Rewrite ping replies to remove port numbers

Author: jhead

internal/proto/proto.go

@@ -0,0 +1,72 @@
+package proto
+
+import (
+	"bytes"
+	"encoding/binary"
+)
+
+var UnconnectedReplyID byte = 0x1C
+
+type UnconnectedReply struct {
+	PingTime   []byte
+	ID         []byte
+	Magic      []byte
+	ServerName string
+}
+
+func ReadUnconnectedReply(in []byte) (reply *UnconnectedReply, err error) {
+	reply = &UnconnectedReply{}
+	buf := bytes.NewBuffer(in)
+
+	// Packet ID
+	buf.ReadByte()
+
+	reply.PingTime = make([]byte, 8)
+	if _, err := buf.Read(reply.PingTime); err != nil {
+		return nil, err
+	}
+
+	reply.ID = make([]byte, 8)
+	if _, err := buf.Read(reply.ID); err != nil {
+		return nil, err
+	}
+
+	reply.Magic = make([]byte, 16)
+	if _, err := buf.Read(reply.Magic); err != nil {
+		return nil, err
+	}
+
+	serverNameLenBytes := make([]byte, 2)
+	if _, err := buf.Read(serverNameLenBytes); err != nil {
+		return nil, err
+	}
+
+	serverNameLen := binary.BigEndian.Uint16(serverNameLenBytes)
+
+	serverNameBytes := make([]byte, serverNameLen)
+	if _, err := buf.Read(serverNameBytes); err != nil {
+		return nil, err
+	}
+
+	reply.ServerName = string(serverNameBytes)
+
+	return
+}
+
+func (r UnconnectedReply) Build() bytes.Buffer {
+	var outBuffer bytes.Buffer
+
+	outBuffer.WriteByte(UnconnectedReplyID)
+	outBuffer.Write(r.PingTime)
+	outBuffer.Write(r.ID)
+	outBuffer.Write(r.Magic)
+
+	serverNameLen := uint16(len(r.ServerName))
+	stringBuf := make([]byte, 2)
+	binary.BigEndian.PutUint16(stringBuf, serverNameLen)
+
+	outBuffer.Write(stringBuf)
+	outBuffer.WriteString(r.ServerName)
+
+	return outBuffer
+}

internal/proxy/proxy.go

@@ -1,13 +1,16 @@
-package proxy // import github.com/jhead/phantom/internal/proxy
+package proxy
 
 import (
 	"fmt"
 	"log"
 	"math/rand"
 	"net"
 	"os"
+	"strings"
 	"time"
 
+	"github.com/jhead/phantom/internal/proto"
+
 	reuse "github.com/libp2p/go-reuseport"
 )
 
@@ -230,12 +233,33 @@ func (proxy *ProxyServer) processDataFromServer(remoteConn *net.UDPConn, client
 	for !proxy.dead {
 		read, _, err := remoteConn.ReadFrom(buffer)
 
+		// Read error
 		if err != nil {
 			fmt.Println(err)
 			break
 		}
 
+		// Empty read
+		if read < 1 {
+			continue
+		}
+
+		// Resize data to byte count from 'read'
 		data := buffer[:read]
+
+		// Rewrite Unconnected Reply packets
+		if packetID := data[0]; packetID == proto.UnconnectedReplyID {
+			if packet, err := proto.ReadUnconnectedReply(data); err == nil {
+				// Rewrite server MOTD to remove ports
+				truncServerName := strings.Split(packet.ServerName, ";")[:9]
+				packet.ServerName = fmt.Sprintf("%v;", strings.Join(truncServerName, ";"))
+				packetBuffer := packet.Build()
+				data = packetBuffer.Bytes()
+			} else {
+				fmt.Printf("Failed to rewrite pong: %v\n", err)
+			}
+		}
+
 		proxy.server.WriteTo(data, client)
 	}
 }