Kotlin implementation of the Valve Source-engine netchannel protocol.
A research project exploring the limits of Source protocol manipulation.

Decodes and encodes game traffic at the bit level — packet structure, fragmentation, compression, checksums, message decoding and encoding. Built to test ideas around message injection, real-time control, and protocol behavior.

Core protocol

• Bit-accurate Source packet parsing (headers, splits, compression, checksums)
• Network message encoding/decoding both directions

Sniffer example

• Capture live game traffic externally
• Can inject chat messages and console commands to the server

Proxy example

• MITM that sits between client and server
• Decodes and rewrites player movement in real-time (mouse look, movement keys)
• Lets you control a player remotely
• More advanced message control without causing a desync

• Source packet header parsing (seq/ack, flags, challenge, checksum)
• Split packet reassembly
• Compression handling (Source flag + Snappy)
• Bit-level reader/writer matching engine wire format
• Full message set: client → server and server → client

• Proxy (StartProxy.kt): MITM that decodes/encodes live traffic
• Sniffer (StartSniffer.kt): capture and decode packets from the wire

src/main/kotlin/
├── StartProxy.kt          # MITM proxy example
├── StartSniffer.kt        # Packet sniffer example (experimental)
├── proxy/                 # Proxy implementation
├── packetloader/          # pcap4j capture + BPF
├── packetparser/           # Core protocol parsing
│   ├── PacketParser.kt    # Main packet processing
│   ├── SplitPacketHandler.kt
│   └── PacketHeaderHandler.kt
├── messages/              # All netchannel message types
├── messagehandler/        # Message dispatch
├── definitions/           # Protocol constants
└── settings/              # Config

• JDK 17+
• libpcap (macOS/Linux) or packet capture permissions
• Local jars in ./lib/ (see build.gradle.kts)

./gradlew build

1. Set target server in StartProxy.kt: CSSserver("ip:port")
2. Set listen port: ProxyServer(listenPort = 6767)
3. Run main() from your IDE
4. Connect your game client via the console command connect localhost:6767

1. Set SnifferSettings.defaultInterfaceIP in AppSettings.kt
2. Set ports to monitor
3. Uncomment sniffer loop in StartSniffer.kt and run

• Unreliable messages: fully implemented
• Reliable messages: parsed but ignored (WIP)
• SVC_PacketEntities: not yet parsed (WIP)
• Config: hard-coded (next: CLI/config file)
• Packet dumper: stubbed

• Kotlin/JVM
• pcap4j (capture)
• snappy-java (compression)
• JNA (native bridging)

For research use only. Only inspect traffic you own or have permission to analyze. This project is intentionally user-unfriendly.