aria2 — Lightweight Multi-Protocol Download Utility

Introduction

aria2 is a lightweight, multi-protocol, multi-source download utility that operates from the command line. It supports HTTP/HTTPS, FTP, SFTP, BitTorrent, and Metalink, allowing you to download files from multiple sources simultaneously to maximize bandwidth usage.

What aria2 Does

• Downloads files via HTTP/HTTPS, FTP, SFTP, BitTorrent, and Metalink protocols in a single binary
• Splits downloads into multiple segments and fetches them in parallel for faster transfer speeds
• Resumes interrupted downloads without re-downloading completed portions
• Provides a JSON-RPC and XML-RPC interface for remote control and integration with web UIs
• Supports downloading from multiple URIs for the same resource simultaneously

Architecture Overview

aria2 is written in C++ for performance and low memory footprint. It uses an event-driven architecture with a single-threaded core and asynchronous I/O to manage thousands of concurrent connections efficiently. The download engine handles protocol negotiation, segment management, and disk I/O while exposing an RPC interface for external control.

Self-Hosting & Configuration

• Runs as a daemon with aria2c --enable-rpc --daemon for persistent download management
• Configuration file at ~/.aria2/aria2.conf stores default options like max connections and download directory
• Pair with web frontends like AriaNg or webui-aria2 for a graphical download manager experience
• Supports proxy configuration via --all-proxy or per-protocol proxy settings
• Session files allow saving and restoring download progress across restarts

Key Features

• Extremely low memory usage (typically 4-9 MiB) compared to other download managers
• Protocol-agnostic segmented downloading accelerates transfers regardless of source type
• BitTorrent support includes DHT, PEX, encryption, magnet URIs, and selective file downloading
• Metalink support enables automatic mirror selection and integrity verification via checksums
• Built-in JSON-RPC/XML-RPC API makes it easy to build custom automation and UIs on top

Comparison with Similar Tools

• wget — single-connection downloads without segmentation; aria2 adds parallel connections and multi-protocol support
• curl — focused on single transfers and scripting; aria2 handles queuing, segmentation, and BitTorrent natively
• axel — lightweight accelerator but lacks BitTorrent, Metalink, and RPC interface
• transmission-cli — BitTorrent only; aria2 handles HTTP/FTP downloads equally well

FAQ

Q: Can aria2 download from multiple mirrors simultaneously? A: Yes. Pass multiple URIs for the same file and aria2 fetches segments from each source in parallel, maximizing throughput.

Q: How does aria2 compare in memory usage to graphical download managers? A: aria2 typically uses 4-9 MiB of RAM, far less than GUI-based tools like JDownloader or Free Download Manager.

Q: Does aria2 support download speed limiting? A: Yes. Use --max-download-limit and --max-upload-limit to cap bandwidth per download or globally.

Q: Can I use aria2 as a daemon with a web interface? A: Yes. Run aria2 with --enable-rpc and connect a frontend like AriaNg for browser-based download management.

Sources

• https://github.com/aria2/aria2
• https://aria2.github.io/