# 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. 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.

Here is a list of features.

- Command-line interface

- Download files through HTTP(S)/FTP/BitTorrent

- Segmented downloading

- Metalink version 4 http://www.ietf.org/id/draft-bryan-metalink-28.txt

- 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 the Firefox3 format 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 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.

- BitTorrent Local Peer Discovery

- 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 and the destination directory and output filename can be specified optionally

- Parameterized URI support

## 3. Dependency

Table 1. 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
Async DNS | C-Ares
Firefox3 cookie | libsqlite3
XML-RPC | libxml2 or Expat.

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.

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

- libc-ares-dev (Required for async DNS support)

- libxml2-dev (Required for Metalink support)

- zlib1g-dev (Required for gzip, deflate decoding support in HTTP)

- libsqlite3-dev (Required for Firefox3 cookie support)

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)

You may also need pkg-config to detect the above mentioned libraries.

The quickest way to build aria2 is just type following commands:

    $ ./configure
    $ make

The configure script checks available libraries and enables the features as much as possible because all the features are enabled by default.

Since 1.1.0, aria2 checks the certificate of HTTPS servers by default. If you build with HTTPS support, I recommend to supply the path to the CA bundle file. For example, in Debian the path to CA bundle file is /etc/ssl/certs/ca-certificates.crt (in ca-certificates package). This may vary depending on your distribution. You can give it to configure script using --with-ca-bundle option:

    $ ./configure --with-ca-bundle='/etc/ssl/certs/ca-certificates.crt'
    $ make

Without --with-ca-bundle option, you will encounter the error when accessing HTTPS servers because the certificate cannot be verified without CA bundle. In such case, you can specify the CA bundle file using aria2’s --ca-certificate option. If you don’t have CA bundle file installed, then the last resort is disable the certificate validation using --check-certificate=false.

The executable is aria2c in src directory.

aria2 uses CppUnit for automated unit testing. To 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.

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

### 5.2. DHT

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

### 5.3. Other things should be noted

- -o option is used to change the filename of .torrent file itself, not a filename of a file in .torrent file. For this purpose, use --index-out option instead.

- 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. This download rate can be adjusted using --bt-request-peer-speed-limit option.

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

## 6. Metalink

The current implementation supports HTTP(S)/FTP/BitTorrent. The other P2P protocols are ignored. Both Metalink4 and Metalink version 3.0 documents are supported.

For checksum verification, MD5, SHA1, and SHA256 are supported. If multiple hash algorithms are provided, aria2 uses stronger one. If whole file checksum verification fails, 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 validates chunks of data during download. 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 completion of the download. The filename is download filename + ".sig". If same file already exists, the signature file is not saved.

In Metalink4, multi-file torrent could appear in metalink:metaurl element. Since aria2 cannot download 2 same torrents at the same time, aria2 groups files in metalink:file element which has same BitTorrent metaurl and downloads them from a single BitTorrent swarm. This is basically multi-file torrent download with file selection, so the adjacent files which is not in Metalink document but shares same piece with selected file are also created.

## 7. netrc

netrc support is enabled by default for HTTP(S)/FTP. To disable netrc support, specify -n command-line option. Your .netrc file should have correct permissions(600).

## 8. References

- man aria2c

- http://aria2.sourceforge.net/

- http://sourceforge.net/apps/trac/aria2/wiki

- http://www.rfc-editor.org/rfc/rfc5854.txt