Backport IPv6 modern forwarding fix to Velocity 1.0.x

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

@@ -21,6 +21,7 @@ import com.velocitypowered.proxy.protocol.packet.SetCompression;
 import io.netty.buffer.ByteBuf;
 import io.netty.buffer.Unpooled;
 import java.io.IOException;
+import java.net.InetSocketAddress;
 import java.security.InvalidKeyException;
 import java.security.NoSuchAlgorithmException;
 import java.util.concurrent.CompletableFuture;
@@ -62,7 +63,7 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
       response.setSuccess(true);
       response.setId(packet.getId());
       response.setData(createForwardingData(configuration.getForwardingSecret(),
-          serverConn.getPlayer().getRemoteAddress().getHostString(),
+          cleanRemoteAddress(serverConn.getPlayer().getRemoteAddress()),
           serverConn.getPlayer().getGameProfile()));
       mc.write(response);
       informationForwarded = true;
@@ -123,6 +124,16 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
         .completeExceptionally(new IOException("Unexpectedly disconnected from remote server"));
   }
 
+  private static String cleanRemoteAddress(InetSocketAddress address) {
+    String addressString = address.getAddress().getHostAddress();
+    int ipv6ScopeIdx = addressString.indexOf('%');
+    if (ipv6ScopeIdx == -1) {
+      return addressString;
+    } else {
+      return addressString.substring(0, ipv6ScopeIdx);
+    }
+  }
+
   private static ByteBuf createForwardingData(byte[] hmacSecret, String address,
       GameProfile profile) {
     ByteBuf dataToForward = Unpooled.buffer();