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Description of the DCC Protocol

Written by Troy Rollo.

Direct Client-to-Client Protocol

(In response for requests for the protocol to be documented).

The first comment I should make is that the DCC protocol was never designed to be portable to clients other than IRCII. As such I take no responsibility for it being difficult to implement for other clients.

Why DCC?

DCC allows the user to overcome some limitations of the IRC server network and to have the ultimate in secure chat connections while still in an IRC oriented protocol.

DCC uses direct TCP connections between the clients taking part to carry data. There is no flood control, so packets can be sent at full speed, and there is no dependance on server links (or load imposed on them). In addition, since only the initial handshake for DCC conections is passed through the IRC network, it is impossible for Operators with cracked servers to spy on DCC messages.


The initial socket for a DCC connection is created by the side that initiates (Offers) the connection. This socket should be a TCP socket bound to INADDR_ANY, listening for connections.

The Initiating client, on creating the socket, should send its details to the target client using the CTCP command DCC. This command takes the form:

DCC type argument address port

typethe connection type
argumentthe connection type dependant argument
addressthe host address of the initiator as an interger.
portthe port or the socket on which the initiator expects to receive the connection.

The address and port should be sent as ascii representations of the decimal integer formed by converting the values to host byte order and treating them as an unsigned long and unsigned short respectively.

The following DCC connection types are known to IRCII:

CHATTo carry a secure conversationthe string "chat"
SENDTo send a file to the receipientthe file name

In addition, the following are included in the IRCII DCC command, although they do not transmit a DCC request via IRC:

TALK - Establishes a TALK connection


The CHAT and SEND connection types should not be accepted automatically as this would create the potential for terrorism. Instead, they should notify the user that an offer has been made, and allow the user to accept it.

The recipient should have the opportunity to rename a file send with the DCC SEND command prior to retrieving it.

The following are the steps which should occur in the clients the clients:


  • DCC command issued.
  • Create a socket, bind it to INADDR_ANY, port 0, and make it passive (a listening socket).
  • Send the recipient a DCC request via CTCP supplying the address and port of the socket. (This is ideally taken from the address of the local side of the socket which is connected to a side of the socket which is connected to a server. This is presumably the interface on the host which is closest to the rest of the net, and results in one less routing hop in the case of gateway nodes).
  • Continue normally until a connection is received.

On a connection:

  • Accept the connection.
  • Close the original passive socket.
  • Conduct transaction on the new socket.


  • CTCP DCC request received.
  • Record information on the DCC request and notify the user.
  • At this point, the USER should be able to abort (close) the request, or accept it. The request should be accepted with a command specifying the sender, type, and argument, or a subset of these where no ambiguity exists.
  • If accepted, create a TCP socket. Connect the new socket to the address and port supplied. Conduct the transaction over the socket.

Type Specific Details

CHAT Data sent across a CHAT connection should be sent line-by-line without any prefixes or commands. A CHAT connection ends when one party issues the DCC CLOSE command to their clients, which causes the socket to be closed and the information on the connection to be discarded.
FILE Data is sent in packets, rather than dumped in a stream manner. This allows the DCC SEND connection to survive where an FTP connection might fail. The size of the packets is up to the client, and may be set by the user. Smaller packets result in a higher probability of survival over bad links. The recipient should acknowledge each packet by transmitting the total number of bytes received as an unsigned, 4 byte integer in network byte order. The sender should not continue to transmit until the recipient has acknowledged all data already transmitted. Additionally, the sender should not close the connection until the last byte has been acknowledged by the recipient.

Note that it is not possible for the recipient to tell if the entire file has been received - only the sender has that information, although IRCII does not report it. Users generally verify the transfer by checking file sizes.

Note also that no provision is made for text translation.

The block size used by IRCII is BIG_BUFFER_SIZE (1024). This should probably be reviewed and reduced.

This page was created by Undernet User Commitee, user-com@undernet.org. Comments are welcomed.
Last modified: March 18, 1997
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