Direct Client-to-Client

Direct Client-to-Client

Direct Client-to-Client (DCC) is an IRC-related sub-protocol enabling peers to interconnect using an IRC server for handshaking in order to exchange files or perform non-relayed chats. Once established, a typical DCC session runs independently from the IRC server. Originally designed to be used with ircII it is now supported by many IRC clients. A variation of the DCC protocol called SDCC (Secure Direct Client-to-Client), also known as DCC SCHAT supports encrypted connections. There exists no RFC specification on the use of DCC.

DCC connections can be initiated in two different ways:

  • The most common way is to use CTCP to initiate a DCC session. The CTCP is sent from one user, over the IRC network, to another user.
  • Another way to initiate a DCC session is for the client to connect directly to the DCC server. Using this method, no traffic will go across the IRC network (the parties involved do not need to be connected to an IRC network in order to DCC).


Common DCC applications


The CHAT service enables users to chat with each other over a DCC connection. The traffic will go directly between the users, and not over the IRC network. When compared to sending messages normally, this reduces IRC network load, allows sending of larger amounts of text at once, due to the lack of flood control, and makes the communication more secure by not exposing the message to the IRC servers (however, the message is still in plaintext).

DCC CHAT is normally initiated using a CTCP handshake. The user wishing to establish the connection sends the following CTCP to the target:

DCC CHAT <protocol> <ip> <port>

<ip> and <port> are those of the sender, and are expressed as integers. <protocol> is "chat" for standard DCC CHAT. The receiving party can then connect to the given port and address.

Once a connection is established, the protocol used for DCC CHAT is very simple: users exchange CRLF-terminated messages. Messages that begin with an ASCII 001 (control-A, represented below by ^A) and the word "ACTION", and are terminated by another ASCII 001, are interpreted as emotes:

^AACTION waves goodbye^A

DCC Whiteboard

This is an extension to DCC CHAT, allowing simple drawing commands to be sent as well as lines of text. DCC Whiteboard is initiated with a handshake similar to DCC CHAT, with the protocol "chat" replaced by "wboard":

DCC CHAT wboard <ip> <port>

Once the connection is established, the two clients exchange CRLF-terminated messages. Messages that begin (and optionally end) with ASCII 001 are interpreted as special commands; the command ACTION represents an emote, while others cause lines to be drawn on the user's whiteboard surface, or allow the two clients to negotiate a set of features.


The SEND service allows users to send files to one another. The original specification for the handshake did not allow the receiver to know the total file size nor to resume a transfer. This has made clients introduce their own extensions to the handshake, many of which have become widely supported.

The original handshake consisted of the sender sending the following CTCP to the receiver:

DCC SEND <filename> <ip> <port>

As with DCC CHAT, <ip> and <port> are the ip address and port where the sending machine will be listening for an incoming connection. Some clients enclose filenames with spaces in double quotes. It is common practice to add the file size as a last argument:

DCC SEND <filename> <ip> <port> <file size>

At this point, the original specification had the receiver either connect to the given address and port and wait for data, or ignore the request, but for clients supporting the DCC RESUME extension, a third alternative is to ask the sender to skip part of the file by sending the CTCP reply:

DCC RESUME <filename> <port> <position>

If the sending client supports DCC RESUME, it will reply with:

DCC ACCEPT <filename> <port> <position>

and the receiver can connect to the given address and port and listen for data to append to an already existing file.

Data is sent in blocks, which the client must acknowledge by sending sizes of incoming data blocks as 32-bit network byte order integers. This slows down connections, and is redundant because such behavior is already implemented by TCP. The send-ahead extension relieves this problem somewhat by not waiting for the acknowledgements, but since the receiver still has to send them for every block it receives, in case the sender expects them, it is not solved completely.

Another extension, TDCC, or turbo DCC, removes the acknowledgements, but requires a slightly modified handshake and is not widely supported. Older versions of TDCC replaced the word SEND in the handshake with TSEND; later versions use the word SEND but append a "T" after the handshake, making this version of TSEND compatible with other clients (as long as they can parse the modified handshake).

DCC SEND exploit

The DCC send exploit can refer to two bugs, a variant buffer overflow error in mIRC triggered by filenames longer than 14 characters[1] and an input validation error in some routers manufactured by Netgear, D-Link and Linksys, triggered by the use of port 0.[2][3] The router exploit, in particular, may be triggered when the phrase 'DCC SEND ' followed by at least 6 characters without spaces or newlines appears anywhere in a TCP stream on port 6667, not just when an actual DCC SEND request has been made.


The XMIT service is a modified version of DCC SEND that allows for resuming files and cuts down on wasteful traffic from the ACK longs. XMIT is not widely supported.

The XMIT handshake differs somewhat from the SEND handshake. The sender sends a CTCP offering a file to the receiver:

DCC XMIT <protocol> <ip> <port>[ <name>[ <size> [<MIME-type>]]]

Square brackets here enclose optional parts. <protocol> is the protocol to use for the transfer; only "clear" is defined presently. Unlike standard DCC SEND, <ip> can be in the additional forms of standard dotted notation for IPv4, or either hexadecimal or mixed notation for IPv6. To leave an early parameter empty, but still supply a later one, the earlier one can be specified as "-". If the receiver does not implement the protocol used, it will send back a CTCP reply of the format:

ERRMSG DCC CHAT <protocol> unavailable

CHAT is used here to maintain compatibility with the error messages sent by the extended DCC CHAT. If the receiver declines the transfer, it sends the following CTCP reply:

ERRMSG DCC CHAT <protocol> declined

Other errors are reported in the same fashion. If the receiver is willing and capable of receiving the file, it will connect to the given address and port. What happens then depends on the protocol used.

In the case of the "clear" protocol, the XMIT server will, upon receiving a connection, send a 32-bit time t in network byte order, representing the file's modification time. Presumably based on the modification time of the local file, the client will then send another network byte order long, an offset which the server should seek to when sending the file. This should be set to zero if the whole file is wanted, or the size of the local file if the client wishes to resume a previous download.

While faster than SEND, XMIT carries one of the same limitations in that it is impossible to tell how big the file is, unless its size is specified in the CTCP negotiation or known beforehand. Furthermore, you can not resume a file past the two gigabyte mark due to the 32-bit offset.

Passive DCC

In a normal DCC connection the initiator acts as the server, and the target is the client. Because of widespread firewalling and reduction of end-to-end transparency because of NAT, the initiator might not be able to act as a server. Various ways of asking the target to act as the server have been devised:

DCC Server

This extension to normal DCC SEND and CHAT was introduced by the IRC client mIRC. DCC Server has moderate support, but is not standard on all clients (see comparison of IRC clients).

It allows the initiation of a DCC connection by IP address, without the need of an IRC server. This is accomplished by the receiving client acting as a server (hence the name) listening (usually on port 59) for a handshake from the sender.

For a CHAT, the initiator sends:

100 <initiator nick>

The target then replies with:

101 <target nick>

and the rest proceeds according to standard DCC CHAT protocol.

For a SEND, the initiator sends:

120 <initiator nick> <filesize> <filename>

The target replies with:

121 <target nick> <resume position>

where <resume position> is the offset in the file from which to start. From here the transfer proceeds as a normal DCC SEND.

DCC Server also supports mIRC-style file servers and DCC GET.


DCC Server provides no way specifying the port to use, so this has to be negotiated manually, which is not always possible, as one of the sides may not be a human. RDCC is a handshake mechanism for DCC Server, which in addition to the port also provides the IP address of the server, which the client might not be able to find otherwise because of host masking. It is not widely supported.

The initiator requests the port the target is listening on by sending the CTCP query:

RDCC <function> <comment>

where <function> is 'c' for chat, 's' for send and 'f' for file server.

The target may then CTCP reply with:

RDCC 0 <ip> <port>

where <ip> and <port> have the same meanings as for normal DCC SEND and CHAT. After this the initiator connects to the ip and port, and a DCC Server handshake follows.


Unlike DCC Server, where the handshake is handled over a direct IP connection, DCC REVERSE has a normal CTCP handshake, similar to the one used by DCC SEND. This is not widely implemented. The sender offers a file to the receiver by sending the CTCP message:

DCC REVERSE <filename> <filesize> <key>

<key> is a 1 to 50 characters long string of ASCII characters in the range 33 to 126, and acts as an identifier for the transfer.

If the receiver accepts, it sends the CTCP reply:

DCC REVERSE <key> <start> <ip> <port>

Here <start> is the position in the file from which to start sending, <ip> is the IP address of the receiver in standard dotted notation for IPv4, or hexadecimal notation for IPv6. The sender then connects to the ip address and port indicated by the receiver, and a normal DCC SEND follows. Both the sender and receiver can cancel the handshake by sending the CTCP reply:



This is the KVIrc client's alternative to DCC REVERSE. The sender offers a file by sending the CTCP:

DCC RSEND <filename> <filesize>

The receiver can then accept by CTCP replying with:

DCC RECV <filename> <ip> <port> <start>

and the sender connects to the receiver and sends as during a normal DCC SEND.

Reverse / Firewall DCC

This passive DCC mechanism is supported by at least mIRC, Visual IRC, XChat, KVIrc, DMDirc, Klient, Konversation, and PhibianIRC. The sender offers a file by sending the CTCP message:

DCC SEND <filename> <ip> 0 <filesize> <token>

<ip> is the IP address of the sender in network byte order, expressed as a single integer (as in standard DCC). The number 0 is sent instead of a valid port, signaling that this is a Reverse DCC request. <token> is a unique integer; if TSEND is being used (by a client that supports it), the letter "T" is appended to the token, letting the receiver know it doesn't need to send acknowledgements.

The receiver can accept the file by opening a listening socket and responding with the CTCP message:

DCC SEND <filename> <ip> <port> <filesize> <token>

This is identical to the original Reverse DCC message, except the <ip> and <port> identify the socket where the receiver is listening. <token> is the same as in the original request, letting the sender know which request is being accepted. (Since this message follows the same format as a regular DCC send request, some servers which filter DCC requests may require the sender to add the receiver to his or her "DCC allow" list.)

The sender then connects to the receiver's socket, sends the content of the file, and waits for the receiver to close the socket when the file is finished.

When the RESUME extension to the SEND protocol is used, the sequence of commands becomes (with '>>' indicating an outgoing message on the initiating side and '<<' response by its peer):

>> DCC SEND <filename> <ip> 0 <filesize> <token>
<< DCC RESUME <filename> 0 <position> <token>
>> DCC ACCEPT <filename> 0 <position> <token>
<< DCC SEND <filename> <peer-ip> <port> <filesize> <token>

After which the protocol proceeds as normal (i.e. the sender connects to the receiver's socket).

File servers (FSERVs)

A DCC fserve, or file server, lets a user browse, read and download files located on a DCC server.

Typically, this is implemented with a DCC CHAT session (which presents the user with a command prompt) or special CTCP commands to request a file. The files are sent over DCC SEND or DCC XMIT. There are many implementations of DCC file servers, among them is the FSERV command in the popular mIRC client.

See also

  • CTCP (Client-to-client protocol)
  • XDCC (eXtended DCC)


External links

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