent.routeAddDefault(name: name, gateway: gateway)
+                    }
                 }
                 if let dns = self.dns {
                     try await agent.configureDNS(config: dns, location: rootfs.destination)

Sources/Containerization/NATInterface.swift

@@ -16,10 +16,10 @@
 
 public struct NATInterface: Interface {
     public var address: String
-    public var gateway: String
+    public var gateway: String?
     public var macAddress: String?
 
-    public init(address: String, gateway: String, macAddress: String? = nil) {
+    public init(address: String, gateway: String?, macAddress: String? = nil) {
         self.address = address
         self.gateway = gateway
         self.macAddress = macAddress

Sources/Containerization/NATNetworkInterface.swift

@@ -32,7 +32,7 @@ public final class NATNetworkInterface: Interface, Sendable {
 
     }
 
-    public var gateway: String {
+    public var gateway: String? {
         get { state.gateway }
         set { state.gateway = newValue }
     }
@@ -49,7 +49,7 @@ public final class NATNetworkInterface: Interface, Sendable {
 
     private struct State {
         fileprivate var address: String
-        fileprivate var gateway: String
+        fileprivate var gateway: String?
         fileprivate var reference: vmnet_network_ref!
         fileprivate var macAddress: String?
     }
@@ -60,7 +60,7 @@ public final class NATNetworkInterface: Interface, Sendable {
     @available(macOS 26, *)
     public init(
         address: String,
-        gateway: String,
+        gateway: String?,
         reference: sending vmnet_network_ref,
         macAddress: String? = nil
     ) {
@@ -75,7 +75,7 @@ public final class NATNetworkInterface: Interface, Sendable {
     @available(macOS, obsoleted: 26, message: "Use init(address:gateway:reference:macAddress:) instead")
     public init(
         address: String,
-        gateway: String,
+        gateway: String?,
         macAddress: String? = nil
     ) {
         self.state = .init(