Add support for HMACed player forwarding data.

This provides a small degree of security but also makes Velocity "secure
by default", especially on shared hosts.

proxy/src/main/java/com/velocitypowered/proxy/config/PlayerInfoForwarding.java

@@ -1,6 +1,6 @@
 package com.velocitypowered.proxy.config;
 
-public enum IPForwardingMode {
+public enum PlayerInfoForwarding {
     NONE,
     LEGACY,
     MODERN

proxy/src/main/java/com/velocitypowered/proxy/config/VelocityConfiguration.java

@@ -5,6 +5,7 @@ import com.moandjiezana.toml.Toml;
 import com.velocitypowered.api.server.Favicon;
 import com.velocitypowered.proxy.util.AddressUtil;
 import com.velocitypowered.api.util.LegacyChatColorUtils;
+import io.netty.buffer.ByteBufUtil;
 import net.kyori.text.Component;
 import net.kyori.text.serializer.ComponentSerializers;
 import org.apache.logging.log4j.LogManager;
@@ -28,7 +29,7 @@ public class VelocityConfiguration {
     private final String motd;
     private final int showMaxPlayers;
     private final boolean onlineMode;
-    private final IPForwardingMode ipForwardingMode;
+    private final PlayerInfoForwarding playerInfoForwardingMode;
     private final Map<String, String> servers;
     private final List<String> attemptConnectionOrder;
     private final int compressionThreshold;
@@ -40,21 +41,25 @@ public class VelocityConfiguration {
     private Component motdAsComponent;
     private Favicon favicon;
 
+    private final byte[] forwardingSecret;
+
     private VelocityConfiguration(String bind, String motd, int showMaxPlayers, boolean onlineMode,
-                                  IPForwardingMode ipForwardingMode, Map<String, String> servers,
+                                  PlayerInfoForwarding playerInfoForwardingMode, Map<String, String> servers,
                                   List<String> attemptConnectionOrder, int compressionThreshold,
-                                  int compressionLevel, boolean queryEnabled, int queryPort) {
+                                  int compressionLevel, boolean queryEnabled, int queryPort,
+                                  byte[] forwardingSecret) {
         this.bind = bind;
         this.motd = motd;
         this.showMaxPlayers = showMaxPlayers;
         this.onlineMode = onlineMode;
-        this.ipForwardingMode = ipForwardingMode;
+        this.playerInfoForwardingMode = playerInfoForwardingMode;
         this.servers = servers;
         this.attemptConnectionOrder = attemptConnectionOrder;
         this.compressionThreshold = compressionThreshold;
         this.compressionLevel = compressionLevel;
         this.queryEnabled = queryEnabled;
         this.queryPort = queryPort;
+        this.forwardingSecret = forwardingSecret;
     }
 
     public boolean validate() {
@@ -76,9 +81,15 @@ public class VelocityConfiguration {
             logger.info("Proxy is running in offline mode!");
         }
 
-        switch (ipForwardingMode) {
+        switch (playerInfoForwardingMode) {
             case NONE:
-                logger.info("IP forwarding is disabled! All players will appear to be connecting from the proxy and will have offline-mode UUIDs.");
+                logger.info("Player info forwarding is disabled! All players will appear to be connecting from the proxy and will have offline-mode UUIDs.");
+                break;
+            case MODERN:
+                if (forwardingSecret.length == 0) {
+                    logger.error("You don't have a forwarding secret set.");
+                    valid = false;
+                }
                 break;
         }
 
@@ -178,8 +189,8 @@ public class VelocityConfiguration {
         return onlineMode;
     }
 
-    public IPForwardingMode getIpForwardingMode() {
+    public PlayerInfoForwarding getPlayerInfoForwardingMode() {
-        return ipForwardingMode;
+        return playerInfoForwardingMode;
     }
 
     public Map<String, String> getServers() {
@@ -202,6 +213,10 @@ public class VelocityConfiguration {
         return favicon;
     }
 
+    public byte[] getForwardingSecret() {
+        return forwardingSecret;
+    }
+
     @Override
     public String toString() {
         return "VelocityConfiguration{" +
@@ -209,7 +224,7 @@ public class VelocityConfiguration {
                 ", motd='" + motd + '\'' +
                 ", showMaxPlayers=" + showMaxPlayers +
                 ", onlineMode=" + onlineMode +
-                ", ipForwardingMode=" + ipForwardingMode +
+                ", playerInfoForwardingMode=" + playerInfoForwardingMode +
                 ", servers=" + servers +
                 ", attemptConnectionOrder=" + attemptConnectionOrder +
                 ", compressionThreshold=" + compressionThreshold +
@@ -218,6 +233,7 @@ public class VelocityConfiguration {
                 ", queryPort=" + queryPort +
                 ", motdAsComponent=" + motdAsComponent +
                 ", favicon=" + favicon +
+                ", forwardingSecret=" + ByteBufUtil.hexDump(forwardingSecret) +
                 '}';
     }
 
@@ -236,18 +252,22 @@ public class VelocityConfiguration {
                 }
             }
 
+            byte[] forwardingSecret = toml.getString("player-info-forwarding-secret", "5up3r53cr3t")
+                    .getBytes(StandardCharsets.UTF_8);
+
             return new VelocityConfiguration(
-                    toml.getString("bind"),
+                    toml.getString("bind", "0.0.0.0:25577"),
-                    toml.getString("motd"),
+                    toml.getString("motd", "&3A Velocity Server"),
-                    toml.getLong("show-max-players").intValue(),
+                    toml.getLong("show-max-players", 500L).intValue(),
-                    toml.getBoolean("online-mode"),
+                    toml.getBoolean("online-mode", true),
-                    IPForwardingMode.valueOf(toml.getString("ip-forwarding").toUpperCase()),
+                    PlayerInfoForwarding.valueOf(toml.getString("player-info-forwarding", "MODERN").toUpperCase()),
                     ImmutableMap.copyOf(servers),
                     toml.getTable("servers").getList("try"),
                     toml.getTable("advanced").getLong("compression-threshold", 1024L).intValue(),
                     toml.getTable("advanced").getLong("compression-level", -1L).intValue(),
-                    toml.getTable("query").getBoolean("enabled"),
+                    toml.getTable("query").getBoolean("enabled", false),
-                    toml.getTable("query").getLong("port", 25577L).intValue());
+                    toml.getTable("query").getLong("port", 25577L).intValue(),
+                    forwardingSecret);
         }
     }
 }

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

@@ -2,7 +2,8 @@ package com.velocitypowered.proxy.connection.backend;
 
 import com.velocitypowered.api.proxy.ConnectionRequestBuilder;
 import com.velocitypowered.proxy.VelocityServer;
-import com.velocitypowered.proxy.config.IPForwardingMode;
+import com.velocitypowered.proxy.config.PlayerInfoForwarding;
+import com.velocitypowered.proxy.config.VelocityConfiguration;
 import com.velocitypowered.proxy.connection.VelocityConstants;
 import com.velocitypowered.proxy.connection.client.ClientPlaySessionHandler;
 import com.velocitypowered.proxy.connection.util.ConnectionRequestResults;
@@ -17,6 +18,11 @@ import io.netty.buffer.Unpooled;
 import io.netty.channel.ChannelPipeline;
 import net.kyori.text.TextComponent;
 
+import javax.crypto.Mac;
+import javax.crypto.SecretKey;
+import javax.crypto.spec.SecretKeySpec;
+import java.security.InvalidKeyException;
+import java.security.NoSuchAlgorithmException;
 import java.util.concurrent.ScheduledFuture;
 import java.util.concurrent.TimeUnit;
 
@@ -30,7 +36,7 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
 
     @Override
     public void activated() {
-        if (VelocityServer.getServer().getConfiguration().getIpForwardingMode() == IPForwardingMode.MODERN) {
+        if (VelocityServer.getServer().getConfiguration().getPlayerInfoForwardingMode() == PlayerInfoForwarding.MODERN) {
             forwardingCheckTask = connection.getMinecraftConnection().getChannel().eventLoop().schedule(() -> {
                 connection.getProxyPlayer().handleConnectionException(connection.getServerInfo(),
                         TextComponent.of("Your server did not send the forwarding request in time. Is it set up correctly?"));
@@ -44,12 +50,14 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
             throw new IllegalStateException("Backend server is online-mode!");
         } else if (packet instanceof LoginPluginMessage) {
             LoginPluginMessage message = (LoginPluginMessage) packet;
-            if (VelocityServer.getServer().getConfiguration().getIpForwardingMode() == IPForwardingMode.MODERN &&
+            VelocityConfiguration configuration = VelocityServer.getServer().getConfiguration();
+            if (configuration.getPlayerInfoForwardingMode() == PlayerInfoForwarding.MODERN &&
                     message.getChannel().equals(VelocityConstants.VELOCITY_IP_FORWARDING_CHANNEL)) {
                 LoginPluginResponse response = new LoginPluginResponse();
                 response.setSuccess(true);
                 response.setId(message.getId());
-                response.setData(createForwardingData(connection.getProxyPlayer().getRemoteAddress().getHostString(),
+                response.setData(createForwardingData(configuration.getForwardingSecret(),
+                        connection.getProxyPlayer().getRemoteAddress().getHostString(),
                         connection.getProxyPlayer().getProfile()));
                 connection.getMinecraftConnection().write(response);
                 cancelForwardingCheck();
@@ -122,23 +130,43 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
         }
     }
 
-    private static ByteBuf createForwardingData(String address, GameProfile profile) {
+    static ByteBuf createForwardingData(byte[] hmacSecret, String address, GameProfile profile) {
-        ByteBuf buf = Unpooled.buffer();
+        ByteBuf dataToForward = Unpooled.buffer();
-        ProtocolUtils.writeString(buf, address);
+        ByteBuf finalData = Unpooled.buffer();
-        ProtocolUtils.writeUuid(buf, profile.idAsUuid());
+        try {
-        ProtocolUtils.writeString(buf, profile.getName());
+            ProtocolUtils.writeString(dataToForward, address);
-        ProtocolUtils.writeVarInt(buf, profile.getProperties().size());
+            ProtocolUtils.writeUuid(dataToForward, profile.idAsUuid());
+            ProtocolUtils.writeString(dataToForward, profile.getName());
+            ProtocolUtils.writeVarInt(dataToForward, profile.getProperties().size());
             for (GameProfile.Property property : profile.getProperties()) {
-            ProtocolUtils.writeString(buf, property.getName());
+                ProtocolUtils.writeString(dataToForward, property.getName());
-            ProtocolUtils.writeString(buf, property.getValue());
+                ProtocolUtils.writeString(dataToForward, property.getValue());
                 String signature = property.getSignature();
                 if (signature != null) {
-                buf.writeBoolean(true);
+                    dataToForward.writeBoolean(true);
-                ProtocolUtils.writeString(buf, signature);
+                    ProtocolUtils.writeString(dataToForward, signature);
                 } else {
-                buf.writeBoolean(false);
+                    dataToForward.writeBoolean(false);
                 }
             }
-        return buf;
+
+            SecretKey key = new SecretKeySpec(hmacSecret, "HmacSHA256");
+            Mac mac = Mac.getInstance("HmacSHA256");
+            mac.init(key);
+            mac.update(dataToForward.array(), dataToForward.arrayOffset(), dataToForward.readableBytes());
+            byte[] sig = mac.doFinal();
+            finalData.writeBytes(sig);
+            finalData.writeBytes(dataToForward);
+            return finalData;
+        } catch (InvalidKeyException e) {
+            finalData.release();
+            throw new RuntimeException("Unable to authenticate data", e);
+        } catch (NoSuchAlgorithmException e) {
+            // Should never happen
+            finalData.release();
+            throw new AssertionError(e);
+        } finally {
+            dataToForward.release();
+        }
     }
 }

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

@@ -1,7 +1,7 @@
 package com.velocitypowered.proxy.connection.backend;
 
 import com.velocitypowered.api.proxy.ConnectionRequestBuilder;
-import com.velocitypowered.proxy.config.IPForwardingMode;
+import com.velocitypowered.proxy.config.PlayerInfoForwarding;
 import com.velocitypowered.proxy.connection.MinecraftConnectionAssociation;
 import com.velocitypowered.proxy.connection.util.ConnectionRequestResults;
 import com.velocitypowered.proxy.protocol.ProtocolConstants;
@@ -97,7 +97,7 @@ public class ServerConnection implements MinecraftConnectionAssociation {
         Handshake handshake = new Handshake();
         handshake.setNextStatus(StateRegistry.LOGIN_ID);
         handshake.setProtocolVersion(proxyPlayer.getConnection().getProtocolVersion());
-        if (VelocityServer.getServer().getConfiguration().getIpForwardingMode() == IPForwardingMode.LEGACY) {
+        if (VelocityServer.getServer().getConfiguration().getPlayerInfoForwardingMode() == PlayerInfoForwarding.LEGACY) {
             handshake.setServerAddress(createBungeeForwardingAddress());
         } else {
             handshake.setServerAddress(serverInfo.getAddress().getHostString());
@@ -111,7 +111,7 @@ public class ServerConnection implements MinecraftConnectionAssociation {
 
         // Send the server login packet for <=1.12.2 and for 1.13+ servers not using "modern" forwarding.
         if (protocolVersion <= ProtocolConstants.MINECRAFT_1_12_2 ||
-                VelocityServer.getServer().getConfiguration().getIpForwardingMode() != IPForwardingMode.MODERN) {
+                VelocityServer.getServer().getConfiguration().getPlayerInfoForwardingMode() != PlayerInfoForwarding.MODERN) {
             ServerLogin login = new ServerLogin();
             login.setUsername(proxyPlayer.getUsername());
             minecraftConnection.write(login);

proxy/src/main/resources/velocity.toml

@@ -19,7 +19,10 @@ online-mode = true
 #             servers using Minecraft 1.12 or lower.
 # - "modern": Forward player IPs and UUIDs as part of the login process using Velocity's native
 #             forwarding. Only applicable for Minecraft 1.13 or higher.
-ip-forwarding = "modern"
+player-info-forwarding = "modern"
+
+# If you are using modern IP forwarding, configure an unique secret here.
+player-info-forwarding-secret = "5up3r53cr3t"
 
 [servers]
 # Configure your servers here.