Everything about Direct Connect File Sharing totally explained
Direct connect is a
peer-to-peer file-sharing protocol. Direct connect clients connect to a central
hub and can download files directly from one user to another.
Hubs feature a list of clients or users connected to them. Users can search for
files and
download them from other clients, as well as
chat with other users.
History
NeoModus was started by
Jonathan Hess in
November,
1999 as a company funded by the adware "Direct Connect" while he was in high school . The first third-party client was called "DClite", which never fully supported the file sharing aspects of the protocol. A new version of Direct Connect was introduced, requiring a simple encryption key to initiate a connection, in hopes that he'd lock out third-party clients. The key was cracked, and the author of DClite released a new version of DClite compatible with the new software from NeoModus. Some time after, DClite was rewritten as Open Direct Connect with goals of having an MDI user interface and using plug-ins for file sharing protocols (similar to MLDonkey). Open Direct Connect also didn't complete support for the full file sharing aspects of the protocol, but a port to Java did. Some time later, other clients such as DCTC (Direct Connect Text Client) and DC++ started popping up.
Protocol
The Direct connect protocol is a text-based protocol, where commands and their information is sent in clear text, without
encryption. As clients connect to a central source of distribution (the hub) of information, the hub is required to have a substantial amount of upload bandwidth available.
There is no official specification of the protocol. This means that every client and hub besides the original Neo-modus client and hub have been forced to
reverse engineer the information. As such, any protocol specification this article may reference is likely either inaccurate and/or incomplete.
The client-server (as well in client-client, where one act as "server") aspect of the protocol stipulates that the server speak first when a connection has been made. For example, when a client connect to a hub's
socket, the hub is first to talk to the client.
The protocol doesn't require a
character encoding or
font for clients or hubs.
The port 411 is the default port for hubs and 412 for client to client connections. If any of these ports are already in use, the port + 1 is used. Eg, if 411, 412 and 413 is in use, the port 414 will be used.
Hub addresses are in the following form: dchub://example.com[:411], where 411 is an optional port.
There is no global identification scheme; users are identified with their nick name on a hub to hub basis.
An incoming request for a client-client connection can't be linked with an actual connection.
A search result can't be linked with a particular search.
Supported by the protocol is the ability to kick or move (redirect) a user to another hub. There is no restriction where a user might be redirected to.
If a user is kicked, the hub isn't required to tell the specific reason as to why the user was kicked. However, if another client in power instructs the hub to kick, that client may send out a notification message before doing so. Redirecting a user must be accompanying with a reason. There is no
HTML referrer equivalent.
Hubs may send out user commands to clients. These commands are only raw protocol commands, and is mostly for making a particular task simpler. Eg, the hub can't send a user command that will trigger the default browser to visit a website. It can however add the command "+rules" (where '+' indicate to the hub that it's a command - this may vary) to display the hub's rules.
The peer-to-peer part of the protocol is based around a concept of "slots" (similar to number of open positions for a job). These slots denote the number of people that are allowed to download from a user at any one time. These slots are controlled by the client.
In client to client connections, the parties negotiate a random number to see who should be allowed to download first. The client with the highest number win.
Downloads are transported using
TCP. Active searches use
UDP. The connection to the hub is with TCP.
There are two kinds of modes a user can be in. Either in active or in passive mode. Clients using active mode can download from anyone else on the network. Passive mode users can only download from active users. In
NeoModus Direct Connect, passive mode users receive other passive mode users' search results, while the user won't be able to download anything. In
DC++, users won't receive those search results. In NeoModus Direct Connect, all users will be sent five search results per query at most. If a user has searched and is in active mode, DC++ will respond with ten search results, and five when the user is in passive mode. Passive clients will be sent search results through the hub, while active clients will receive the results directly.
An active client have a listening port for TCP respectively UDP, though the ports don't depend on each other.
Protocol
delimiters are '$', '|' and ' ' (space). There is no
escape sequence, so these characters can't be sent without possibly compromising the integrity of the message. However, a workaround was devised in DC++: send the
HTML equivalent if these characters are to be viewed by the user.
Continued interest exists in features such as ratings and language packs. However, the authors of DC++ have been actively working on a complete replacement of the Direct connect protocol called
Advanced Direct Connect.
One example of an added feature to the protocol, in compared to the original protocol, is the broadcasting of
Tiger-Tree Hashing of shared files (TTH). The advantages of this include verifying that a file is downloaded correctly, and the ability to find files independent of their names.
Hubs
Direct connect hubs are central
servers to which clients connect, thus the networks are not as de-centralised as
Gnutella or
FastTrack. Hubs provide information about the clients, as well as file searching and
chat capabilities.
File transfers are done directly between clients, in true
peer-to-peer fashion.
Hubs often have special areas of interest. Many have requirements on the total size of the files that their members share (share size), and restrictions on the content and quality of shares. A hub can have any arbitrary rule. Hubs can allow users to register and provide user
authentication. It should be noted that the authentication is also in clear text. The hub may choose certain individiuals as operators (similar to
IRC operators) to enforce said rules if the hub itself cannot.
While not directly supported by the protocol, hub linking software exist. The software allow multiple hubs to be connected, allowing users to share and chat with people on the other linked hubs.
Direct connect hubs have difficulty
scaling, due to the broadcast-centricity of the protocol.
Hub software
Direct Connect Hub link
DC-hublink
is open source software that links hub chat. It supports the NMDC-protocol, ADC-protocol (version 1.1.0 and up), as well as the IRC-protocol (version 1.2.0 and up)
Hexhub
HeXHub is an IOCP-based hub, written in
Assembly, for
Windows.
Open Direct Connect Hub
Open Direct Connect Hub is open source hub software for Unix/Linux.
PtokaX
PtokaX is hub software for Linux/ *BSD/Windows.
VerliHub
Verlihub is an open source hub server that runs on Linux/Windows/Solaris/Mac OS X.
YnHub
YnHub (Yoshi and Nev Hub) is hub software for
Windows. It is the successor to YHub (Yoshi Hub).
DConnect Daemon
DCD open source software that runs on Linux/ *BSD/ Solaris/ Mac OS X/ Windows(cygwin)
DSHub
DSHub open source hub software written in Java, multiplatform, for the ADC protocol.
Client software
While not mandated by the protocol, most clients send a "tag". This is part of the client's description and display information ranging from client name and version to number of total available slots to if the user is using a proxy server. It was originally added to DC++, due to its ability to be in multiple hubs with the same instance. The information is arbitrary.
The original client's file list (a comprehensive list of the files a user share) was compressed using
Huffman's compression algorithm. Newer clients (among them DC++) serve a
XML based list, compressed with
bzip2.
DC++
DC++ is an
open source alternative to NMDC written in
C++ mainly for Windows, but cross platform forks exist like LinuxDC++.
MLDonkey
MLDonkey is a multiprotocol open source peer to peer client for Linux, Unix and Windows.
NeoModus Direct connect
NeoModus Direct Connect (abbreviated to NMDC) was the original Direct connect client. It was written by Jonathan Hess at the company
NeoModus. The first version was written in
Visual Basic and released in November
1999. It only ran on
Windows. The second version was written in C++ and C# and had
Mac OS X support.
ShakesPeer
ShakesPeer is a client specifically written for
Mac OS X.
Linux DC++
LinuxDC++ is a port of DC++ to Linux. It has the same core with a rewritten user interface.
Valknut
Valknut is a C++ and
Qt written client that's designed to run on
Linux,
FreeBSD and Mac OS X.
DCTC
Direct Connect Text Client (DCTC)
(External Link) for Linux. They also provide a GUI (dc_gui) for the client and a hub program (dchub).
DC#
DC# is a client primary for Linux that uses
GTK+. It has as a goal to be as easy to use as possible.
Hublists
Direct connect used for DDoS attacks
As the protocol allow hubs to redirect users to another hub, malicious hubs have redirected users to other places than real Direct connect hubs, effectively causing a
Distributed Denial of Service attack. As well, the hubs may alter the
IP in client to client connections, pointing to a potential victim.
As of recently, there are articles being shown by a programming group called TeamElite which displays advice on how to prevent and effectively stop the ability for CTM attacks. The following collection of articles show how this is possible:
CTM Guides. One of the first contributors to the Anti-CTM movement brought forth by hublists is a hublist/client/server developer and member of the group TeamElite who goes by the alias The_Architect.
Further Information
Get more info on 'Direct Connect File Sharing'.
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