Merge pull request #136 from dualspiral/forge-handshake

Refactor Forge support to track the handshake & include keepalives

proxy/src/main/java/com/velocitypowered/proxy/connection/ConnectionType.java

@@ -0,0 +1,47 @@
+package com.velocitypowered.proxy.connection;
+
+import com.velocitypowered.api.util.GameProfile;
+import com.velocitypowered.proxy.config.PlayerInfoForwarding;
+import com.velocitypowered.proxy.connection.backend.BackendConnectionPhase;
+import com.velocitypowered.proxy.connection.client.ClientConnectionPhase;
+
+/**
+ * The types of connection that may be selected.
+ */
+public interface ConnectionType {
+
+  /**
+   * The initial {@link ClientConnectionPhase} for this connection type.
+   *
+   * @return The {@link ClientConnectionPhase}
+   */
+  ClientConnectionPhase getInitialClientPhase();
+
+  /**
+   * The initial {@link BackendConnectionPhase} for this connection type.
+   *
+   * @return The {@link BackendConnectionPhase}
+   */
+  BackendConnectionPhase getInitialBackendPhase();
+
+  /**
+   * Adds properties to the {@link GameProfile} if required. If any properties
+   * are added, the returned {@link GameProfile} will be a copy.
+   *
+   * @param original The original {@link GameProfile}
+   * @param forwardingType The Velocity {@link PlayerInfoForwarding}
+   * @return The {@link GameProfile} with the properties added in.
+   */
+  GameProfile addGameProfileTokensIfRequired(GameProfile original,
+                                             PlayerInfoForwarding forwardingType);
+
+  /**
+   * Tests whether the hostname is the handshake packet is valid.
+   *
+   * @param address The address to check
+   * @return true if valid.
+   */
+  default boolean checkServerAddressIsValid(String address) {
+    return !address.contains("\0");
+  }
+}

proxy/src/main/java/com/velocitypowered/proxy/connection/ConnectionTypes.java

@@ -0,0 +1,36 @@
+package com.velocitypowered.proxy.connection;
+
+import com.velocitypowered.proxy.connection.backend.BackendConnectionPhases;
+import com.velocitypowered.proxy.connection.client.ClientConnectionPhases;
+import com.velocitypowered.proxy.connection.forge.legacy.LegacyForgeConnectionType;
+import com.velocitypowered.proxy.connection.util.ConnectionTypeImpl;
+
+/**
+ * The connection types supported by Velocity.
+ */
+public final class ConnectionTypes {
+
+  /**
+   * Indicates that the connection has yet to reach the
+   * point where we have a definitive answer as to what
+   * type of connection we have.
+   */
+  public static final ConnectionType UNDETERMINED =
+      new ConnectionTypeImpl(ClientConnectionPhases.VANILLA, BackendConnectionPhases.UNKNOWN);
+
+  /**
+   * Indicates that the connection is a Vanilla connection.
+   */
+  public static final ConnectionType VANILLA =
+      new ConnectionTypeImpl(ClientConnectionPhases.VANILLA, BackendConnectionPhases.VANILLA);
+
+  /**
+   * Indicates that the connection is a 1.8-1.12 Forge
+   * connection.
+   */
+  public static final ConnectionType LEGACY_FORGE = new LegacyForgeConnectionType();
+
+  private ConnectionTypes() {
+    throw new AssertionError();
+  }
+}

proxy/src/main/java/com/velocitypowered/proxy/connection/MinecraftConnection.java

@@ -56,9 +56,8 @@ public class MinecraftConnection extends ChannelInboundHandlerAdapter {
   private ProtocolVersion protocolVersion;
   private ProtocolVersion nextProtocolVersion;
   private @Nullable MinecraftConnectionAssociation association;
-  private boolean isLegacyForge;
   private final VelocityServer server;
-  private boolean canSendLegacyFmlResetPacket = false;
+  private ConnectionType connectionType = ConnectionTypes.UNDETERMINED;
 
   public MinecraftConnection(Channel channel, VelocityServer server) {
     this.channel = channel;
@@ -279,22 +278,6 @@ public class MinecraftConnection extends ChannelInboundHandlerAdapter {
     this.association = association;
   }
 
-  public boolean isLegacyForge() {
-    return isLegacyForge;
-  }
-
-  public void setLegacyForge(boolean isForge) {
-    this.isLegacyForge = isForge;
-  }
-
-  public boolean canSendLegacyFmlResetPacket() {
-    return canSendLegacyFmlResetPacket;
-  }
-
-  public void setCanSendLegacyFmlResetPacket(boolean canSendLegacyFMLResetPacket) {
-    this.canSendLegacyFmlResetPacket = isLegacyForge && canSendLegacyFMLResetPacket;
-  }
-
   public ProtocolVersion getNextProtocolVersion() {
     return this.nextProtocolVersion;
   }
@@ -302,4 +285,20 @@ public class MinecraftConnection extends ChannelInboundHandlerAdapter {
   public void setNextProtocolVersion(ProtocolVersion nextProtocolVersion) {
     this.nextProtocolVersion = nextProtocolVersion;
   }
+
+  /**
+   * Gets the detected {@link ConnectionType}
+   * @return The {@link ConnectionType}
+   */
+  public ConnectionType getType() {
+    return connectionType;
+  }
+
+  /**
+   * Sets the detected {@link ConnectionType}
+   * @param connectionType The {@link ConnectionType}
+   */
+  public void setType(ConnectionType connectionType) {
+    this.connectionType = connectionType;
+  }
 }

proxy/src/main/java/com/velocitypowered/proxy/connection/backend/BackendConnectionPhase.java

@@ -0,0 +1,46 @@
+package com.velocitypowered.proxy.connection.backend;
+
+import com.velocitypowered.proxy.connection.client.ConnectedPlayer;
+import com.velocitypowered.proxy.connection.forge.legacy.LegacyForgeHandshakeBackendPhase;
+import com.velocitypowered.proxy.protocol.packet.PluginMessage;
+
+/**
+ * Provides connection phase specific actions.
+ *
+ * <p>Note that Forge phases are found in the enum
+ * {@link LegacyForgeHandshakeBackendPhase}.</p>
+ */
+public interface BackendConnectionPhase {
+
+  /**
+   * Handle a plugin message in the context of
+   * this phase.
+   *
+   * @param message The message to handle
+   * @return true if handled, false otherwise.
+   */
+  default boolean handle(VelocityServerConnection server,
+                         ConnectedPlayer player,
+                         PluginMessage message) {
+    return false;
+  }
+
+  /**
+   * Indicates whether the connection is considered complete
+   * @return true if so
+   */
+  default boolean consideredComplete() {
+    return true;
+  }
+
+  /**
+   * Fired when the provided server connection is about to be terminated
+   * because the provided player is connecting to a new server.
+   *
+   * @param serverConnection The server the player is disconnecting from
+   * @param player The player
+   */
+  default void onDepartForNewServer(VelocityServerConnection serverConnection,
+                                    ConnectedPlayer player) {
+  }
+}

proxy/src/main/java/com/velocitypowered/proxy/connection/backend/BackendConnectionPhases.java

@@ -0,0 +1,42 @@
+package com.velocitypowered.proxy.connection.backend;
+
+import com.velocitypowered.proxy.connection.client.ConnectedPlayer;
+import com.velocitypowered.proxy.connection.forge.legacy.LegacyForgeHandshakeBackendPhase;
+import com.velocitypowered.proxy.protocol.packet.PluginMessage;
+
+/**
+ * Contains Vanilla {@link BackendConnectionPhase}s.
+ *
+ * <p>See {@link LegacyForgeHandshakeBackendPhase} for Legacy Forge
+ * versions</p>
+ */
+public final class BackendConnectionPhases {
+
+  /**
+   * The backend connection is vanilla.
+   */
+  public static final BackendConnectionPhase VANILLA = new BackendConnectionPhase() {};
+
+  /**
+   * The backend connection is unknown at this time.
+   */
+  public static final BackendConnectionPhase UNKNOWN = new BackendConnectionPhase() {
+    @Override
+    public boolean consideredComplete() {
+      return false;
+    }
+
+    @Override
+    public boolean handle(VelocityServerConnection serverConn,
+                          ConnectedPlayer player,
+                          PluginMessage message) {
+      // The connection may be legacy forge. If so, the Forge handler will deal with this
+      // for us. Otherwise, we have nothing to do.
+      return LegacyForgeHandshakeBackendPhase.NOT_STARTED.handle(serverConn, player, message);
+    }
+  };
+
+  private BackendConnectionPhases() {
+    throw new AssertionError();
+  }
+}

proxy/src/main/java/com/velocitypowered/proxy/connection/backend/BackendPlaySessionHandler.java

@@ -7,7 +7,7 @@ import com.velocitypowered.proxy.VelocityServer;
 import com.velocitypowered.proxy.connection.MinecraftConnection;
 import com.velocitypowered.proxy.connection.MinecraftSessionHandler;
 import com.velocitypowered.proxy.connection.client.ClientPlaySessionHandler;
-import com.velocitypowered.proxy.connection.forge.ForgeConstants;
+import com.velocitypowered.proxy.connection.forge.legacy.LegacyForgeConstants;
 import com.velocitypowered.proxy.connection.util.ConnectionMessages;
 import com.velocitypowered.proxy.protocol.MinecraftPacket;
 import com.velocitypowered.proxy.protocol.packet.BossBar;
@@ -94,18 +94,9 @@ public class BackendPlaySessionHandler implements MinecraftSessionHandler {
       return true;
     }
 
-    if (!serverConn.hasCompletedJoin() && packet.getChannel()
-        .equals(ForgeConstants.FORGE_LEGACY_HANDSHAKE_CHANNEL)) {
-      if (!serverConn.isLegacyForge()) {
-        serverConn.setLegacyForge(true);
-
-        // We must always reset the handshake before a modded connection is established if
-        // we haven't done so already.
-        serverConn.getPlayer().sendLegacyForgeHandshakeResetPacket();
-      }
-
-      // Always forward these messages during login.
-      return false;
+    if (serverConn.getPhase().handle(serverConn, serverConn.getPlayer(), packet)) {
+      // Handled.
+      return true;
     }
 
     ChannelIdentifier id = server.getChannelRegistrar().getFromId(packet.getChannel());
@@ -173,7 +164,7 @@ public class BackendPlaySessionHandler implements MinecraftSessionHandler {
     if (mc.getProtocolVersion().compareTo(ProtocolVersion.MINECRAFT_1_12_2) <= 0) {
       String channel = message.getChannel();
       minecraftOrFmlMessage = channel.startsWith("MC|") || channel
-          .startsWith(ForgeConstants.FORGE_LEGACY_HANDSHAKE_CHANNEL);
+          .startsWith(LegacyForgeConstants.FORGE_LEGACY_HANDSHAKE_CHANNEL);
     } else {
       minecraftOrFmlMessage = message.getChannel().startsWith("minecraft:");
     }

proxy/src/main/java/com/velocitypowered/proxy/connection/backend/LoginSessionHandler.java

@@ -117,14 +117,15 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
       serverConn.getPlayer().getConnection()
           .setSessionHandler(new ClientPlaySessionHandler(server, serverConn.getPlayer()));
     } else {
-      // If the server we are departing is modded, we must always reset the client's handshake.
-      if (existingConnection.isLegacyForge()) {
-        serverConn.getPlayer().sendLegacyForgeHandshakeResetPacket();
-      }
+      // For Legacy Forge
+      existingConnection.getPhase().onDepartForNewServer(serverConn, serverConn.getPlayer());
 
       // Shut down the existing server connection.
       serverConn.getPlayer().setConnectedServer(null);
       existingConnection.disconnect();
+
+      // Send keep alive to try to avoid timeouts
+      serverConn.getPlayer().sendKeepAlive();
     }
 
     smc.getChannel().config().setAutoRead(false);

proxy/src/main/java/com/velocitypowered/proxy/connection/backend/VelocityServerConnection.java

@@ -17,10 +17,12 @@ import com.velocitypowered.api.proxy.server.ServerInfo;
 import com.velocitypowered.api.network.ProtocolVersion;
 import com.velocitypowered.proxy.VelocityServer;
 import com.velocitypowered.proxy.config.PlayerInfoForwarding;
+import com.velocitypowered.proxy.connection.ConnectionTypes;
 import com.velocitypowered.proxy.connection.MinecraftConnection;
 import com.velocitypowered.proxy.connection.MinecraftConnectionAssociation;
 import com.velocitypowered.proxy.connection.client.ConnectedPlayer;
 import com.velocitypowered.proxy.protocol.ProtocolUtils;
+import com.velocitypowered.proxy.connection.forge.legacy.LegacyForgeConstants;
 import com.velocitypowered.proxy.protocol.StateRegistry;
 import com.velocitypowered.proxy.protocol.netty.MinecraftDecoder;
 import com.velocitypowered.proxy.protocol.netty.MinecraftEncoder;
@@ -44,9 +46,9 @@ public class VelocityServerConnection implements MinecraftConnectionAssociation,
   private final ConnectedPlayer proxyPlayer;
   private final VelocityServer server;
   private @Nullable MinecraftConnection connection;
-  private boolean legacyForge = false;
   private boolean hasCompletedJoin = false;
   private boolean gracefulDisconnect = false;
+  private BackendConnectionPhase connectionPhase = BackendConnectionPhases.UNKNOWN;
   private long lastPingId;
   private long lastPingSent;
 
@@ -93,6 +95,10 @@ public class VelocityServerConnection implements MinecraftConnectionAssociation,
             // Kick off the connection process
             connection.setSessionHandler(
                 new LoginSessionHandler(server, VelocityServerConnection.this, result));
+
+            // Set the connection phase, which may, for future forge (or whatever), be determined
+            // at this point already
+            connectionPhase = connection.getType().getInitialBackendPhase();
             startHandshake();
           } else {
             // We need to remember to reset the in-flight connection to allow connect() to work
@@ -133,8 +139,8 @@ public class VelocityServerConnection implements MinecraftConnectionAssociation,
     handshake.setProtocolVersion(proxyPlayer.getConnection().getNextProtocolVersion());
     if (forwardingMode == PlayerInfoForwarding.LEGACY) {
       handshake.setServerAddress(createLegacyForwardingAddress());
-    } else if (proxyPlayer.getConnection().isLegacyForge()) {
-      handshake.setServerAddress(handshake.getServerAddress() + "\0FML\0");
+    } else if (proxyPlayer.getConnection().getType() == ConnectionTypes.LEGACY_FORGE) {
+      handshake.setServerAddress(handshake.getServerAddress() + LegacyForgeConstants.HANDSHAKE_HOSTNAME_TOKEN);
     } else {
       handshake.setServerAddress(registeredServer.getServerInfo().getAddress().getHostString());
     }
@@ -198,20 +204,17 @@ public class VelocityServerConnection implements MinecraftConnectionAssociation,
     return true;
   }
 
-  public boolean isLegacyForge() {
-    return legacyForge;
-  }
-
-  void setLegacyForge(boolean modded) {
-    legacyForge = modded;
-  }
-
-  public boolean hasCompletedJoin() {
-    return hasCompletedJoin;
-  }
-
-  public void setHasCompletedJoin(boolean hasCompletedJoin) {
-    this.hasCompletedJoin = hasCompletedJoin;
+  public void completeJoin() {
+    if (!hasCompletedJoin) {
+      hasCompletedJoin = true;
+      if (connectionPhase == BackendConnectionPhases.UNKNOWN) {
+        // Now we know
+        connectionPhase = BackendConnectionPhases.VANILLA;
+        if (connection != null) {
+          connection.setType(ConnectionTypes.VANILLA);
+        }
+      }
+    }
   }
 
   boolean isGracefulDisconnect() {
@@ -245,4 +248,35 @@ public class VelocityServerConnection implements MinecraftConnectionAssociation,
     return connection != null && !connection.isClosed() && !gracefulDisconnect
         && proxyPlayer.isActive();
   }
+
+  /**
+   * Gets the current "phase" of the connection, mostly used for tracking
+   * modded negotiation for legacy forge servers and provides methods
+   * for performing phase specific actions.
+   *
+   * @return The {@link BackendConnectionPhase}
+   */
+  public BackendConnectionPhase getPhase() {
+    return connectionPhase;
+  }
+
+  /**
+   * Sets the current "phase" of the connection. See {@link #getPhase()}
+   *
+   * @param connectionPhase The {@link BackendConnectionPhase}
+   */
+  public void setConnectionPhase(BackendConnectionPhase connectionPhase) {
+    this.connectionPhase = connectionPhase;
+  }
+
+  /**
+   * Gets whether the {@link com.velocitypowered.proxy.protocol.packet.JoinGame}
+   * packet has been sent by this server.
+   *
+   * @return Whether the join has been completed.
+   */
+  public boolean hasCompletedJoin() {
+    return hasCompletedJoin;
+  }
+
 }

proxy/src/main/java/com/velocitypowered/proxy/connection/client/ClientConnectionPhase.java

@@ -0,0 +1,55 @@
+package com.velocitypowered.proxy.connection.client;
+
+import com.velocitypowered.proxy.connection.forge.legacy.LegacyForgeHandshakeClientPhase;
+import com.velocitypowered.proxy.protocol.packet.PluginMessage;
+
+/**
+ * Provides connection phase specific actions.
+ *
+ * <p>Note that Forge phases are found in the enum
+ * {@link LegacyForgeHandshakeClientPhase}.</p>
+ */
+public interface ClientConnectionPhase {
+
+  /**
+   * Handle a plugin message in the context of
+   * this phase.
+   *
+   * @param player The player
+   * @param handler The {@link ClientPlaySessionHandler} that is handling
+   *                packets
+   * @param message The message to handle
+   * @return true if handled, false otherwise.
+   */
+  default boolean handle(ConnectedPlayer player,
+                         ClientPlaySessionHandler handler,
+                         PluginMessage message) {
+    return false;
+  }
+
+  /**
+   * Instruct Velocity to reset the connection phase
+   * back to its default for the connection type.
+   *
+   * @param player The player
+   */
+  default void resetConnectionPhase(ConnectedPlayer player) {
+  }
+
+  /**
+   * Perform actions just as the player joins the
+   * server.
+   *
+   * @param player The player
+   */
+  default void onFirstJoin(ConnectedPlayer player) {
+  }
+
+  /**
+   * Indicates whether the connection is considered complete.
+   * @return true if so
+   */
+  default boolean consideredComplete() {
+    return true;
+  }
+}

proxy/src/main/java/com/velocitypowered/proxy/connection/client/ClientConnectionPhases.java

@@ -0,0 +1,19 @@
+package com.velocitypowered.proxy.connection.client;
+
+/**
+ * The vanilla {@link ClientConnectionPhase}s.
+ *
+ * <p>See {@link com.velocitypowered.proxy.connection.forge.legacy.LegacyForgeHandshakeClientPhase}
+ * for Legacy Forge phases</p>
+ */
+public final class ClientConnectionPhases {
+
+  /**
+   * The client is connecting with a vanilla client (as far as we can tell).
+   */
+  public static final ClientConnectionPhase VANILLA = new ClientConnectionPhase() {};
+
+  private ClientConnectionPhases() {
+    throw new AssertionError();
+  }
+}

proxy/src/main/java/com/velocitypowered/proxy/connection/client/ClientPlaySessionHandler.java

@@ -3,15 +3,13 @@ package com.velocitypowered.proxy.connection.client;
 import com.velocitypowered.api.event.connection.PluginMessageEvent;
 import com.velocitypowered.api.event.player.PlayerChatEvent;
 import com.velocitypowered.api.proxy.messages.ChannelIdentifier;
-import com.velocitypowered.api.util.ModInfo;
 import com.velocitypowered.api.network.ProtocolVersion;
 import com.velocitypowered.proxy.VelocityServer;
 import com.velocitypowered.proxy.connection.MinecraftConnection;
 import com.velocitypowered.proxy.connection.MinecraftSessionHandler;
 import com.velocitypowered.proxy.connection.backend.VelocityServerConnection;
-import com.velocitypowered.proxy.connection.forge.ForgeConstants;
-import com.velocitypowered.proxy.connection.forge.ForgeUtil;
 import com.velocitypowered.proxy.protocol.MinecraftPacket;
+import com.velocitypowered.proxy.protocol.StateRegistry;
 import com.velocitypowered.proxy.protocol.packet.BossBar;
 import com.velocitypowered.proxy.protocol.packet.Chat;
 import com.velocitypowered.proxy.protocol.packet.ClientSettings;
@@ -160,7 +158,9 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
     VelocityServerConnection serverConn = player.getConnectedServer();
     MinecraftConnection backendConn = serverConn != null ? serverConn.getConnection() : null;
     if (serverConn != null && backendConn != null) {
-      if (PluginMessageUtil.isRegister(packet)) {
+      if (backendConn.getState() != StateRegistry.PLAY) {
+        logger.warn("Plugin message was sent while the backend was in PLAY. Channel: {}. Packet discarded.");
+      } else if (PluginMessageUtil.isRegister(packet)) {
         List<String> actuallyRegistered = new ArrayList<>();
         List<String> channels = PluginMessageUtil.getChannels(packet);
         for (String channel : channels) {
@@ -184,33 +184,26 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
       } else if (PluginMessageUtil.isMcBrand(packet)) {
         PluginMessage rewritten = PluginMessageUtil.rewriteMinecraftBrand(packet, server.getVersion());
         backendConn.write(rewritten);
-      } else if (backendConn.isLegacyForge() && !serverConn.hasCompletedJoin()) {
-        if (packet.getChannel().equals(ForgeConstants.FORGE_LEGACY_HANDSHAKE_CHANNEL)) {
-          if (!player.getModInfo().isPresent()) {
-            List<ModInfo.Mod> mods = ForgeUtil.readModList(packet);
-            if (!mods.isEmpty()) {
-              player.setModInfo(new ModInfo("FML", mods));
-            }
-          }
+      } else if (!player.getPhase().handle(player, this, packet)) {
 
-          // Always forward the FML handshake to the remote server.
-          backendConn.write(packet);
+        if (!player.getPhase().consideredComplete()
+            || !serverConn.getPhase().consideredComplete()) {
+
+            // The client is trying to send messages too early. This is primarily caused by mods, but
+            // it's further aggravated by Velocity. To work around these issues, we will queue any
+            // non-FML handshake messages to be sent once the FML handshake has completed or the JoinGame
+            // packet has been received by the proxy, whichever comes first.
+            loginPluginMessages.add(packet);
         } else {
-          // The client is trying to send messages too early. This is primarily caused by mods, but
-          // it's further aggravated by Velocity. To work around these issues, we will queue any
-          // non-FML handshake messages to be sent once the JoinGame packet has been received by the
-          // proxy.
-          loginPluginMessages.add(packet);
-        }
-      } else {
-        ChannelIdentifier id = server.getChannelRegistrar().getFromId(packet.getChannel());
-        if (id == null) {
-          backendConn.write(packet);
-        } else {
-          PluginMessageEvent event = new PluginMessageEvent(player, serverConn, id,
-              packet.getData());
-          server.getEventManager().fire(event).thenAcceptAsync(pme -> backendConn.write(packet),
-              backendConn.eventLoop());
+          ChannelIdentifier id = server.getChannelRegistrar().getFromId(packet.getChannel());
+          if (id == null) {
+            backendConn.write(packet);
+          } else {
+            PluginMessageEvent event = new PluginMessageEvent(player, serverConn, id,
+                packet.getData());
+            server.getEventManager().fire(event).thenAcceptAsync(pme -> backendConn.write(packet),
+                backendConn.eventLoop());
+          }
         }
       }
     }
@@ -227,7 +220,7 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
     }
 
     MinecraftConnection smc = serverConnection.getConnection();
-    if (smc != null && serverConnection.hasCompletedJoin()) {
+    if (smc != null && serverConnection.getPhase().consideredComplete()) {
       smc.write(packet);
     }
   }
@@ -241,7 +234,7 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
     }
 
     MinecraftConnection smc = serverConnection.getConnection();
-    if (smc != null && serverConnection.hasCompletedJoin()) {
+    if (smc != null && serverConnection.getPhase().consideredComplete()) {
       smc.write(buf.retain());
     }
   }
@@ -289,15 +282,8 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
       spawned = true;
       player.getConnection().delayedWrite(joinGame);
 
-      // We have something special to do for legacy Forge servers - during first connection the FML
-      // handshake will transition to complete regardless. Thus, we need to ensure that a reset
-      // packet is ALWAYS sent on first switch.
-      //
-      // As we know that calling this branch only happens on first join, we set that if we are a
-      // Forge client that we must reset on the next switch.
-      //
-      // The call will handle if the player is not a Forge player appropriately.
-      player.getConnection().setCanSendLegacyFmlResetPacket(true);
+      // Required for Legacy Forge
+      player.getPhase().onFirstJoin(player);
     } else {
       // Clear tab list to avoid duplicate entries
       player.getTabList().clearAll();
@@ -359,21 +345,7 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
     // Flush everything
     player.getConnection().flush();
     serverMc.flush();
-    serverConn.setHasCompletedJoin(true);
-    if (serverConn.isLegacyForge()) {
-      // We only need to indicate we can send a reset packet if we complete a handshake, that is,
-      // logged onto a Forge server.
-      //
-      // The special case is if we log onto a Vanilla server as our first server, FML will treat
-      // this  as complete and **will** need a reset packet sending at some point. We will handle
-      // this during initial player connection if the player is detected to be forge.
-      //
-      // We do not use the result of VelocityServerConnection#isLegacyForge() directly because we
-      // don't want to set it false if this is a first connection to a Vanilla server.
-      //
-      // See LoginSessionHandler#handle for where the counterpart to this method is
-      player.getConnection().setCanSendLegacyFmlResetPacket(true);
-    }
+    serverConn.completeJoin();
   }
 
   public List<UUID> getServerBossBars() {
@@ -401,4 +373,20 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
       player.getConnection().write(response);
     }
   }
+
+  /**
+   * Immediately send any queued messages to the server.
+   */
+  public void flushQueuedMessages() {
+    VelocityServerConnection serverConnection = player.getConnectedServer();
+    if (serverConnection != null) {
+      MinecraftConnection connection = serverConnection.getConnection();
+      if (connection != null) {
+        PluginMessage pm;
+        while ((pm = loginPluginMessages.poll()) != null) {
+          connection.write(pm);
+        }
+      }
+    }
+  }
 }

proxy/src/main/java/com/velocitypowered/proxy/connection/client/ConnectedPlayer.java

@@ -30,12 +30,13 @@ import com.velocitypowered.proxy.VelocityServer;
 import com.velocitypowered.proxy.connection.MinecraftConnection;
 import com.velocitypowered.proxy.connection.MinecraftConnectionAssociation;
 import com.velocitypowered.proxy.connection.backend.VelocityServerConnection;
-import com.velocitypowered.proxy.connection.forge.ForgeConstants;
 import com.velocitypowered.proxy.connection.util.ConnectionMessages;
 import com.velocitypowered.proxy.connection.util.ConnectionRequestResults;
+import com.velocitypowered.proxy.protocol.StateRegistry;
 import com.velocitypowered.proxy.protocol.packet.Chat;
 import com.velocitypowered.proxy.protocol.packet.ClientSettings;
 import com.velocitypowered.proxy.protocol.packet.Disconnect;
+import com.velocitypowered.proxy.protocol.packet.KeepAlive;
 import com.velocitypowered.proxy.protocol.packet.PluginMessage;
 import com.velocitypowered.proxy.protocol.packet.TitlePacket;
 import com.velocitypowered.proxy.server.VelocityRegisteredServer;
@@ -48,6 +49,7 @@ import java.util.Optional;
 import java.util.UUID;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.CompletionException;
+import java.util.concurrent.ThreadLocalRandom;
 import net.kyori.text.Component;
 import net.kyori.text.TextComponent;
 import net.kyori.text.TranslatableComponent;
@@ -79,6 +81,7 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player {
   private @Nullable ModInfo modInfo;
   private final VelocityTabList tabList;
   private final VelocityServer server;
+  private ClientConnectionPhase connectionPhase;
 
   @MonotonicNonNull
   private List<String> serversToTry = null;
@@ -91,6 +94,7 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player {
     this.connection = connection;
     this.virtualHost = virtualHost;
     this.permissionFunction = PermissionFunction.ALWAYS_UNDEFINED;
+    this.connectionPhase = connection.getType().getInitialClientPhase();
   }
 
   @Override
@@ -139,7 +143,7 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player {
     return Optional.ofNullable(modInfo);
   }
 
-  void setModInfo(ModInfo modInfo) {
+  public void setModInfo(ModInfo modInfo) {
     this.modInfo = modInfo;
     server.getEventManager().fireAndForget(new PlayerModInfoEvent(this, modInfo));
   }
@@ -409,10 +413,7 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player {
   }
 
   public void sendLegacyForgeHandshakeResetPacket() {
-    if (connection.canSendLegacyFmlResetPacket()) {
-      connection.write(ForgeConstants.resetPacket());
-      connection.setCanSendLegacyFmlResetPacket(false);
-    }
+    connectionPhase.resetConnectionPhase(this);
   }
 
   private MinecraftConnection ensureBackendConnection() {
@@ -469,6 +470,39 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player {
     ensureBackendConnection().write(Chat.createServerbound(input));
   }
 
+  /**
+   * Sends a {@link KeepAlive} packet to the player with a random ID.
+   * The response will be ignored by Velocity as it will not match the
+   * ID last sent by the server.
+   */
+  public void sendKeepAlive() {
+    if (connection.getState() == StateRegistry.PLAY) {
+      KeepAlive keepAlive = new KeepAlive();
+      keepAlive.setRandomId(ThreadLocalRandom.current().nextLong());
+      connection.write(keepAlive);
+    }
+  }
+
+  /**
+   * Gets the current "phase" of the connection, mostly used for tracking
+   * modded negotiation for legacy forge servers and provides methods
+   * for performing phase specific actions.
+   *
+   * @return The {@link ClientConnectionPhase}
+   */
+  public ClientConnectionPhase getPhase() {
+    return connectionPhase;
+  }
+
+  /**
+   * Sets the current "phase" of the connection. See {@link #getPhase()}
+   *
+   * @param connectionPhase The {@link ClientConnectionPhase}
+   */
+  public void setPhase(ClientConnectionPhase connectionPhase) {
+    this.connectionPhase = connectionPhase;
+  }
+
   private class ConnectionRequestBuilderImpl implements ConnectionRequestBuilder {
 
     private final RegisteredServer toConnect;

proxy/src/main/java/com/velocitypowered/proxy/connection/client/HandshakeSessionHandler.java

@@ -10,8 +10,11 @@ import com.velocitypowered.api.network.ProtocolVersion;
 import com.velocitypowered.proxy.VelocityServer;
 import com.velocitypowered.proxy.config.PlayerInfoForwarding;
 import com.velocitypowered.proxy.config.VelocityConfiguration;
+import com.velocitypowered.proxy.connection.ConnectionType;
+import com.velocitypowered.proxy.connection.ConnectionTypes;
 import com.velocitypowered.proxy.connection.MinecraftConnection;
 import com.velocitypowered.proxy.connection.MinecraftSessionHandler;
+import com.velocitypowered.proxy.connection.forge.legacy.LegacyForgeConstants;
 import com.velocitypowered.proxy.protocol.MinecraftPacket;
 import com.velocitypowered.proxy.protocol.StateRegistry;
 import com.velocitypowered.proxy.protocol.packet.Disconnect;
@@ -95,13 +98,11 @@ public class HandshakeSessionHandler implements MinecraftSessionHandler {
           return true;
         }
 
-        // Determine if we're using Forge (1.8 to 1.12, may not be the case in 1.13).
-        boolean isForge = handshake.getServerAddress().endsWith("\0FML\0");
-        connection.setLegacyForge(isForge);
+        ConnectionType type = checkForForge(handshake);
+        connection.setType(type);
 
-        // Make sure legacy forwarding is not in use on this connection. Make sure that we do _not_
-        // reject Forge.
-        if (handshake.getServerAddress().contains("\0") && !isForge) {
+        // Make sure legacy forwarding is not in use on this connection.
+        if (!type.checkServerAddressIsValid(handshake.getServerAddress())) {
           connection.closeWith(Disconnect
               .create(TextComponent.of("Running Velocity behind Velocity is unsupported.")));
           return true;
@@ -124,6 +125,18 @@ public class HandshakeSessionHandler implements MinecraftSessionHandler {
     }
   }
 
+  private ConnectionType checkForForge(Handshake handshake) {
+    // Determine if we're using Forge (1.8 to 1.12, may not be the case in 1.13).
+    if (handshake.getServerAddress().endsWith(LegacyForgeConstants.HANDSHAKE_HOSTNAME_TOKEN)
+        && handshake.getProtocolVersion().getProtocol() < ProtocolVersion.MINECRAFT_1_13.getProtocol()) {
+      return ConnectionTypes.LEGACY_FORGE;
+    } else {
+      // For later: See if we can determine Forge 1.13+ here, else this will need to be UNDETERMINED
+      // until later in the cycle (most likely determinable during the LOGIN phase)
+      return ConnectionTypes.VANILLA;
+    }
+  }
+
   private String cleanVhost(String hostname) {
     int zeroIdx = hostname.indexOf('\0');
     return zeroIdx == -1 ? hostname : hostname.substring(0, zeroIdx);

proxy/src/main/java/com/velocitypowered/proxy/connection/client/LoginSessionHandler.java

@@ -17,7 +17,6 @@ import com.velocitypowered.api.proxy.server.RegisteredServer;
 import com.velocitypowered.api.util.GameProfile;
 import com.velocitypowered.api.network.ProtocolVersion;
 import com.velocitypowered.proxy.VelocityServer;
-import com.velocitypowered.proxy.config.PlayerInfoForwarding;
 import com.velocitypowered.proxy.connection.MinecraftConnection;
 import com.velocitypowered.proxy.connection.MinecraftSessionHandler;
 
@@ -54,8 +53,6 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
   private static final Logger logger = LogManager.getLogger(LoginSessionHandler.class);
   private static final String MOJANG_HASJOINED_URL =
       "https://sessionserver.mojang.com/session/minecraft/hasJoined?username=%s&serverId=%s&ip=%s";
-  private static final GameProfile.Property IS_FORGE_CLIENT_PROPERTY =
-      new GameProfile.Property("forgeClient", "true", "");
 
   private final VelocityServer server;
   private final MinecraftConnection inbound;
@@ -214,14 +211,9 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
   }
 
   private void initializePlayer(GameProfile profile, boolean onlineMode) {
-    if (inbound.isLegacyForge() && server.getConfiguration().getPlayerInfoForwardingMode()
-        == PlayerInfoForwarding.LEGACY) {
-      // We can't forward the FML token to the server when we are running in legacy forwarding mode,
-      // since both use the "hostname" field in the handshake. We add a special property to the
-      // profile instead, which will be ignored by non-Forge servers and can be intercepted by a
-      // Forge coremod, such as SpongeForge.
-      profile = profile.addProperty(IS_FORGE_CLIENT_PROPERTY);
-    }
+    // Some connection types may need to alter the game profile.
+    profile = inbound.getType().addGameProfileTokensIfRequired(profile,
+        server.getConfiguration().getPlayerInfoForwardingMode());
     GameProfileRequestEvent profileRequestEvent = new GameProfileRequestEvent(apiInbound, profile,
         onlineMode);
 

proxy/src/main/java/com/velocitypowered/proxy/connection/forge/ForgeConstants.java

@@ -1,22 +0,0 @@
-package com.velocitypowered.proxy.connection.forge;
-
-import com.velocitypowered.proxy.protocol.packet.PluginMessage;
-
-public class ForgeConstants {
-
-  public static final String FORGE_LEGACY_HANDSHAKE_CHANNEL = "FML|HS";
-  public static final String FORGE_LEGACY_CHANNEL = "FML";
-  public static final String FORGE_MULTIPART_LEGACY_CHANNEL = "FML|MP";
-  private static final byte[] FORGE_LEGACY_HANDSHAKE_RESET_DATA = new byte[]{-2, 0};
-
-  private ForgeConstants() {
-    throw new AssertionError();
-  }
-
-  public static PluginMessage resetPacket() {
-    PluginMessage msg = new PluginMessage();
-    msg.setChannel(FORGE_LEGACY_HANDSHAKE_CHANNEL);
-    msg.setData(FORGE_LEGACY_HANDSHAKE_RESET_DATA.clone());
-    return msg;
-  }
-}

proxy/src/main/java/com/velocitypowered/proxy/connection/forge/ForgeUtil.java

@@ -1,46 +0,0 @@
-package com.velocitypowered.proxy.connection.forge;
-
-import com.google.common.base.Preconditions;
-import com.google.common.collect.ImmutableList;
-import com.velocitypowered.api.util.ModInfo;
-import com.velocitypowered.proxy.protocol.ProtocolUtils;
-import com.velocitypowered.proxy.protocol.packet.PluginMessage;
-import io.netty.buffer.ByteBuf;
-import io.netty.buffer.Unpooled;
-import java.util.List;
-
-public class ForgeUtil {
-
-  private ForgeUtil() {
-    throw new AssertionError();
-  }
-
-  public static List<ModInfo.Mod> readModList(PluginMessage message) {
-    Preconditions.checkNotNull(message, "message");
-    Preconditions
-        .checkArgument(message.getChannel().equals(ForgeConstants.FORGE_LEGACY_HANDSHAKE_CHANNEL),
-            "message is not a FML HS plugin message");
-
-    ByteBuf byteBuf = Unpooled.wrappedBuffer(message.getData());
-    try {
-      byte discriminator = byteBuf.readByte();
-
-      if (discriminator == 2) {
-        ImmutableList.Builder<ModInfo.Mod> mods = ImmutableList.builder();
-        int modCount = ProtocolUtils.readVarInt(byteBuf);
-
-        for (int index = 0; index < modCount; index++) {
-          String id = ProtocolUtils.readString(byteBuf);
-          String version = ProtocolUtils.readString(byteBuf);
-          mods.add(new ModInfo.Mod(id, version));
-        }
-
-        return mods.build();
-      }
-
-      return ImmutableList.of();
-    } finally {
-      byteBuf.release();
-    }
-  }
-}

proxy/src/main/java/com/velocitypowered/proxy/connection/forge/legacy/LegacyForgeConnectionType.java

@@ -0,0 +1,38 @@
+package com.velocitypowered.proxy.connection.forge.legacy;
+
+import com.velocitypowered.api.util.GameProfile;
+import com.velocitypowered.proxy.config.PlayerInfoForwarding;
+import com.velocitypowered.proxy.connection.ConnectionTypes;
+import com.velocitypowered.proxy.connection.util.ConnectionTypeImpl;
+
+/**
+ * Contains extra logic for {@link ConnectionTypes#LEGACY_FORGE}
+ */
+public class LegacyForgeConnectionType extends ConnectionTypeImpl {
+
+  private static final GameProfile.Property IS_FORGE_CLIENT_PROPERTY =
+      new GameProfile.Property("forgeClient", "true", "");
+
+  public LegacyForgeConnectionType() {
+    super(LegacyForgeHandshakeClientPhase.NOT_STARTED, LegacyForgeHandshakeBackendPhase.NOT_STARTED);
+  }
+
+  @Override
+  public GameProfile addGameProfileTokensIfRequired(GameProfile original, PlayerInfoForwarding forwardingType) {
+    // We can't forward the FML token to the server when we are running in legacy forwarding mode,
+    // since both use the "hostname" field in the handshake. We add a special property to the
+    // profile instead, which will be ignored by non-Forge servers and can be intercepted by a
+    // Forge coremod, such as SpongeForge.
+    if (forwardingType == PlayerInfoForwarding.LEGACY) {
+      return original.addProperty(IS_FORGE_CLIENT_PROPERTY);
+    }
+
+    return original;
+  }
+
+  @Override
+  public boolean checkServerAddressIsValid(String address) {
+    return super.checkServerAddressIsValid(
+        address.replaceAll(LegacyForgeConstants.HANDSHAKE_HOSTNAME_TOKEN, ""));
+  }
+}

proxy/src/main/java/com/velocitypowered/proxy/connection/forge/legacy/LegacyForgeConstants.java

@@ -0,0 +1,59 @@
+package com.velocitypowered.proxy.connection.forge.legacy;
+
+/**
+ * Constants for use with Legacy Forge systems.
+ */
+public class LegacyForgeConstants {
+
+  /**
+   * Clients attempting to connect to 1.8+ Forge servers will have
+   * this token appended to the hostname in the initial handshake
+   * packet.
+   */
+  public static final String HANDSHAKE_HOSTNAME_TOKEN = "\0FML\0";
+
+  /**
+   * The channel for legacy forge handshakes.
+   */
+  public static final String FORGE_LEGACY_HANDSHAKE_CHANNEL = "FML|HS";
+
+  /**
+   * The reset packet discriminator.
+   */
+  private static final int RESET_DATA_DISCRIMINATOR = -2;
+
+  /**
+   * The acknowledgement packet discriminator.
+   */
+  static final int ACK_DISCRIMINATOR = -1;
+
+  /**
+   * The Server -> Client Hello discriminator.
+   */
+  static final int SERVER_HELLO_DISCRIMINATOR = 0;
+
+  /**
+   * The Client -> Server Hello discriminator.
+   */
+  static final int CLIENT_HELLO_DISCRIMINATOR = 1;
+
+  /**
+   * The Mod List discriminator.
+   */
+  static final int MOD_LIST_DISCRIMINATOR = 2;
+
+  /**
+   * The Registry discriminator.
+   */
+  static final int REGISTRY_DISCRIMINATOR = 3;
+
+  /**
+   * The form of the data for the reset packet
+   */
+  static final byte[] FORGE_LEGACY_HANDSHAKE_RESET_DATA = new byte[]{RESET_DATA_DISCRIMINATOR, 0};
+
+  private LegacyForgeConstants() {
+    throw new AssertionError();
+  }
+
+}

proxy/src/main/java/com/velocitypowered/proxy/connection/forge/legacy/LegacyForgeHandshakeBackendPhase.java

@@ -0,0 +1,173 @@
+package com.velocitypowered.proxy.connection.forge.legacy;
+
+import com.velocitypowered.proxy.connection.ConnectionTypes;
+import com.velocitypowered.proxy.connection.backend.BackendConnectionPhase;
+import com.velocitypowered.proxy.connection.backend.BackendConnectionPhases;
+import com.velocitypowered.proxy.connection.backend.VelocityServerConnection;
+import com.velocitypowered.proxy.connection.client.ConnectedPlayer;
+import com.velocitypowered.proxy.protocol.packet.PluginMessage;
+import javax.annotation.Nullable;
+
+/**
+ * Allows for simple tracking of the phase that the Legacy
+ * Forge handshake is in (server side).
+ */
+public enum LegacyForgeHandshakeBackendPhase implements BackendConnectionPhase {
+
+  /**
+   * Dummy phase for use with {@link BackendConnectionPhases#UNKNOWN}
+   */
+  NOT_STARTED(LegacyForgeConstants.SERVER_HELLO_DISCRIMINATOR) {
+    @Override
+    LegacyForgeHandshakeBackendPhase nextPhase() {
+      return HELLO;
+    }
+  },
+
+  /**
+   * Sent a hello to the client, waiting for a hello back before sending
+   * the mod list.
+   */
+  HELLO(LegacyForgeConstants.MOD_LIST_DISCRIMINATOR) {
+    @Override
+    LegacyForgeHandshakeBackendPhase nextPhase() {
+      return SENT_MOD_LIST;
+    }
+
+    @Override
+    void onTransitionToNewPhase(VelocityServerConnection connection) {
+      // We must always reset the handshake before a modded connection is established if
+      // we haven't done so already.
+      if (connection.getConnection() != null) {
+        connection.getConnection().setType(ConnectionTypes.LEGACY_FORGE);
+      }
+      connection.getPlayer().sendLegacyForgeHandshakeResetPacket();
+    }
+  },
+
+  /**
+   * The mod list from the client has been accepted and a server mod list
+   * has been sent. Waiting for the client to acknowledge.
+   */
+  SENT_MOD_LIST(LegacyForgeConstants.REGISTRY_DISCRIMINATOR) {
+    @Override
+    LegacyForgeHandshakeBackendPhase nextPhase() {
+      return SENT_SERVER_DATA;
+    }
+  },
+
+  /**
+   * The server data is being sent or has been sent, and is waiting for
+   * the client to acknowledge it has processed this.
+   */
+  SENT_SERVER_DATA(LegacyForgeConstants.ACK_DISCRIMINATOR) {
+    @Override
+    LegacyForgeHandshakeBackendPhase nextPhase() {
+      return WAITING_ACK;
+    }
+  },
+
+  /**
+   * Waiting for the client to acknowledge before completing handshake.
+   */
+  WAITING_ACK(LegacyForgeConstants.ACK_DISCRIMINATOR) {
+    @Override
+    LegacyForgeHandshakeBackendPhase nextPhase() {
+      return COMPLETE;
+    }
+  },
+
+  /**
+   * The server has completed the handshake and will continue after the client ACK.
+   */
+  COMPLETE(null) {
+    @Override
+    public boolean consideredComplete() {
+      return true;
+    }
+  };
+
+  @Nullable private final Integer packetToAdvanceOn;
+
+  /**
+   * Creates an instance of the {@link LegacyForgeHandshakeClientPhase}.
+   *
+   * @param packetToAdvanceOn The ID of the packet discriminator that indicates
+   *                          that the server has moved onto a new phase, and
+   *                          as such, Velocity should do so too (inspecting
+   *                          {@link #nextPhase()}. A null indicates there is no
+   *                          further phase to transition to.
+   */
+  LegacyForgeHandshakeBackendPhase(@Nullable Integer packetToAdvanceOn) {
+    this.packetToAdvanceOn = packetToAdvanceOn;
+  }
+
+  @Override
+  public final boolean handle(VelocityServerConnection serverConnection,
+                              ConnectedPlayer player,
+                              PluginMessage message) {
+    if (message.getChannel().equals(LegacyForgeConstants.FORGE_LEGACY_HANDSHAKE_CHANNEL)) {
+      // Get the phase and check if we need to start the next phase.
+      LegacyForgeHandshakeBackendPhase newPhase = getNewPhase(serverConnection, message);
+
+      // Update phase on server
+      serverConnection.setConnectionPhase(newPhase);
+
+      // Write the packet to the player, we don't need it now.
+      player.getConnection().write(message);
+      return true;
+    }
+
+    // Not handled, fallback
+    return false;
+  }
+
+  @Override
+  public boolean consideredComplete() {
+    return false;
+  }
+
+  @Override
+  public void onDepartForNewServer(VelocityServerConnection serverConnection,
+                                   ConnectedPlayer player) {
+    // If the server we are departing is modded, we must always reset the client's handshake.
+    player.getPhase().resetConnectionPhase(player);
+  }
+
+  /**
+   * Performs any specific tasks when moving to a new phase.
+   *
+   * @param connection The server connection
+   */
+  void onTransitionToNewPhase(VelocityServerConnection connection) {
+  }
+
+  /**
+   * Gets the next phase, if any (will return self if we are at the end
+   * of the handshake).
+   *
+   * @return The next phase
+   */
+  LegacyForgeHandshakeBackendPhase nextPhase() {
+    return this;
+  }
+
+  /**
+   * Get the phase to act on, depending on the packet that has been sent.
+   *
+   * @param serverConnection The server Velocity is connecting to
+   * @param packet The packet
+   * @return The phase to transition to, which may be the same as before.
+   */
+  private LegacyForgeHandshakeBackendPhase getNewPhase(VelocityServerConnection serverConnection,
+                                                       PluginMessage packet) {
+    if (packetToAdvanceOn != null
+        && LegacyForgeUtil.getHandshakePacketDiscriminator(packet) == packetToAdvanceOn) {
+      LegacyForgeHandshakeBackendPhase phaseToTransitionTo = nextPhase();
+      phaseToTransitionTo.onTransitionToNewPhase(serverConnection);
+      return phaseToTransitionTo;
+    }
+
+    return this;
+  }
+}

proxy/src/main/java/com/velocitypowered/proxy/connection/forge/legacy/LegacyForgeHandshakeClientPhase.java

@@ -0,0 +1,255 @@
+package com.velocitypowered.proxy.connection.forge.legacy;
+
+import com.velocitypowered.api.util.ModInfo;
+import com.velocitypowered.proxy.connection.MinecraftConnection;
+import com.velocitypowered.proxy.connection.backend.VelocityServerConnection;
+import com.velocitypowered.proxy.connection.client.ClientConnectionPhase;
+import com.velocitypowered.proxy.connection.client.ClientPlaySessionHandler;
+import com.velocitypowered.proxy.connection.client.ConnectedPlayer;
+import com.velocitypowered.proxy.protocol.packet.PluginMessage;
+import java.util.List;
+import javax.annotation.Nullable;
+
+/**
+ * Allows for simple tracking of the phase that the Legacy
+ * Forge handshake is in
+ */
+public enum LegacyForgeHandshakeClientPhase implements ClientConnectionPhase {
+
+  /**
+   * No handshake packets have yet been sent.
+   * Transition to {@link #HELLO} when the ClientHello is sent.
+   */
+  NOT_STARTED(LegacyForgeConstants.CLIENT_HELLO_DISCRIMINATOR) {
+    @Override
+    LegacyForgeHandshakeClientPhase nextPhase() {
+      return HELLO;
+    }
+
+    @Override
+    public void onFirstJoin(ConnectedPlayer player) {
+      // We have something special to do for legacy Forge servers - during first connection the FML
+      // handshake will getNewPhase to complete regardless. Thus, we need to ensure that a reset
+      // packet is ALWAYS sent on first switch.
+      //
+      // As we know that calling this branch only happens on first join, we set that if we are a
+      // Forge client that we must reset on the next switch.
+      player.setPhase(LegacyForgeHandshakeClientPhase.COMPLETE);
+    }
+
+    @Override
+    boolean onHandle(ConnectedPlayer player,
+                     ClientPlaySessionHandler handler,
+                     PluginMessage message,
+                     MinecraftConnection backendConn) {
+      // If we stay in this phase, we do nothing because it means the packet wasn't handled.
+      // Returning false indicates this
+      return false;
+    }
+  },
+
+  /**
+   * Client and Server exchange pleasantries.
+   * Transition to {@link #MOD_LIST} when the ModList is sent.
+   */
+  HELLO(LegacyForgeConstants.MOD_LIST_DISCRIMINATOR) {
+    @Override
+    LegacyForgeHandshakeClientPhase nextPhase() {
+      return MOD_LIST;
+    }
+  },
+
+
+
+  /**
+   * The Mod list is sent to the server, captured by Velocity.
+   * Transition to {@link #WAITING_SERVER_DATA} when an ACK is sent, which
+   * indicates to the server to start sending state data.
+   */
+  MOD_LIST(LegacyForgeConstants.ACK_DISCRIMINATOR) {
+    @Override
+    LegacyForgeHandshakeClientPhase nextPhase() {
+      return WAITING_SERVER_DATA;
+    }
+
+    @Override
+    boolean onHandle(ConnectedPlayer player,
+                  ClientPlaySessionHandler handler,
+                  PluginMessage message,
+                  MinecraftConnection backendConn) {
+      // Read the mod list if we haven't already.
+      if (!player.getModInfo().isPresent()) {
+        List<ModInfo.Mod> mods = LegacyForgeUtil.readModList(message);
+        if (!mods.isEmpty()) {
+          player.setModInfo(new ModInfo("FML", mods));
+        }
+      }
+
+      return super.onHandle(player, handler, message, backendConn);
+    }
+  },
+
+  /**
+   * Waiting for state data to be received.
+   * Transition to {@link #WAITING_SERVER_COMPLETE} when this is complete
+   * and the client sends an ACK packet to confirm
+   */
+  WAITING_SERVER_DATA(LegacyForgeConstants.ACK_DISCRIMINATOR) {
+    @Override
+    LegacyForgeHandshakeClientPhase nextPhase() {
+      return WAITING_SERVER_COMPLETE;
+    }
+  },
+
+  /**
+   * Waiting on the server to send another ACK.
+   * Transition to {@link #PENDING_COMPLETE} when client sends another
+   * ACK
+   */
+  WAITING_SERVER_COMPLETE(LegacyForgeConstants.ACK_DISCRIMINATOR) {
+    @Override
+    LegacyForgeHandshakeClientPhase nextPhase() {
+      return PENDING_COMPLETE;
+    }
+  },
+
+  /**
+   * Waiting on the server to send yet another ACK.
+   * Transition to {@link #COMPLETE} when client sends another
+   * ACK
+   */
+  PENDING_COMPLETE(LegacyForgeConstants.ACK_DISCRIMINATOR) {
+    @Override
+    LegacyForgeHandshakeClientPhase nextPhase() {
+      return COMPLETE;
+    }
+  },
+
+  /**
+   * The handshake is complete. The handshake can be reset.
+   *
+   * <p>Note that a successful connection to a server does not mean that
+   * we will be in this state. After a handshake reset, if the next server
+   * is vanilla we will still be in the {@link #NOT_STARTED} phase,
+   * which means we must NOT send a reset packet. This is handled by
+   * overriding the {@link #resetConnectionPhase(ConnectedPlayer)} in this
+   * element (it is usually a no-op).</p>
+   */
+  COMPLETE(null) {
+    @Override
+    public void resetConnectionPhase(ConnectedPlayer player) {
+      player.getConnection().write(LegacyForgeUtil.resetPacket());
+      player.setPhase(LegacyForgeHandshakeClientPhase.NOT_STARTED);
+    }
+
+    @Override
+    public boolean consideredComplete() {
+      return true;
+    }
+
+    @Override
+    boolean onHandle(ConnectedPlayer player,
+                     ClientPlaySessionHandler handler,
+                     PluginMessage message,
+                     MinecraftConnection backendConn) {
+      super.onHandle(player, handler, message, backendConn);
+
+      // just in case the timing is awful
+      player.sendKeepAlive();
+      handler.flushQueuedMessages();
+
+      return true;
+    }
+  };
+
+  @Nullable private final Integer packetToAdvanceOn;
+
+  /**
+   * Creates an instance of the {@link LegacyForgeHandshakeClientPhase}.
+   *
+   * @param packetToAdvanceOn The ID of the packet discriminator that indicates
+   *                          that the client has moved onto a new phase, and
+   *                          as such, Velocity should do so too (inspecting
+   *                          {@link #nextPhase()}. A null indicates there is no
+   *                          further phase to transition to.
+   */
+  LegacyForgeHandshakeClientPhase(Integer packetToAdvanceOn) {
+    this.packetToAdvanceOn = packetToAdvanceOn;
+  }
+
+  @Override
+  public final boolean handle(ConnectedPlayer player,
+                              ClientPlaySessionHandler handler,
+                              PluginMessage message) {
+    VelocityServerConnection serverConn = player.getConnectedServer();
+    if (serverConn != null) {
+      MinecraftConnection backendConn = serverConn.getConnection();
+      if (backendConn != null
+          && message.getChannel().equals(LegacyForgeConstants.FORGE_LEGACY_HANDSHAKE_CHANNEL)) {
+        // Get the phase and check if we need to start the next phase.
+        LegacyForgeHandshakeClientPhase newPhase = getNewPhase(message);
+
+        // Update phase on player
+        player.setPhase(newPhase);
+
+        // Perform phase handling
+        return newPhase.onHandle(player, handler, message, backendConn);
+      }
+    }
+
+    // Not handled, fallback
+    return false;
+  }
+
+  /**
+   * Handles the phase tasks
+   *
+   * @param player The player
+   * @param handler The {@link ClientPlaySessionHandler} that is handling
+   *                packets
+   * @param message The message to handle
+   * @param backendConn The backend connection to write to, if required.
+   *
+   * @return true if handled, false otherwise.
+   */
+  boolean onHandle(ConnectedPlayer player,
+                   ClientPlaySessionHandler handler,
+                   PluginMessage message,
+                   MinecraftConnection backendConn) {
+    // Send the packet on to the server.
+    backendConn.write(message);
+
+    // We handled the packet. No need to continue processing.
+    return true;
+  }
+
+  @Override
+  public boolean consideredComplete() {
+    return false;
+  }
+
+  /**
+   * Gets the next phase, if any (will return self if we are at the end
+   * of the handshake).
+   *
+   * @return The next phase
+   */
+  LegacyForgeHandshakeClientPhase nextPhase() {
+    return this;
+  }
+
+  /**
+   * Get the phase to act on, depending on the packet that has been sent.
+   *
+   * @param packet The packet
+   * @return The phase to transition to, which may be the same as before.
+   */
+  private LegacyForgeHandshakeClientPhase getNewPhase(PluginMessage packet) {
+    if (packetToAdvanceOn != null
+        && LegacyForgeUtil.getHandshakePacketDiscriminator(packet) == packetToAdvanceOn) {
+      return nextPhase();
+    }
+
+    return this;
+  }
+}

proxy/src/main/java/com/velocitypowered/proxy/connection/forge/legacy/LegacyForgeUtil.java

@@ -0,0 +1,81 @@
+package com.velocitypowered.proxy.connection.forge.legacy;
+
+import com.google.common.base.Preconditions;
+import com.google.common.collect.ImmutableList;
+import com.velocitypowered.api.util.ModInfo;
+import com.velocitypowered.proxy.protocol.ProtocolUtils;
+import com.velocitypowered.proxy.protocol.packet.PluginMessage;
+import io.netty.buffer.ByteBuf;
+import io.netty.buffer.Unpooled;
+import java.util.List;
+
+class LegacyForgeUtil {
+
+  private LegacyForgeUtil() {
+    throw new AssertionError();
+  }
+
+  /**
+   * Gets the discriminator from the FML|HS packet (the first byte in the data)
+   *
+   * @param message The message to analyse
+   * @return The discriminator
+   */
+  static byte getHandshakePacketDiscriminator(PluginMessage message) {
+    Preconditions.checkArgument(
+        message.getChannel().equals(LegacyForgeConstants.FORGE_LEGACY_HANDSHAKE_CHANNEL));
+    ByteBuf buf = Unpooled.wrappedBuffer(message.getData());
+    try {
+      return buf.readByte();
+    } finally {
+      buf.release();
+    }
+  }
+
+  /**
+   * Gets the mod list from the mod list packet and parses it.
+   *
+   * @param message The message
+   * @return The list of mods. May be empty.
+   */
+  static List<ModInfo.Mod> readModList(PluginMessage message) {
+    Preconditions.checkNotNull(message, "message");
+    Preconditions
+        .checkArgument(message.getChannel().equals(LegacyForgeConstants.FORGE_LEGACY_HANDSHAKE_CHANNEL),
+            "message is not a FML HS plugin message");
+
+    ByteBuf byteBuf = Unpooled.wrappedBuffer(message.getData());
+    try {
+      byte discriminator = byteBuf.readByte();
+
+      if (discriminator == LegacyForgeConstants.MOD_LIST_DISCRIMINATOR) {
+        ImmutableList.Builder<ModInfo.Mod> mods = ImmutableList.builder();
+        int modCount = ProtocolUtils.readVarInt(byteBuf);
+
+        for (int index = 0; index < modCount; index++) {
+          String id = ProtocolUtils.readString(byteBuf);
+          String version = ProtocolUtils.readString(byteBuf);
+          mods.add(new ModInfo.Mod(id, version));
+        }
+
+        return mods.build();
+      }
+
+      return ImmutableList.of();
+    } finally {
+      byteBuf.release();
+    }
+  }
+
+  /**
+   * Creates a reset packet.
+   *
+   * @return A copy of the reset packet
+   */
+  static PluginMessage resetPacket() {
+    PluginMessage msg = new PluginMessage();
+    msg.setChannel(LegacyForgeConstants.FORGE_LEGACY_HANDSHAKE_CHANNEL);
+    msg.setData(LegacyForgeConstants.FORGE_LEGACY_HANDSHAKE_RESET_DATA.clone());
+    return msg;
+  }
+}

proxy/src/main/java/com/velocitypowered/proxy/connection/util/ConnectionTypeImpl.java

@@ -0,0 +1,39 @@
+package com.velocitypowered.proxy.connection.util;
+
+import com.velocitypowered.api.util.GameProfile;
+import com.velocitypowered.proxy.config.PlayerInfoForwarding;
+import com.velocitypowered.proxy.connection.ConnectionType;
+import com.velocitypowered.proxy.connection.backend.BackendConnectionPhase;
+import com.velocitypowered.proxy.connection.client.ClientConnectionPhase;
+
+/**
+ * Indicates the type of connection that has been made
+ */
+public class ConnectionTypeImpl implements ConnectionType {
+
+  private final ClientConnectionPhase initialClientPhase;
+  private final BackendConnectionPhase initialBackendPhase;
+
+  public ConnectionTypeImpl(ClientConnectionPhase initialClientPhase,
+                            BackendConnectionPhase initialBackendPhase) {
+    this.initialClientPhase = initialClientPhase;
+    this.initialBackendPhase = initialBackendPhase;
+  }
+
+  @Override
+  public final ClientConnectionPhase getInitialClientPhase() {
+    return initialClientPhase;
+  }
+
+  @Override
+  public final BackendConnectionPhase getInitialBackendPhase() {
+    return initialBackendPhase;
+  }
+
+  @Override
+  public GameProfile addGameProfileTokensIfRequired(GameProfile original,
+                                                    PlayerInfoForwarding forwardingType) {
+    return original;
+  }
+}
+