Send explicit exit notification if UDP

Implement --explicit-exit-notify by default.

Fixes #29

CHANGELOG.md

@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## Unreleased
+
+### Fixed
+
+- Send explicit exit notification if UDP. [#29](https://github.com/keeshux/tunnelkit/issues/29)
+
 ## 1.5.0 (2019-03-20)
 
 ### Added

TunnelKit/Sources/Core/Packet.swift

@@ -128,6 +128,22 @@ class DataPacket {
     static let pingString = Data(hex: "2a187bf3641eb4cb07ed2d0a981fc748")
 }
 
+enum OCCPacket: UInt8 {
+    case exit = 0x06
+    
+    private static let magicString = Data(hex: "287f346bd4ef7a812d56b8d3afc5459c")
+
+    func serialized(_ info: Any? = nil) -> Data {
+        var data = OCCPacket.magicString
+        data.append(rawValue)
+        switch self {
+        case .exit:
+            break // nothing more
+        }
+        return data
+    }
+}
+
 /// :nodoc:
 extension PacketCode: CustomStringConvertible {
     public var description: String {

TunnelKit/Sources/Core/SessionProxy.swift

@@ -1176,6 +1176,11 @@ public class SessionProxy {
     
     private func deferStop(_ method: StopMethod, _ error: Error?) {
         isStopping = true
+
+        // send exit notification if socket is unreliable (normally UDP)
+        if let link = link, !link.isReliable {
+            sendDataPackets([OCCPacket.exit.serialized()])
+        }
         
         switch method {
         case .shutdown: