# aria2 - The ultra fast download utility

Author: | Tatsuhiro Tsujikawa
Email: | tatsuhiro.t_at_gmail_dot_com

# Disclaimer

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

# Introduction

aria2 is a utility for downloading files. The supported protocols are HTTP(S), FTP, SFTP, 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/SFTP and BitTorrent at the same time, while the data downloaded from HTTP(S)/FTP/SFTP is uploaded to the BitTorrent swarm. Using Metalink's chunk checksums, aria2 automatically validates chunks of data while downloading a file like BitTorrent.

The project page is located at https://aria2.github.io/.

See aria2 Online Manual (Russian translation, Portuguese translation) to learn how to use aria2.

# Features

Here is a list of features:

- 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
- 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, 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

# Versioning and release schedule

We use 3 numbers for aria2 version: MAJOR.MINOR.PATCH. We will ship MINOR update on 15th of every month. We may skip a release if we have no changes since the last release. The feature and documentation freeze happens 10 days before the release day (5th day of the month) for translation teams. We will raise an issue about the upcoming release around that day.

We may release PATCH releases between regular releases if we have security issues.

MAJOR version will stay at 1 for the time being.

# How to get source code

We maintain the source code at Github: https://github.com/aria2/aria2

To get the latest source code, run following command:

    $ git clone https://github.com/aria2/aria2.git

This will create aria2 directory in your current directory and source files are stored there.

# Dependency

features | dependency
--- | ---
HTTPS | OSX or GnuTLS or OpenSSL or Windows
SFTP | libssh2
BitTorrent | None. Optional: libnettle+libgmp or libgcrypt or OpenSSL (see note)
Metalink | libxml2 or Expat.
Checksum | None. Optional: OSX or libnettle or libgcrypt or OpenSSL or Windows (see note)
gzip, deflate in HTTP | zlib
Async DNS | C-Ares
Firefox3/Chromium cookie | libsqlite3
XML-RPC | libxml2 or Expat.
JSON-RPC over WebSocket | libnettle or libgcrypt or OpenSSL

Note

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

Note

On Apple OSX the OS-level SSL/TLS support will be preferred. Hence neither GnuTLS nor OpenSSL are required on that platform. If you'd like to disable this behavior, run configure with `--without-appletls`.

GnuTLS has precedence over OpenSSL if both libraries are installed. If you prefer OpenSSL, run configure with `--without-gnutls` `--with-openssl`.

On Windows there is SSL implementation available that is based on the native Windows SSL capabilities (Schannel) and it will be preferred. Hence neither GnuTLS nor OpenSSL are required on that platform. If you'd like to disable this behavior, run configure with `--without-wintls`.

Note

On Apple OSX the OS-level checksum support will be preferred, unless aria2 is configured with `--without-appletls`.

libnettle has precedence over libgcrypt if both libraries are installed. If you prefer libgcrypt, run configure with `--without-libnettle --with-libgcrypt`. If OpenSSL is selected over GnuTLS, neither libnettle nor libgcrypt will be used.

If none of the optional dependencies are installed, an internal implementation that only supports md5 and sha1 will be used.

On Windows there is SSL implementation available that is based on the native Windows capabilities and it will be preferred, unless aria2 is configured with `--without-wintls`.

A user can have one of the following configurations for SSL and crypto libraries:

- libgcrypt
- libnettle
- OpenSSL
- GnuTLS + libgcrypt
- GnuTLS + libnettle

You can disable BitTorrent and Metalink support by providing `--disable-bittorrent` and `--disable-metalink` to the configure script respectively.

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

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

# How to build

aria2 is primarily written in C++. Initially it was written based on C++98/C++03 standard features. We are now migrating aria2 to C++11 standard. The current source code requires C++11 aware compiler. For well-known compilers, such as g++ and clang, the `-std=c++11` or `-std=c++0x` flag must be supported.

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)
- nettle-dev (Required for BitTorrent, Checksum support)
- libgmp-dev (Required for BitTorrent)
- libssh2-1-dev (Required for SFTP 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/Chromium cookie support)
- pkg-config (Required to detect installed libraries)

You can use libgcrypt-dev instead of nettle-dev and libgmp-dev:

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

You can use libssl-dev instead of libgnutls-dev, nettle-dev, libgmp-dev, libgpg-error-dev and 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)

On Fedora you need the following packages: gcc, gcc-c++, kernel-devel, libgcrypt-devel, libxml2-devel, openssl-devel, gettext-devel, cppunit

If you downloaded source code from git repository, you have to install following packages to get autoconf macros:

- libxml2-dev
- libcppunit-dev
- autoconf
- automake
- autotools-dev
- autopoint
- libtool

And run following command to generate configure script and other files necessary to build the program:

    $ autoreconf -i

Also you need Sphinx to build man page.

If you are building aria2 for Mac OS X, take a look at the make-release-os.mk GNU Make makefile.

The quickest way to build aria2 is first run configure script:

    $ ./configure

To build statically linked aria2, use `ARIA2_STATIC=yes` command-line option:

    $ ./configure ARIA2_STATIC=yes

After configuration is done, run `make` to compile the program:

    $ make

See Cross-compiling Windows binary to create a Windows binary. See Cross-compiling Android binary to create an Android binary.

The configure script checks available libraries and enables as many features as possible except for experimental features not enabled by default.

Since 1.1.0, aria2 checks the certificate of HTTPS servers by default. If you build with OpenSSL or the recent version of GnuTLS which has `gnutls_certificate_set_x509_system_trust()` function and the library is properly configured to locate the system-wide CA certificates store, aria2 will automatically load those certificates at the startup. If it is not the case, 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`.

Using the native OSX (AppleTLS) and/or Windows (WinTLS) implementation will automatically use the system certificate store, so `--with-ca-bundle` is not necessary and will be ignored when using these implementations.

By default, the bash_completion file named `aria2c` is installed to the directory `$prefix/share/doc/aria2/bash_completion`. To change the install directory of the file, use `--with-bashcompletiondir` option.

After a `make` the executable is located at `src/aria2c`.

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

    $ make check

# Cross-compiling Windows binary

In this section, we describe how to build a Windows binary using a mingw-w64 (http://mingw-w64.org/doku.php) cross-compiler on Debian Linux. The MinGW (http://www.mingw.org/) may not be able to build aria2.

The easiest way to build Windows binary is use Dockerfile.mingw. See Dockerfile.mingw how to build binary. If you cannot use Dockerfile, then continue to read following paragraphs.

Basically, after compiling and installing depended libraries, you can do cross-compile just passing appropriate `--host` option and specifying `CPPFLAGS`, `LDFLAGS` and `PKG_CONFIG_LIBDIR` variables to configure. For convenience and lowering our own development cost, we provide easier way to configure the build settings.

`mingw-config` script is a configure script wrapper for mingw-w64. We use it to create official Windows build. This script assumes following libraries have been built for cross-compile:

- c-ares
- expat
- sqlite3
- zlib
- libssh2
- cppunit

Some environment variables can be adjusted to change build settings:

`HOST`
  cross-compile to build programs to run on `HOST`. It defaults to `i686-w64-mingw32`. To build 64bit binary, specify `x86_64-w64-mingw32`.
`PREFIX`
  Prefix to the directory where dependent libraries are installed. It defaults to `/usr/local/$HOST`. `-I$PREFIX/include` will be added to `CPPFLAGS`. `-L$PREFIX/lib` will be added to `LDFLAGS`. `$PREFIX/lib/pkgconfig` will be set to `PKG_CONFIG_LIBDIR`.

For example, to build 64bit binary do this:

    $ HOST=x86_64-w64-mingw32 ./mingw-config

If you want libaria2 dll with `--enable-libaria2`, then don't use `ARIA2_STATIC=yes` and prepare the DLL version of external libraries.

# Cross-compiling Android binary

In this section, we describe how to build Android binary using Android NDK cross-compiler on Debian Linux.

At the time of this writing, android-ndk-r9 should compile aria2 without errors.

`android-config` script is a configure script wrapper for Android build. We use it to create official Android build. This script assumes the following libraries have been built for cross-compile:

- c-ares
- openssl
- expat
- zlib
- libssh2

When building the above libraries, make sure that disable shared library and enable only static library. We are going to link those libraries statically.

We use zlib which comes with Android NDK, so we don't have to build it by ourselves.

`android-config` assumes the existence of `$ANDROID_HOME` environment variable which must fulfill the following conditions:

- Android NDK toolchain is installed under `$ANDROID_HOME/toolchain`. Refer to "4/ Invoking the compiler (the easy way):" section in Android NDK `docs/STANDALONE-TOOLCHAIN.html` to install custom toolchain.

  For example, to install toolchain under `$ANDROID_HOME/toolchain`, do this:

      $NDK/build/tools/make-standalone-toolchain.sh \
        --install-dir=$ANDROID_HOME/toolchain \
        --toolchain=arm-linux-androideabi-4.9 \
        --platform=android-16

  You may need to add `--system=linux-x86_64` to the above command-line for x86_64 Linux host.

- The dependent libraries must be installed under `$ANDROID_HOME/usr/local`.

Before running `android-config` and `android-make`, `$ANDROID_HOME` environment variable must be set to point to the correct path.

After `android-config`, run `android-make` to compile sources.

# Building documentation

Sphinx is used to build the documentation. aria2 man pages will be build when you run `make` if they are not up-to-date. You can also build HTML version of aria2 man page by `make html`. The HTML version manual is also available at online (Russian translation, Portuguese translation).

# BitTorrent

## About file names

The file name of the downloaded file is determined as follows:

single-file mode
  If "name" key is present in .torrent file, file name is the value of "name" key. Otherwise, file name is the base name of .torrent file appended by ".file". For example, .torrent file is "test.torrent", then file name 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.

## DHT

aria2 supports mainline compatible DHT. By default, the routing table for IPv4 DHT is saved to `$XDG_CACHE_HOME/aria2/dht.dat` and the routing table for IPv6 DHT is saved to `$XDG_CACHE_HOME/aria2/dht6.dat` unless files exist at `$HOME/.aria2/dht.dat` or `$HOME/.aria2/dht6.dat`. aria2 uses same port number to listen on for both IPv4 and IPv6 DHT.

## UDP tracker

UDP tracker support is enabled when IPv4 DHT is enabled. The port number of UDP tracker is shared with DHT. Use `--dht-listen-port` option to change the port number.

## Other things should be noted

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

# Metalink

The current implementation supports HTTP(S)/FTP/SFTP/BitTorrent. The other P2P protocols are ignored. Both Metalink4 (RFC 5854) and Metalink version 3.0 documents are supported.

For checksum verification, md5, sha-1, sha-224, sha-256, sha-384 and sha-512 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 file name is download file name + ".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.

If relative URI is specified in metalink:url or metalink:metaurl element, aria2 uses the URI of Metalink file as base URI to resolve the relative URI. If relative URI is found in Metalink file which is read from local disk, aria2 uses the value of `--metalink-base-uri` option as base URI. If this option is not specified, the relative URI will be ignored.

# Metalink/HTTP

The current implementation only uses rel=duplicate links only. aria2 understands Digest header fields and check whether it matches the digest value from other sources. If it differs, drop connection. aria2 also uses this digest value to perform checksum verification after download finished. aria2 recognizes geo value. To tell aria2 which location you prefer, you can use `--metalink-location` option.

# netrc

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

# WebSocket

The WebSocket server embedded in aria2 implements the specification defined in RFC 6455. The supported protocol version is 13.

# libaria2

The libaria2 is a C++ library which offers aria2 functionality to the client code. Currently, libaria2 is not built by default. To enable libaria2, use `--enable-libaria2` configure option. By default, only the shared library is built. To build static library, use `--enable-static` configure option as well. See libaria2 documentation to know how to use API.

# References

- aria2 Online Manual
- https://aria2.github.io/
- RFC 959 FILE TRANSFER PROTOCOL (FTP)
- RFC 1738 Uniform Resource Locators (URL)
- RFC 2428 FTP Extensions for IPv6 and NATs
- RFC 2616 Hypertext Transfer Protocol -- HTTP/1.1
- RFC 3659 Extensions to FTP
- RFC 3986 Uniform Resource Identifier (URI): Generic Syntax
- RFC 4038 Application Aspects of IPv6 Transition
- RFC 5854 The Metalink Download Description Format
- RFC 6249 Metalink/HTTP: Mirrors and Hashes
- RFC 6265 HTTP State Management Mechanism
- RFC 6266 Use of the Content-Disposition Header Field in the Hypertext Transfer Protocol (HTTP)
- RFC 6455 The WebSocket Protocol
- RFC 6555 Happy Eyeballs: Success with Dual-Stack Hosts
- The BitTorrent Protocol Specification
- BitTorrent: DHT Protocol
- BitTorrent: Fast Extension
- BitTorrent: IPv6 Tracker Extension
- BitTorrent: Extension for Peers to Send Metadata Files
- BitTorrent: Extension Protocol
- BitTorrent: Multitracker Metadata Extension
- BitTorrent: UDP Tracker Protocol for BitTorrent and BitTorrent udp-tracker protocol specification.
- BitTorrent: WebSeed - HTTP/FTP Seeding (GetRight style)
- BitTorrent: Private Torrents
- BitTorrent: BitTorrent DHT Extensions for IPv6
- BitTorrent: Message Stream Encryption
- Kademlia: A Peer-to-peer Information System Based on the XOR Metric