Make sure to deny 1.12.2 and below clients in "modern" IP forwarding.

proxy/src/main/java/com/velocitypowered/proxy/VelocityServer.java

@@ -57,7 +57,7 @@ public class VelocityServer implements ProxyServer {
             .registerTypeHierarchyAdapter(Favicon.class, new FaviconSerializer())
             .create();
 
-    private final ConnectionManager cm = new ConnectionManager();
+    private final ConnectionManager cm = new ConnectionManager(this);
     private VelocityConfiguration configuration;
     private NettyHttpClient httpClient;
     private KeyPair serverKeyPair;

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

@@ -6,6 +6,7 @@ import com.velocitypowered.api.event.connection.ConnectionHandshakeEvent;
 import com.velocitypowered.api.event.proxy.ProxyPingEvent;
 import com.velocitypowered.api.proxy.InboundConnection;
 import com.velocitypowered.proxy.VelocityServer;
+import com.velocitypowered.proxy.config.PlayerInfoForwarding;
 import com.velocitypowered.proxy.config.VelocityConfiguration;
 import com.velocitypowered.proxy.connection.MinecraftConnection;
 import com.velocitypowered.proxy.connection.MinecraftSessionHandler;
@@ -26,9 +27,11 @@ import java.util.Optional;
 
 public class HandshakeSessionHandler implements MinecraftSessionHandler {
     private final MinecraftConnection connection;
+    private final VelocityServer server;
 
-    public HandshakeSessionHandler(MinecraftConnection connection) {
+    public HandshakeSessionHandler(MinecraftConnection connection, VelocityServer server) {
         this.connection = Preconditions.checkNotNull(connection, "connection");
+        this.server = server;
     }
 
     @Override
@@ -74,6 +77,14 @@ public class HandshakeSessionHandler implements MinecraftSessionHandler {
                     return;
                 }
 
+                // If the proxy is configured for modern forwarding, we must deny connections from 1.12.2 and lower,
+                // otherwise IP information will never get forwarded.
+                if (server.getConfiguration().getPlayerInfoForwardingMode() == PlayerInfoForwarding.MODERN && handshake.getProtocolVersion() <
+                        ProtocolConstants.MINECRAFT_1_13) {
+                    connection.closeWith(Disconnect.create(TextComponent.of("This server is only compatible with 1.13 and above.")));
+                    return;
+                }
+
                 VelocityServer.getServer().getEventManager().fireAndForget(new ConnectionHandshakeEvent(ic));
                 connection.setSessionHandler(new LoginSessionHandler(connection, ic));
                 break;

proxy/src/main/java/com/velocitypowered/proxy/network/ConnectionManager.java

@@ -2,6 +2,7 @@ package com.velocitypowered.proxy.network;
 
 import com.google.common.util.concurrent.ThreadFactoryBuilder;
 import com.velocitypowered.natives.util.Natives;
+import com.velocitypowered.proxy.VelocityServer;
 import com.velocitypowered.proxy.connection.MinecraftConnection;
 import com.velocitypowered.proxy.connection.client.HandshakeSessionHandler;
 import com.velocitypowered.proxy.protocol.ProtocolConstants;
@@ -53,8 +54,10 @@ public final class ConnectionManager {
     private final TransportType transportType;
     private final EventLoopGroup bossGroup;
     private final EventLoopGroup workerGroup;
+    private final VelocityServer server;
 
-    public ConnectionManager() {
+    public ConnectionManager(VelocityServer server) {
+        this.server = server;
         this.transportType = TransportType.bestType();
         this.bossGroup = transportType.createEventLoopGroup(true);
         this.workerGroup = transportType.createEventLoopGroup(false);
@@ -83,7 +86,7 @@ public final class ConnectionManager {
 
                         final MinecraftConnection connection = new MinecraftConnection(ch);
                         connection.setState(StateRegistry.HANDSHAKE);
-                        connection.setSessionHandler(new HandshakeSessionHandler(connection));
+                        connection.setSessionHandler(new HandshakeSessionHandler(connection, server));
                         ch.pipeline().addLast(Connections.HANDLER, connection);
                     }
                 })