Add rudimentary NetworkSettings generation

Apple/NetworkExtension/NetworkSettingsConverter.swift

@@ -0,0 +1,56 @@
+//
+//  NetworkSettingsConverter.swift
+//  NetworkExtension
+//
+//  Created by Jett Chen on 2023/7/7.
+//
+
+import Foundation
+import NetworkExtension
+
+public struct TunCrateNetworkSettings {
+    let addr: Int64
+    let netmask: Int64
+    let mtu: Int32
+}
+
+extension TunCrateNetworkSettings {
+    var decodedIPv4Addr: String? {
+        return decodeIPv4(addr)
+    }
+
+    var decodedIPv4Netmask: String? {
+        return decodeIPv4(netmask)
+    }
+
+    var decodedMTU: Int? {
+        return mtu >= 0 ? Int(mtu) : nil
+    }
+
+    private func decodeIPv4(_ addr: Int64) -> String? {
+        if addr < 0 {
+            return nil
+        }
+        let bytes = (
+            UInt8((addr & 0xFF000000) >> 24),
+            UInt8((addr & 0x00FF0000) >> 16),
+            UInt8((addr & 0x0000FF00) >> 8),
+            UInt8(addr & 0x000000FF)
+        )
+        return "\(bytes.0).\(bytes.1).\(bytes.2).\(bytes.3)"
+    }
+    
+    func generateNetworkSettings() -> NEPacketTunnelNetworkSettings {
+            let neSettings = NEPacketTunnelNetworkSettings()
+
+            if let addr = decodedIPv4Addr, let netmask = decodedIPv4Netmask {
+                neSettings.ipv4Settings = NEIPv4Settings(addresses: [addr], subnetMasks: [netmask])
+            }
+            if let mtuValue = decodedMTU {
+                neSettings.mtu = NSNumber(value: mtuValue)
+            }
+            return neSettings
+        }
+
+}
+