Add FileHandleNetworkInterface

FileHandleNetworkInterface uses VZFileHandleNetworkDeviceAttachment to
attach the container to a file handle where another service can
provide an arbitrary network. This might be an entirely simulated
network, a virtual network like a VPN, e.g. using Wireguard, but it
could be also a bridged physical network from the host where the service
will then take over the responsibility of bridging the traffic between
the container and host.

We do not make any assumption about IP addresses for now, but instead
also assume that the IP address can get assigned via DHCP.

Author: curdbecker

Sources/Containerization/BridgedNetworkInterface.swift

@@ -28,7 +28,6 @@ public final class BridgedNetworkInterface: Interface, Sendable {
     public let macAddress: MACAddress?
     public let ipv4Address: CIDRv4? = nil
     public let ipv4Gateway: IPv4Address? = nil
-    public let mtu: UInt32 = 1500
 
     public init(hostInterfaceName: String, macAddress: MACAddress? = nil) {
         self.hostInterfaceName = hostInterfaceName

Sources/Containerization/FileHandleNetworkInterface.swift

@@ -0,0 +1,51 @@
+//===----------------------------------------------------------------------===//
+// Copyright © 2026 Apple Inc. and the Containerization project authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//===----------------------------------------------------------------------===//
+
+#if os(macOS)
+
+import ContainerizationError
+import ContainerizationExtras
+import Virtualization
+
+/// A network interface that connects the container to an arbitrary FileHandle-backed
+/// network service. The IP address might be assigned by the upstream DHCP server or
+/// configured inside the container; `ipv4Address` is always nil.
+@available(macOS 26, *)
+public final class FileHandleNetworkInterface: Interface, Sendable {
+    public let macAddress: MACAddress?
+    public let ipv4Address: CIDRv4? = nil
+    public let ipv4Gateway: IPv4Address? = nil
+    public let fileHandle: FileHandle
+
+    public init(fileHandle: FileHandle, macAddress: MACAddress? = nil) {
+        self.macAddress = macAddress
+        self.fileHandle = fileHandle
+    }
+}
+
+@available(macOS 26, *)
+extension FileHandleNetworkInterface: VZInterface {
+    public func device() throws -> VZVirtioNetworkDeviceConfiguration {
+        let config = VZVirtioNetworkDeviceConfiguration()
+        config.attachment = VZFileHandleNetworkDeviceAttachment(fileHandle: fileHandle)
+        if let mac = macAddress, let vzMac = VZMACAddress(string: mac.description) {
+            config.macAddress = vzMac
+        }
+        return config
+    }
+}
+
+#endif