Aria2 1.6.3

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aria2 is a multi-protocol & multi-source, cross platform download utility. The supported protocols are HTTP(S), FTP, BitTorrent (DHT, PEX, MSE/PE and WEB-Seeding), and Metalink.

aria2 can download a file from multiple sources/protocols and tries to utilize your maximum download bandwidth. It supports downloading a file from HTTP(S)/FTP and BitTorrent at the same time, while the data downloaded from HTTP(S)/FTP is uploaded to the BitTorrent swarm. Using Metalink's chunk checksums, aria2 automatically validates chunks of data while downloading a file like BitTorrent.

There are other alternative applications. But aria2 has 2 distinctive features: (1) aria2 can download a file from several URIs(HTTP(S)/FTP/BitTorrent) and (2) If you give aria2 a list of URIs, aria2 downloads them concurrently. You don't have to wait for the current download queue to finish one file at a time anymore. aria2 tries to utilize your maximum download bandwidth and downloads files quickly.

There are also some applications that have the ability of segmented downloading. Typically these applications split a file by the number of threads and download them parallel and wait for all threads to finish. In other words, they don't split unfinished segments adaptively. Normally(if things goes well) there is no problem for this strategy, but if one thread is very slow(i.e. one of the server is very slow), then you have to wait for it to finish.
aria2 can cope with this peculiar situation. aria2 can split segment adaptively all the way down to 1MiB. So you don't have to worry about the above problem. But you might complain: if a slow server is downloading last 1MiB, then you have to wait for that none the less. The nswer is "no". Even in such case, aria2 does the job quite well: aria2 cancels slow server and use the faster server to finish download. In other words, aria2 is very clever and reliable in many situations.

Unlike original Aria, which has GTK+ interface, aria2 provides command-line interface only. But it brings lower resource requirement. The physical memory usage is typically 3MB(normal HTTP/FTP downloads) to 6MB(BitTorrent downloads). CPU usage in BitTorrent with download speed of 1500KB/sec is around 6%.
Command-line interface,Download files through HTTP(S)/FTP/BitTorrent,Segmented downloading,Metalink version 3.0 support(HTTP/FTP/BitTorrent,HTTP/1.1 implementation,HTTP Proxy support,HTTP BASIC authentication support,HTTP Proxy authentication support,Well-known environment variables for proxy: http_proxy, https_proxy, ftp_proxy, all_proxy and no_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 Firefox3 and the Mozilla/Firefox(1.x/2.x) /Netscape format
Save Cookies from file in the Mozilla/Firefox(1.x/2.x)/Netscape format
Custom HTTP Header support,Persistent Connections support,FTP through HTTP Proxy,Download/Upload speed throttling,BitTorrent extensions: Fast extension, DHT, PEX, MSE/PSE, Multi-Tracker,BitTorrent WEB-Seeding. aria2 requests chunks more than piece size to reduce the request overhead. It also supports pipelined requests with piece size. Rename/change the directory structure of BitTorrent downloads completely,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. The destination directory and output filename can be specified optionally.Parameterized URI support