EMule

Free peer to peer file sharing application for Microsoft Windows. eMule is a free peer to peer file sharing application for Microsoft Windows. Started in May 2002 as an alternative to eDonkey2000, eMule now connects to both the eDonkey network and the Kad network. The distinguishing features of eMule are the direct exchange of sources between client nodes, fast recovery of corrupted downloads, and the use of a credit system to reward frequent uploaders. Furthermore, eMule transmits data in zlib compressed form to save bandwidth. eMule is coded in C++ using the Microsoft Foundation Classes. Since July 2002 eMule has been free software, released under the GNU General Public License; its popularity has led to eMule's codebase being used as the basis of cross platform clients aMule, JMule, xMule, along with the release of many eMule mods (modifications of the original eMule) on the Internet. as of 2023, it is the fifth most downloaded project on SourceForge, with over 693 million downloads. Development was later restarted by the community as eMule Community; the latest stable version is 0.70a. The 0.47b version adds protocol obfuscation and eMule will automatically select two port numbers at random in the startup wizard.

Each file that is shared using eMule is hashed as a hash list comprising separate 9500 KiB chunks using the MD4 algorithm. The top level MD4 hash, file size, filename, and several secondary search attributes such as bit rate and codec are stored on eD2k servers and the serverless Kad network. Users can search for filenames in the servers/kad and are presented with the filenames and the unique identifier consisting of the top level MD4 hash for the file and the file's size that can be added to their downloads. The client then asks the servers where the other clients are using that hash. The servers return a set of IP/ports that indicate the locations of the clients that share the file. eMule then asks the peers for the file. eMule will then be queued until an upload slot becomes available. When a complete chunk of 9,728,000 bytes (9500 KiB) is downloaded and verified, this data is also shared by the downloader, helping others to download the file as well. It is also possible that a client knows other clients that are also sharing that same file. In that case a source exchange between the clients is made. This exchange of known peers is done directly between the peers. Newer versions of eMule support AICH (Advanced Intelligent Corruption Handling). It is meant to make eMule's corruption handling competitive with BitTorrent. SHA 1 hashes are computed for each 180 KiB sub chunk and a whole SHA 1 hash tree is formed. AICH is processed purely with peer to peer source exchanges. eMule requires 10 agreeing peers regarding the SHA 1 hash, so rare files generally do not benefit from AICH.

Users who cannot be reached from the outside because they are firewalled, behind a NAT device that has not been correctly port forwarded, or whose IP address ends with a zero (e. g. 123.45.67.0) get a "Low ID" from the servers. They are still able to upload and download but need the help of servers or other kad clients to be reached by other clients. Since they cannot be notified that they are in front of an upload queue, they have to poll peers if an upload slot is available. Since they cannot connect to any other Low ID clients, they see only 40–60% of the clients that a High ID can see. Their IP/ports are not exchanged between other peers, limiting their possibilities for finding sources via eMule's pure P2P source exchange. A Low ID client also consumes a lot more data on an eserver than a High ID client due to the lowidcallbacks. Also, a releaser or heavy uploader that uses a releaser mod such as MorphXT or Xtreme that is forced to operate on a Low ID (hotel room, job) also will find that they will have little control over their upload priorities (especially powershares) as the servers appear to limit their connection forwarding for each client, thus turning their upload queue to a contention situation where the first to be able to get forwarding and find an open slot gets it.

eMule is said to be the most complete implementation of the eD2k protocol and its extensions. eMule supports AICH, making its corruption handling competitive with BitTorrent. eMule also supports source exchanges, allowing it to substantially reduce the loads on the servers and Kad. With a High ID and well sourced downloads pre acquired by server and/or Kad, eMule is able to sustain the peer sources on these files independent longer after disconnection from eD2k and Kad.

As a popular open source program, eMule has many variants, usually called mods. Some mods started as forks from official eMule versions and then continued to develop independently rather than modifying newer official versions. An example of this type of mod is the obsolete eMule Plus. Since eMule Plus forked off before the release of v0.30, the first official version to include Kad, eMule Plus does not support this feature mainly because the project development has been abandoned for about 4 years. Other current mods follow official eMule releases and make their own releases based on each new release of the official version. Since distributed mods are required to publicly share their source code by the GNU General Public License, useful features created by mod developers can be quickly incorporated into an official version.

Due to the popularity and open source nature of eMule, some third parties have created modified versions of it, which frequently contain spyware and other malware programs. Some fake sites ask for credit card information or require the user to sign up for a paid membership. The official eMule is free and does not ask for such information. These versions are usually found via rotating advertisements sometimes placed on legitimate sites.

VeryCD's easyMule is a popular eMule client among Chinese users. It has a simplified interface and lacks some advanced settings available in the standard eMule client. As of version 1.1 it only supports searching through the VeryCD database, though external eD2k links are accepted. Some criticized VeryCD for their misleading name "Dianlv" (; generally the Chinese name for eDonkey or eMule) and the site emule. org. cn, which is named "Dianlv (eMule) Chinese Site" (电驴(eMule)中文网站).

On July 29, 2017 was released a "Community Version" of eMule maintained by the official forum users. This version is available to download in a GitHub repository and is based on the latest official release or beta but contains additional features and bug fixes made by the community, prioritizing a more up to date version.