feat: add a fake status response when autoscaling is up

Author: alexfrs69

cmd/mc-router/main.go

@@ -65,6 +65,9 @@ type Config struct {
 
 	SimplifySRV bool `default:"false" usage:"Simplify fully qualified SRV records for mapping"`
 
+	FakeOnline 		bool   	`default:"false" usage:"Enable fake online MOTD when backend is offline and auto-scale-up is enabled"`
+    FakeOnlineMOTD 	string 	`default:"Server is sleeping\nJoin to wake it up" usage:"Custom MOTD to show when backend is offline and auto-scale-up is enabled"`
+
 	Webhook WebhookConfig `usage:"Webhook configuration"`
 }
 
@@ -152,7 +155,7 @@ func main() {
 		trustedIpNets = append(trustedIpNets, ipNet)
 	}
 
-	connector := server.NewConnector(metricsBuilder.BuildConnectorMetrics(), config.UseProxyProtocol, config.ReceiveProxyProtocol, trustedIpNets, config.RecordLogins, autoScaleUpAllowDenyConfig)
+	connector := server.NewConnector(metricsBuilder.BuildConnectorMetrics(), config.UseProxyProtocol, config.ReceiveProxyProtocol, trustedIpNets, config.RecordLogins, autoScaleUpAllowDenyConfig, config.FakeOnline, config.FakeOnlineMOTD)
 
 	clientFilter, err := server.NewClientFilter(config.ClientsToAllow, config.ClientsToDeny)
 	if err != nil {

mcproto/types.go

@@ -84,3 +84,30 @@ type ByteReader interface {
 const (
 	PacketLengthFieldBytes = 1
 )
+
+type StatusResponse struct {
+    Version     StatusVersion   `json:"version"`
+    Players     StatusPlayers   `json:"players"`
+    Description StatusText      `json:"description"`
+    Favicon     string          `json:"favicon,omitempty"`
+}
+
+type StatusVersion struct {
+    Name     string `json:"name"`
+    Protocol int    `json:"protocol"`
+}
+
+type StatusPlayers struct {
+    Max    int           `json:"max"`
+    Online int           `json:"online"`
+    Sample []PlayerEntry `json:"sample,omitempty"`
+}
+
+type PlayerEntry struct {
+    Name string `json:"name"`
+    ID   string `json:"id"`
+}
+
+type StatusText struct {
+    Text string `json:"text"`
+}

mcproto/write.go

@@ -0,0 +1,54 @@
+package mcproto
+
+import (
+	"encoding/json"
+	"io"
+)
+
+func WriteStatusResponse(w io.Writer, motd string) error {
+    resp := StatusResponse{
+        Version: StatusVersion{
+            Name:     "1.21.5",
+            Protocol: 770,
+        },
+        Players: StatusPlayers{
+            Max:    0,
+            Online: 0,
+        },
+        Description: StatusText{
+            Text: motd,
+        },
+    }
+    data, err := json.Marshal(resp)
+    if err != nil {
+        return err
+    }
+
+    jsonLen := encodeVarInt(len(data))
+    payload := append(jsonLen, data...)
+    return WritePacket(w, 0x00, payload)
+}
+
+func WritePacket(w io.Writer, packetID int, data []byte) error {
+    packet := append(encodeVarInt(packetID), data...)
+    length := encodeVarInt(len(packet))
+    _, err := w.Write(append(length, packet...))
+    return err
+}
+
+// encodeVarInt encodes an int as a Minecraft VarInt.
+func encodeVarInt(value int) []byte {
+    var buf []byte
+    for {
+        temp := byte(value & 0x7F)
+        value >>= 7
+        if value != 0 {
+            temp |= 0x80
+        }
+        buf = append(buf, temp)
+        if value == 0 {
+            break
+        }
+    }
+    return buf
+}

server/connector.go

@@ -25,6 +25,7 @@ import (
 
 const (
 	handshakeTimeout = 5 * time.Second
+	backendTimeout = 1 * time.Second
 )
 
 var noDeadline time.Time
@@ -67,7 +68,7 @@ func (p *PlayerInfo) String() string {
 	return fmt.Sprintf("%s/%s", p.Name, p.Uuid)
 }
 
-func NewConnector(metrics *ConnectorMetrics, sendProxyProto bool, receiveProxyProto bool, trustedProxyNets []*net.IPNet, recordLogins bool, autoScaleUpAllowDenyConfig *AllowDenyConfig) *Connector {
+func NewConnector(metrics *ConnectorMetrics, sendProxyProto bool, receiveProxyProto bool, trustedProxyNets []*net.IPNet, recordLogins bool, autoScaleUpAllowDenyConfig *AllowDenyConfig, fakeOnline bool, fakeOnlineMOTD string) *Connector {
 	return &Connector{
 		metrics:                    metrics,
 		sendProxyProto:             sendProxyProto,
@@ -76,6 +77,8 @@ func NewConnector(metrics *ConnectorMetrics, sendProxyProto bool, receiveProxyPr
 		trustedProxyNets:           trustedProxyNets,
 		recordLogins:               recordLogins,
 		autoScaleUpAllowDenyConfig: autoScaleUpAllowDenyConfig,
+		fakeOnline:      			fakeOnline,
+		fakeOnlineMOTD:             fakeOnlineMOTD,
 	}
 }
 
@@ -93,6 +96,9 @@ type Connector struct {
 	clientFilter               *ClientFilter
 	autoScaleUpAllowDenyConfig *AllowDenyConfig
 
+	fakeOnline bool
+    fakeOnlineMOTD string
+
 	connectionNotifier ConnectionNotifier
 }
 
@@ -394,7 +400,7 @@ func (c *Connector) findAndConnectBackend(ctx context.Context, frontendConn net.
 		WithField("player", playerInfo).
 		Info("Connecting to backend")
 
-	backendConn, err := net.Dial("tcp", backendHostPort)
+	backendConn, err := net.DialTimeout("tcp", backendHostPort, backendTimeout)
 	if err != nil {
 		logrus.
 			WithError(err).
@@ -412,6 +418,27 @@ func (c *Connector) findAndConnectBackend(ctx context.Context, frontendConn net.
 			}
 		}
 
+		// Verify that the packet is a status request && autoScaleUp is enabled
+		if c.fakeOnline && waker != nil && nextState == mcproto.StateStatus {
+			logrus.Info("Server is offline, sending fakeOnlineMOTD")
+
+			// Send a response to the client indicating that the server is sleeping
+			writeStatusErr := mcproto.WriteStatusResponse(
+				frontendConn, 
+				c.fakeOnlineMOTD,
+			)
+
+			if writeStatusErr != nil {
+				logrus.
+					WithError(writeStatusErr).
+					WithField("client", clientAddr).
+					WithField("serverAddress", serverAddress).
+					WithField("backend", backendHostPort).
+					WithField("player", playerInfo).
+					Error("Failed to write status response")
+			}
+		}
+
 		return
 	}