Properly block connecting to Velocity with legacy forwarding.

I also took the opportunity to improve the disconnect handling for
Disconnect messages from the server.

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

@@ -208,7 +208,11 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player {
         } else {
             logger.error("{}: disconnected while connecting to {}: {}", this, info.getName(), plainTextReason);
         }
-        handleConnectionException(info, disconnectReason);
+        handleConnectionException(info, TextComponent.builder()
+                .content("Unable to connect to " + info.getName() + ": ")
+                .color(TextColor.RED)
+                .append(disconnectReason)
+                .build());
     }
 
     public void handleConnectionException(ServerInfo info, Component disconnectReason) {

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

@@ -55,18 +55,27 @@ public class HandshakeSessionHandler implements MinecraftSessionHandler {
             case StateRegistry.LOGIN_ID:
                 connection.setState(StateRegistry.LOGIN);
                 connection.setProtocolVersion(handshake.getProtocolVersion());
+
                 if (!ProtocolConstants.isSupported(handshake.getProtocolVersion())) {
                     connection.closeWith(Disconnect.create(TranslatableComponent.of("multiplayer.disconnect.outdated_client")));
                     return;
-                } else {
+                }
+
                 InetAddress address = ((InetSocketAddress) connection.getChannel().remoteAddress()).getAddress();
                 if (!VelocityServer.getServer().getIpAttemptLimiter().attempt(address)) {
                     connection.closeWith(Disconnect.create(TextComponent.of("You are logging in too fast, try again later.")));
                     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")) {
+                    connection.closeWith(Disconnect.create(TextComponent.of("Running Velocity behind Velocity is unsupported.")));
+                    return;
+                }
+
                 VelocityServer.getServer().getEventManager().fireAndForget(new ConnectionHandshakeEvent(ic));
                 connection.setSessionHandler(new LoginSessionHandler(connection, ic));
-                }
                 break;
             default:
                 throw new IllegalArgumentException("Invalid state " + handshake.getNextStatus());

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/Handshake.java

@@ -56,7 +56,7 @@ public class Handshake implements MinecraftPacket {
     @Override
     public void decode(ByteBuf buf, ProtocolConstants.Direction direction, int protocolVersion) {
         this.protocolVersion = ProtocolUtils.readVarInt(buf);
-        this.serverAddress = ProtocolUtils.readString(buf, 255);
+        this.serverAddress = ProtocolUtils.readString(buf);
         this.port = buf.readUnsignedShort();
         this.nextStatus = ProtocolUtils.readVarInt(buf);
     }