First attempt at getting Forge working nicely with Velocity.

api/src/main/java/com/velocitypowered/api/util/GameProfile.java

@@ -57,7 +57,7 @@ public final class GameProfile {
                 '}';
     }
 
-    public final class Property {
+    public final static class Property {
         private final String name;
         private final String value;
         private final String signature;

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

@@ -45,6 +45,7 @@ public class MinecraftConnection extends ChannelInboundHandlerAdapter {
     private MinecraftSessionHandler sessionHandler;
     private int protocolVersion;
     private MinecraftConnectionAssociation association;
+    private boolean isLegacyForge;
     private final VelocityServer server;
 
     public MinecraftConnection(Channel channel, VelocityServer server) {
@@ -222,4 +223,12 @@ public class MinecraftConnection extends ChannelInboundHandlerAdapter {
     public void setAssociation(MinecraftConnectionAssociation association) {
         this.association = association;
     }
+
+    public boolean isLegacyForge() {
+        return isLegacyForge;
+    }
+
+    public void setLegacyForge(boolean isForge) {
+        this.isLegacyForge = isForge;
+    }
 }

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

@@ -6,4 +6,8 @@ public class VelocityConstants {
     }
 
     public static final String VELOCITY_IP_FORWARDING_CHANNEL = "velocity:player_info";
+
+    public static final String FORGE_LEGACY_HANDSHAKE_CHANNEL = "FML|HS";
+
+    public static final byte[] FORGE_LEGACY_HANDSHAKE_RESET_DATA = new byte[] { -2, 0 };
 }

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

@@ -4,6 +4,7 @@ import com.velocitypowered.api.event.player.ServerConnectedEvent;
 import com.velocitypowered.api.proxy.messages.ChannelSide;
 import com.velocitypowered.api.proxy.messages.MessageHandler;
 import com.velocitypowered.proxy.VelocityServer;
+import com.velocitypowered.proxy.connection.VelocityConstants;
 import com.velocitypowered.proxy.connection.client.ClientPlaySessionHandler;
 import com.velocitypowered.proxy.protocol.MinecraftPacket;
 import com.velocitypowered.proxy.protocol.ProtocolConstants;
@@ -46,6 +47,7 @@ public class BackendPlaySessionHandler implements MinecraftSessionHandler {
             connection.getPlayer().handleConnectionException(connection.getServerInfo(), original);
         } else if (packet instanceof JoinGame) {
             playerHandler.handleBackendJoinGame((JoinGame) packet);
+            connection.setHasCompletedJoin(true);
         } else if (packet instanceof BossBar) {
             BossBar bossBar = (BossBar) packet;
             switch (bossBar.getAction()) {
@@ -68,6 +70,19 @@ public class BackendPlaySessionHandler implements MinecraftSessionHandler {
                 return;
             }
 
+            if (!connection.hasCompletedJoin() && pm.getChannel().equals(VelocityConstants.FORGE_LEGACY_HANDSHAKE_CHANNEL)) {
+                if (!connection.isModded()) {
+                    connection.setModded(true);
+
+                    // We must always reset the handshake before a modded connection is established.
+                    connection.getPlayer().sendLegacyForgeHandshakeResetPacket();
+                }
+
+                // Always forward these messages during login
+                connection.getPlayer().getConnection().write(pm);
+                return;
+            }
+
             MessageHandler.ForwardStatus status = server.getChannelRegistrar().handlePluginMessage(connection,
                     ChannelSide.FROM_SERVER, pm);
             if (status == MessageHandler.ForwardStatus.FORWARD) {

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

@@ -84,6 +84,10 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
                 connection.getPlayer().getConnection().setSessionHandler(new ClientPlaySessionHandler(server, connection.getPlayer()));
             } else {
                 // The previous server connection should become obsolete.
+                // Before we remove it, if the server we are departing is modded, we must always reset the client state.
+                if (existingConnection.isModded()) {
+                    connection.getPlayer().sendLegacyForgeHandshakeResetPacket();
+                }
                 existingConnection.disconnect();
             }
 

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

@@ -43,6 +43,8 @@ public class VelocityServerConnection implements MinecraftConnectionAssociation,
     private final ConnectedPlayer proxyPlayer;
     private final VelocityServer server;
     private MinecraftConnection minecraftConnection;
+    private boolean isModded = false;
+    private boolean hasCompletedJoin = false;
 
     public VelocityServerConnection(ServerInfo target, ConnectedPlayer proxyPlayer, VelocityServer server) {
         this.serverInfo = target;
@@ -154,4 +156,20 @@ public class VelocityServerConnection implements MinecraftConnectionAssociation,
         message.setData(data);
         minecraftConnection.write(message);
     }
+
+    public boolean isModded() {
+        return isModded;
+    }
+
+    public void setModded(boolean modded) {
+        isModded = modded;
+    }
+
+    public boolean hasCompletedJoin() {
+        return hasCompletedJoin;
+    }
+
+    public void setHasCompletedJoin(boolean hasCompletedJoin) {
+        this.hasCompletedJoin = hasCompletedJoin;
+    }
 }

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

@@ -235,6 +235,12 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
             return;
         }
 
+        if (player.getConnectedServer().isModded() && !player.getConnectedServer().hasCompletedJoin()) {
+            // Ensure that the messages are forwarded
+            player.getConnectedServer().getMinecraftConnection().write(packet);
+            return;
+        }
+
         MessageHandler.ForwardStatus status = server.getChannelRegistrar().handlePluginMessage(player,
                 ChannelSide.FROM_CLIENT, packet);
         if (status == MessageHandler.ForwardStatus.FORWARD) {

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

@@ -15,6 +15,7 @@ import com.velocitypowered.api.util.MessagePosition;
 import com.velocitypowered.api.proxy.Player;
 import com.velocitypowered.proxy.VelocityServer;
 import com.velocitypowered.proxy.connection.MinecraftConnectionAssociation;
+import com.velocitypowered.proxy.connection.VelocityConstants;
 import com.velocitypowered.proxy.connection.util.ConnectionMessages;
 import com.velocitypowered.proxy.connection.util.ConnectionRequestResults;
 import com.velocitypowered.api.util.GameProfile;
@@ -294,6 +295,13 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player {
         this.connectedServer = serverConnection;
     }
 
+    public void sendLegacyForgeHandshakeResetPacket() {
+        PluginMessage resetPacket = new PluginMessage();
+        resetPacket.setChannel(VelocityConstants.FORGE_LEGACY_HANDSHAKE_CHANNEL);
+        resetPacket.setData(VelocityConstants.FORGE_LEGACY_HANDSHAKE_RESET_DATA);
+        connection.write(resetPacket);
+    }
+
     public void close(TextComponent reason) {
         connection.closeWith(Disconnect.create(reason));
     }

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

@@ -70,9 +70,12 @@ public class HandshakeSessionHandler implements MinecraftSessionHandler {
                     return;
                 }
 
-                // Make sure legacy forwarding is not in use on this connection. Make sure that we do _not_ reject Forge,
-                // although Velocity does not yet support Forge.
-                if (handshake.getServerAddress().contains("\0") && !handshake.getServerAddress().endsWith("\0FML\0")) {
+                // Determine if we're using Forge (1.8 to 1.12, may not be the case in 1.13) and store that in the connection
+                boolean isForge = handshake.getServerAddress().endsWith("\0FML\0");
+                connection.setLegacyForge(isForge);
+
+                // 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) {
                     connection.closeWith(Disconnect.create(TextComponent.of("Running Velocity behind Velocity is unsupported.")));
                     return;
                 }

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

@@ -8,6 +8,7 @@ import com.velocitypowered.api.event.permission.PermissionsSetupEvent;
 import com.velocitypowered.api.event.player.GameProfileRequestEvent;
 import com.velocitypowered.api.proxy.InboundConnection;
 import com.velocitypowered.api.proxy.server.ServerInfo;
+import com.velocitypowered.proxy.config.PlayerInfoForwarding;
 import com.velocitypowered.proxy.connection.VelocityConstants;
 import com.velocitypowered.api.util.GameProfile;
 import com.velocitypowered.proxy.protocol.MinecraftPacket;
@@ -31,7 +32,9 @@ import java.net.MalformedURLException;
 import java.net.URL;
 import java.security.GeneralSecurityException;
 import java.security.KeyPair;
+import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.List;
 import java.util.Optional;
 import java.util.concurrent.ThreadLocalRandom;
 
@@ -176,6 +179,12 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
     }
 
     private void initializePlayer(GameProfile profile, boolean onlineMode) {
+        if (inbound.isLegacyForge() && server.getConfiguration().getPlayerInfoForwardingMode() == PlayerInfoForwarding.LEGACY) {
+            // We want to add the FML token to the properties
+            List<GameProfile.Property> properties = new ArrayList<>(profile.getProperties());
+            properties.add(new GameProfile.Property("forgeClient", "true", ""));
+            profile = new GameProfile(profile.getId(), profile.getName(), properties);
+        }
         GameProfileRequestEvent profileRequestEvent = new GameProfileRequestEvent(apiInbound, profile, onlineMode);
 
         server.getEventManager().fire(profileRequestEvent).thenCompose(profileEvent -> {