aria2 - The high speed download utility

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

2. About aria2
--------------
aria2 has segmented downloading engine in its core. By segmented downloading,
it can download files very much faster than ordinary browsers.

aria2 is in very early development stage. Currently it has following features:
* HTTP/HTTPS GET support
* HTTP Proxy support
* HTTP BASIC authentication support
* HTTP Proxy authentication support
* FTP support(active, passive mode)
* FTP through HTTP proxy(GET command or tunneling)
* Segmented download
* Cookie support(currently aria2 ignores "expires")
* It can run as a daemon process.
* BitTorrent protocol support with fast extension.
* Selective download in multi-file torrent
* Metalink version 3.0 support(HTTP/FTP only).

3. How to build
---------------
$ ./configure
$ make

The executable is aria2c in src directory.

4. Dependency
-------------
In order to enable HTTPS support, you need GNU TLS or OpenSSL.
In order to enable BitTorrent support, you need GNU TLS+libgcrypt or OpenSSL.
In order to enable Metalink support, you need libxml2. Optionally GNU TLS+
libgcrypt or OpenSSL are required for checksum checking support(MD5, SHA1).

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

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

5. BitTorrrent
--------------
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. Then aria2 opens all files mentioned in .torrent file,
directly writes to and reads from these files.
NOTE: Even in selective download, all files are opened.

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.

Note:
*  -o option is used to change the filename of downloaded .torrent file.
* This version only supports compact peers list format.
* The ports aria2c uses are 6881-6999.
* The maximum number of peers is 55.
* After selective download completes, aria2 is going to download rest of the
files.

6. Metalink
-----------
The current implementation only supports HTTP/FTP downloading in Metalink.
BitTorrent and other P2P protocols are ignored.

For checksum checking, both MD5 and SHA1 are supported. If both values are
provided, then 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 and os.