Add standard relay server

.idea/inspectionProfiles/Project_Default.xml

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+<component name="InspectionProjectProfileManager">
+  <profile version="1.0">
+    <option name="myName" value="Project Default" />
+    <inspection_tool class="WeakerAccess" enabled="false" level="WARNING" enabled_by_default="false">
+      <option name="SUGGEST_PACKAGE_LOCAL_FOR_MEMBERS" value="true" />
+      <option name="SUGGEST_PACKAGE_LOCAL_FOR_TOP_CLASSES" value="true" />
+      <option name="SUGGEST_PRIVATE_FOR_INNERS" value="false" />
+    </inspection_tool>
+  </profile>
+</component>

.idea/kotlinc.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="Kotlin2JvmCompilerArguments">
+    <option name="jvmTarget" value="1.8" />
+  </component>
+  <component name="KotlinCommonCompilerArguments">
+    <option name="apiVersion" value="1.2" />
+    <option name="languageVersion" value="1.2" />
+  </component>
+</project>

.idea/libraries/KotlinJavaRuntime.xml

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+<component name="libraryTable">
+  <library name="KotlinJavaRuntime">
+    <CLASSES>
+      <root url="jar://$KOTLIN_BUNDLED$/lib/kotlin-stdlib.jar!/" />
+      <root url="jar://$KOTLIN_BUNDLED$/lib/kotlin-reflect.jar!/" />
+      <root url="jar://$KOTLIN_BUNDLED$/lib/kotlin-test.jar!/" />
+    </CLASSES>
+    <JAVADOC />
+    <SOURCES>
+      <root url="jar://$KOTLIN_BUNDLED$/lib/kotlin-stdlib-sources.jar!/" />
+      <root url="jar://$KOTLIN_BUNDLED$/lib/kotlin-reflect-sources.jar!/" />
+      <root url="jar://$KOTLIN_BUNDLED$/lib/kotlin-test-sources.jar!/" />
+    </SOURCES>
+  </library>
+</component>

.idea/misc.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="ProjectRootManager" version="2" project-jdk-name="11" project-jdk-type="JavaSDK">
+    <output url="file://$PROJECT_DIR$/out" />
+  </component>
+</project>

.idea/modules.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="ProjectModuleManager">
+    <modules>
+      <module fileurl="file://$PROJECT_DIR$/Bungee.iml" filepath="$PROJECT_DIR$/Bungee.iml" />
+    </modules>
+  </component>
+</project>

Bungee.iml

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+<?xml version="1.0" encoding="UTF-8"?>
+<module type="JAVA_MODULE" version="4">
+  <component name="NewModuleRootManager" inherit-compiler-output="true">
+    <exclude-output />
+    <content url="file://$MODULE_DIR$">
+      <sourceFolder url="file://$MODULE_DIR$/src" isTestSource="false" />
+    </content>
+    <orderEntry type="inheritedJdk" />
+    <orderEntry type="sourceFolder" forTests="false" />
+    <orderEntry type="library" name="KotlinJavaRuntime" level="project" />
+  </component>
+</module>

src/Test.kt

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+import dev.w1zzrd.bungee.BungeeServer
+import java.net.InetAddress
+
+fun main(args: Array<String>){
+    // Route localhost:80 -> google.com
+    val server = BungeeServer(
+            InetAddress.getByName("0.0.0.0"), 25565 .. 25565,
+            InetAddress.getByName("192.168.1.145"), 25565
+    )
+    server.start()
+    server.waitFor()
+}

src/dev/w1zzrd/bungee/BungeeServer.kt

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+package dev.w1zzrd.bungee
+
+import java.net.*
+
+const val SERVER_CONNECTION_BACKLOG = 65536
+const val BUFFER_SIZE = 16777216       // 16 MiB communication buffer
+
+class BungeeServer(
+        private val listenAddr: InetAddress,
+        private val port: Int,
+        private val routeTo: InetAddress,
+        private val routePort: Int
+){
+    private var canStart = true
+    private val serverSocket = ServerSocket()
+    private val buffer = ByteArray(BUFFER_SIZE)
+    private val listenThread = Thread(this::acceptSocket)
+    private val clients = HashMap<Socket, Socket>()
+    private var alive = false
+
+    init {
+        serverSocket.soTimeout = 1
+    }
+
+    /**
+     * Start listening for incoming connections.
+     */
+    fun start(){
+        if(!canStart) throw IllegalStateException("Already started/stopped")
+        canStart = false
+
+        // Set up server socket
+        serverSocket.bind(InetSocketAddress(listenAddr, port), SERVER_CONNECTION_BACKLOG)
+
+        listenThread.start()
+    }
+
+    fun waitFor() = listenThread.join()
+
+    private fun acceptSocket(){
+        alive = true
+        while(alive) {
+            // Accept new clients
+            serverSocket.acceptPassthrough()
+
+            // Remove connections that have been dropped
+            purgeDeadConnections()
+
+            // Forward data
+            for ((clientSocket, routeSocket) in clients) {
+                // Bidirectional communication
+                clientSocket.forwardConnection(routeSocket)
+                routeSocket.forwardConnection(clientSocket)
+            }
+        }
+    }
+
+    private fun purgeDeadConnections(){
+        for((clientSocket, routeSocket) in clients){
+            if(
+                    !(clientSocket.isConnected && routeSocket.isConnected) ||
+                    clientSocket.isClosed ||
+                    routeSocket.isClosed
+            ){
+                purgeConnectionRoute(clientSocket, routeSocket)
+            }
+        }
+    }
+
+    private fun purgeConnectionRoute(clientSocket: Socket, routeSocket: Socket){
+        clientSocket.forceClose()
+        routeSocket.forceClose()
+        clients.remove(clientSocket, routeSocket)
+    }
+
+    private fun Socket.forceClose() = try{ close() }catch(t: Throwable){}
+
+    private fun Socket.forwardConnection(dest: Socket){
+        val from = getInputStream()
+        val to = dest.getOutputStream()
+
+        try {
+            if (from.available() > 0) {
+                val read = from.read(buffer, 0, buffer.size)
+                if (read > 0) to.write(buffer, 0, read)
+            }
+        }catch(t: Throwable){
+            // Probably a disconnection tbh
+            purgeConnectionRoute(this, dest)
+        }
+    }
+
+
+    private fun ServerSocket.acceptPassthrough(){
+        val client: Socket
+        try {
+            client = accept()
+        } catch (e: SocketTimeoutException) {
+            // Accepting clients timed out: no new clients
+            return
+        }
+
+        // Create a corresponding, outgoing connection for the new client
+        clients[client] = Socket(routeTo, routePort)
+    }
+}