# aria2 - The ultra fast download utility

Tatsuhiro Tsujikawa
`<tujikawa_at_users_dot_sourceforge_dot_net>`

## 1. Disclaimer

This program comes with no warranty. You must use this program at your own risk.

## 2. Introduction

aria2 is a utility for downloading files. The supported protocols are HTTP(S), FTP, BitTorrent, and Metalink. It 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.

Here is a list of features.

- 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

- HTTP gzip, deflate content encoding support

- Chunked transfer encoding support

- Load Cookies from file using the Mozilla/Firefox(1.x/2.x) and 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

- 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

- Parameterized URI support

## 3. Dependency

Table: External Library Dependency
features | dependency
--- | ---
HTTPS | GnuTLS or OpenSSL
BitTorrent | GnuTLS+Libgcrypt or OpenSSL
Metalink | libxml2 or Expat.
Checksum | GnuTLS+Libgcrypt or OpenSSL
gzip, deflate in HTTP | zlib

Note

  GNU TLS has precedence over OpenSSL if both libraries are installed. If you prefer OpenSSL, run configure with "—without-gnutls".

Note

  libxml2 has precedence over Expat if both libraries are installed. If you prefer Expat, run configure with "—without-libxml2".

You can disable BitTorrent, Metalink support by providing —disable-bittorrent, —disable-metalink respectively to configure script.

In order to enable async DNS support, you need c-ares or ares.

- c-ares: http://daniel.haxx.se/projects/c-ares/

- ares: ftp://athena-dist.mit.edu/pub/ATHENA/ares

## 4. How to build

In order to build aria2 from the source package, you need following development packages(package name may vary depending on the distribution you use):

- libgnutls-dev (Required for HTTPS, BitTorrent, Checksum support)

- libgpg-error-dev (Required for BitTorrent, Checksum support)

- libgcrypt-dev (Required for BitTorrent, Checksum support)

- libares-dev (Required for async DNS support)

- libxml2-dev (Required for Metalink support)

- libz1g-dev (Required for gzip, deflate decoding support in HTTP)

You can use libssl-dev instead of libgnutls-dev,libgpg-error-dev,libgcrypt-dev:

- libssl-dev (Required for HTTPS, BitTorrent, Checksum support)

You can use libexpat1-dev instead of libxml2-dev:

- libexpat1-dev (Required for Metalink support)

The build process is fairly standard way for *nix programs:

    $ ./configure
    $ make

The executable is aria2c in src directory.

aria2 uses CppUnit for automated unit testing. To run run the unit test:

    $ make check

## 5. BitTorrrent

### 5.1. About filename

The filename of the downloaded file is determined as follows:

single-file mode

  If "name" key is present in .torrent file, filename is the value of "name" key. Otherwise, filename is the basename of .torrent file appended by ".file". For example, .torrent file is "test.torrrent", then filename is "test.torrent.file". The directory to store the downloaded file can be specified by -d option.

multi-file mode

  The complete directory/file structure mentioned in .torrent file is created. The directory to store the top directory of downloaded files can be specified by -d option.

In the default behavior, before download starts, complete directory structure is created if needed. By default, aria2 opens at most 100 files mentioned in .torrent file, directly writes to and reads from these files. The number of files to open can be controlled by "—bt-max-open-files" option.

If "—direct-file-mapping" option set to be false, aria2 creates temporary file in the store directory. The length of this file is the sum of length of the files in .torrent file, so at least 2 times more disk space than the file size itself is required. Writing and reading is done against this file. After download completes, aria2 creates complete directory structure if needed, and copies whole file or a part of it to the destination.

### 5.2. DHT

As of release 0.13.0, aria2 supports DHT. The routing table is saved to $HOME/.aria2/dht.dat.

Note

  - -o option is used to change the filename of .torrent file itself, not a filename of a file in .torrent file.

  - The port numbers that aria2 uses by default are 6881-6999 for TCP and UDP.

  - aria2 doesn't configure port-forwarding automatically. Please configure your router or firewall manually.

  - The maximum number of peers is 55. This limit may be exceeded when download rate is low.

  - As of release 0.10.0, aria2 stops sending request message after selective download completes,

## 6. Metalink

The current implementation supports HTTP/FTP/BitTorrent. The other P2P protocols are ignored.

For checksum checking, MD5, SHA1, SHA256 are supported. If multiple hash algorithms are provided, aria2 uses SHA1. If checksum checking is failed, aria2 doesn't retry the download and just exits with non-zero return code.

The supported user preferences are version, language, location, protocol and os.

If chunk checksums are provided in Metalink file, aria2 automatically validate a chunk of file while downloading it. This behavior can be turned off by a command-line option.

If signature is included in a Metalink file, aria2 saves it as a file after the download completed. The filename is download filename + ".sig". If same file already exists, the signature file is not saved.

## 7. netrc

netrc support is enabled by default in ftp. To disable netrc support, specify -n command-line option. Your .netrc file should have correct permissions(600).

## 8. Configuration file

Configuration file must be placed under $HOME/.aria2 and must be named as aria2.conf. In each line, there is 1 parameter whose syntax is name=value pair, where name is the long command-line option name without — prefix. The lines beginning # are treated as comments.

Example:

    # sample configuration file for aria2c
    file-allocation=prealloc
    listen-port=60000
    seed-ratio=1.0
    max-upload-limit=40K
    ftp-pasv=true