LUFT — Lightweight UDP File Transfer

About the Project

LUFT is a client-server application for file transfer over UDP, built from scratch on top of the operating system's socket API. The goal is to manually implement the reliability mechanisms that TCP provides natively — segmentation, ordering, error detection, and retransmission — over a protocol that, by nature, offers none of these guarantees.

Reliable file transfer over UDP using raw sockets, without any network abstraction libraries.

Dependencies

• CMake >= 3.25
• Ninja
• GCC with C++23 support
• clang-tidy >= 19 (required — build fails without it)
• zlib

sudo apt install cmake ninja-build g++ clang-tidy-19 zlib1g-dev

Build

Configure and compile using one of the available presets:

Preset	Description
debug	All targets, with debug symbols
release	All targets, with optimizations
server-debug	Server only (debug)
client-debug	Client only (debug)
server-release	Server only (release)
client-release	Client only (release)

# Configure
cmake --preset debug

# Build
ninja -C build/debug

To build only a specific target (e.g. server or client):

ninja -C build/debug luft_server
ninja -C build/debug luft_client

For a release build:

cmake --preset release
ninja -C build/release

Binaries are generated in build/debug/ or build/release/.

Usage

Server

The server requires a port and a directory to search files in.

./build/debug/luft_server <port> <search_directory>

Example: start the server on port 9000, serving files from /tmp/files:

./build/debug/luft_server 9000 /tmp/files

Server listening on port 9000

Errors on startup:

Situation	Output
Wrong number of arguments	Usage: server <port> <search_directory>
Directory does not exist	Failed to create server: invalid directory
Port already in use	Server error: bind failed

---

Client

The client connects to a server and downloads a file.

./build/debug/luft_client <ip> <port> <filename> [output_dir]

• output_dir is optional. If omitted, the file is saved in the current directory.
• If a file with the same name already exists, the client automatically renames the new file to filename (N).ext, incrementing N until a free name is found.

Example: download hello.txt from a server running on 127.0.0.1:9000:

./build/debug/luft_client 127.0.0.1 9000 hello.txt

Example: download to a specific directory:

./build/debug/luft_client 127.0.0.1 9000 hello.txt /tmp/downloads

Errors:

Situation	Output
Wrong number of arguments	Usage: client <ip> <port> <filename> [output_dir]
Output directory does not exist	Failed to create client: invalid output directory
Connection or receive failure	Failed to fetch file: receive failed
Corrupted packet	Failed to fetch file: deserialize failed

---

Running both together

Open two terminals:

# Terminal 1 — start the server
./build/debug/luft_server 9000 /tmp/files

# Terminal 2 — download a file
./build/debug/luft_client 127.0.0.1 9000 hello.txt