🏠 local_rcc

Selfhosted game servers for the latest versions of Roblox.

local_rcc is a library that works with Roblox Studio (currently Windows only) and allows the Roblox Player to connect to a Roblox Studio hosted server.

🛠️ Building

This guide is for building on Microsoft Windows.

Make sure you have the following prerequisites:

• Microsoft Visual 2022 with the base C++ development pack
  • Includes necessary compiler and linker, plus a build of CMake
• Ninja build system
  • Easily install with winget install Ninja-build.ninja

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Clone the repository:

git clone https://github.com/rsblox/local_rcc.git

There are two methods for building:

1. Visual Studio Code with the CMake Tools extension
2. Visual Studio's built-in CMake features

With Visual Studio Code & CMake Tools extension

Make sure you have Visual Studio Code installed with the CMake Tools extention installed.

Open the cloned repository:

code local_rcc

If prompted to select a build kit, select the amd64 one.

CMake Tools will automatically configure the project; be patient and wait in the output for configuration to complete; this includes the fetching of dependencies from the Internet.

Navigate to the CMake tab and set the build type to RelWithDebInfo.

In the project outline in the same CMake tab, build the local_rcc [local_rcc.dll] target. Once completed, the library should be located at bin/local_rcc.dll.

With Visual Studio

Open Visual Studio 2022 and navigate to File -> Open -> CMake... and navigate to the CMakeLists.txt file in the repository cloned earlier. Open the Output and watch for fetching the dependencies and configuring to complete. This may take a bit.

Make sure the configuration on the top is set to x64-RelWithDebInfo. In the Solution Explorer window, click the icon with the caption "Switch between solutions and available views". Click on the CMake Targets view. This list should be populated once CMake configuration is successful. Right click on the local_rcc target, and click "Build local_rcc". The resultant build should be located at out/build/x64-RelWithDebInfo/local_rcc.dll.

📖 Explanation

There are four steps to make a team test compatible with Player: setting a fast flag, blocking an unhandled item, compiling compatible bytecode, and replicating the bytecode.

DebugLocalRccServerConnection

Overriding the fast flag, DebugLocalRccServerConnection, on both Player and Studio will allow a connection to be made between the client and the server. It also will force the client to connect to localhost|53640.

With this alone, you can already join a Studio team test from Player - however, it is not playable yet.

ClientQoSItem

Shortly after Player joins the team test server, it will be kicked after sending a ClientQoSItem - which is unhandled by Studio. This can be resolved by refusing to deserialize the item if a ClientQoSItem is passed to RBX::Network::Replicator::deserializeItem.

BytecodeEncoderClient

Opening the developer console in Player after connecting to the team test server shows some strange errors with the path of LocalScript instances that are replicated to Player and none of them are executed. This is partly due to how Studio populates the contents of the server's legalScripts vector. A LegalScript is a 'whitelisted' script for replication, containing source and bytecode. The function RBX::Network::Server::registerLegalSharedScript is used to register and compile these scripts.

Studio already calls this function and even invokes LuaVM::compile - however, on Studio it just returns an empty std::string. This can be fixed by hooking LuaVM::compile and compiling source with LuaVM::BytecodeEncoderClient.

NetworkSchema

Even after compiling scripts correctly and populating each LegalScript with bytecode, the same error appears in the developer console. When a server is started, a NetworkSchema is generated, which contains info for every class in the reflection database. This includes info on how to send properties, like Script.Source, which is determined by their NetworkValueFormat. By default, Studio selects the type for Script.Source to be ProtectedStringSource, and sends the source code of scripts to other Studio clients for them to compile themselves.

This can be fixed by forcing the server to use ProtectedStringBytecode, which will make it get the stored bytecode from the legalScripts vector and send it over when serializing the property. The generation of the NetworkSchema occurs in RBX::Network::NetworkSchema::generateSchemaDefinitionPacket, and the portion of interest is when it chooses the corresponding NetworkValueFormat for the reflection type ProtectedString.