Aria2 是一个多平台轻量级，支持 HTTP、FTP、BitTorrent 等多协议、多来源的命令行下载工具。Aria2 可以从多个来源、多个协议下载资源，最大的程度上利用了你的带宽。Aria2 有着非常小的资源占用，在关闭磁盘缓存的情况下，物理内存占用通常为 4M（正常 HTTP/FTP 下载的情况下），BitTorrent 下载每秒2.8M/S的情况下，CPU 占有率约为 6%。Aria2 支持 JSON-RPC 和 XML-RPC 接口远程调用。
很多人在 Windows 可能用过 Internet Download Manager，是很好用的多线程下载工具。Aria2 跟 IDM 类似，不仅可以多线程下载，还可以通过多来源进行下载，简单的说就是从多个镜像服务器同时下载一个文件，Aria2 还支持 BT 协议，弥补了 IDM 只支持 HTTP 和 FTP 的痛点。
Linux下的软件截图：
Windows 下的软件截图
特性：
- Command-line interface
- Download files through HTTP(S)/FTP/SFTP/BitTorrent
- Segmented downloading
- Metalink version 4 (RFC 5854) support(HTTP/FTP/SFTP/BitTorrent)
- Metalink version 3.0 support(HTTP/FTP/SFTP/BitTorrent)
- Metalink/HTTP (RFC 6249) support
- HTTP/1.1 implementation
- HTTP Proxy support
- HTTP BASIC authentication support
- HTTP Proxy authentication support
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Well-known environment variables for proxy:
http_proxy,https_proxy,ftp_proxy,all_proxyandno_proxy - HTTP gzip, deflate content encoding support
- Verify peer using given trusted CA certificate in HTTPS
- Client certificate authentication in HTTPS
- Chunked transfer encoding support
- Load Cookies from file using the Firefox3 format, Chromium/Google Chrome and the Mozilla/Firefox (1.x/2.x)/Netscape format.
- Save Cookies in the Mozilla/Firefox (1.x/2.x)/Netscape format.
- Custom HTTP Header support
- Persistent Connections support
- FTP/SFTP through HTTP Proxy
- Download/Upload speed throttling
- BitTorrent extensions: Fast extension, DHT, PEX, MSE/PSE, Multi-Tracker, UDP tracker
- BitTorrent WEB-Seeding. aria2 requests chunks more than piece size to reduce the request overhead. It also supports pipelined requests with piece size.
- BitTorrent Local Peer Discovery
- Rename/change the directory structure of BitTorrent downloads completely
- JSON-RPC (over HTTP and WebSocket)/XML-RPC interface
- Run as a daemon process
- Selective download in multi-file torrent/Metalink
- Chunk checksum validation in Metalink
- Can disable segmented downloading in Metalink
- Netrc support
- Configuration file support
- Download URIs found in a text file or stdin and the destination directory and output file name can be specified optionally
- Parameterized URI support
- IPv6 support with Happy Eyeballs
- Disk cache to reduce disk activity
