Example IRC Communications

In this section, we will describe three example IRC communications. Before diving into the IRC RFCs, we suggest you read through these examples to get a better sense for what a typical conversation between an IRC client and server looks like. These examples will also serve to clarify the format of messages, prefixes, and replies.

Logging into an IRC server

Connecting to an IRC server

Connecting to an IRC server

When an IRC client connects to an IRC server, it must first register its connection. This is done by sending two messages: NICK and USER (messages 1 and 2 in the figure above). NICK specifies the user’s nick (amy in this case), and USER provides additional information about the user. More specifically, USER specifies the user’s username (amy) and the user’s full name (Amy Pond) (we will not be implementing the second and third parameters of USER). The username is typically obtained automatically by the IRC client based on the user’s identity. For example, if you’re logged into a UNIX machine as user jrandom, then most IRC clients will, by default, use that as your username. However, there is no requirement that your nick must match your username.

Assuming you’ve chosen a nick that is not already taken, the IRC server will send back a RPL_WELCOME reply (which is assigned code 001). This reply has the following components:

  • :bar.example.com: The prefix. Remember that prefixes are used to indicate the origin of a message. Since this reply originates in server bar.example.com, the prefix simply includes that hostname. This may seem redundant, given that the client presumably already knows it is connected to that server; however, in IRC networks, a reply could originate in a server other than the one the client is connected to.

  • 001: The numeric code for RPL_WELCOME.

  • amy: The first parameter which, in reply messages, must always be the nick of the user this reply is intended for.

  • :Welcome to the Internet Relay Network borja!borja@polaris.cs.uchicago.edu: The second parameter. The content of this parameter is specified in [RFC2812 §5]:

    001    RPL_WELCOME
           "Welcome to the Internet Relay Network

    Notice how the specification of replies omits the first parameter, which is always the recipient of the reply. So, the specification lists the second and subsequent (if any) parameters.

    One of the things sent back in the RPL_WELCOME reply is the full client identifier (<nick>!<user>@<host>), which is also used in other types of messages. It is composed of the nick as specified in the NICK command, the username as specified in the USER, and the client’s hostname (if the server cannot resolve the client’s hostname, the IP address is used).

The following figure shows a variant of the communication described above:

Connecting to an IRC server when the chosen nick is taken.

Connecting to an IRC server when the chosen nick is taken.

If a user tries to register with a nick that is already taken, the server will send back a ERR_NICKNAMEINUSE reply (code 433). Notice how the parameters in this reply are slightly different:

  • *: The first parameter should be the nick of the user this reply is intended for. However, since the user does not yet have a nick, the asterisk character is used instead.
  • amy: In the ERR_NICKNAMEINUSE reply, the second parameter is the “offending nick” (i.e., the nick that could not be chosen, because it is already taken).
  • :Nickname is already in use: The third parameter simply includes a human readable description of the error. IRC clients will typically print these out verbatim.

So, notice how there is no uniform set of parameters sent back in all replies (other than the first parameter, which is always the recipient nick). When implementing a reply, you must consult [RFC2812 §5] to determine exactly what you should be sending back in the reply.

Messaging between users

Sending a message to another user

Sending a message to another user

Once several users are connected, it is possible for them to send messages to each other, even in the absence of channels. In fact, most of the second assignment focuses on implementing messaging between users, whereas the third assignment will focus on adding support for channels.

To send a message to a specific nick, a user must send a PRIVMSG to the server. The figure above shows two users, with nicks amy and rory, exchanging three messages. In message 1, user amy sends a message to rory. The parameters for PRIVMSG are very simple: the first parameter is the nick of the user the message is intended for, and the second parameter is the message itself.

When the server receives that message, and assuming there is a rory user, it will forward the message to the IRC client that is registered with that nick. This is done in message 2, and notice how it is simply a verbatim copy of message 1, but prefixed with amy’s full client identifier (otherwise, the recipient IRC client would have no idea who the message was from). Messages 3 and 4 show a similar exchange, except going from rory to amy, and messages 5 and 6 show another message going from amy to rory.

Notice how all messages are relayed through the IRC server (hence the name of the protocol: Internet Relay Chat). Non-relayed messaging is not supported in the IRC specification, and we will not be implementing such a functionality in this project. However, there are two extensions to IRC (CTCP, the Client-to-Client Protocol, and DCC, Direct Client-to-Client) that are the de facto standard for non-relayed chat on IRC. Most IRC servers and clients support these extensions, even though they have never been formally specified as an RFC (the closest thing to a specification is this document: http://www.irchelp.org/irchelp/rfc/ctcpspec.html).

Joining, talking in, and leaving a channel


Users connected to an IRC server can join existing channels by using the JOIN message. The format of the message itself is pretty simple (its only parameter is the name of the channel the user wants to join), but it results in several replies being sent not just to the user joining the channel, but also to all the users currently in the channel. The figure above shows what happens when user amy joins channel #tardis, where two users (doctor and river) are already present.

Message 1 is amy’s JOIN message to the server. When this message is received, the server relays it to the users who are already in the channel (doctor and river) to make them aware that there is a new user in the channel (messages 2a and 2b). Notice how the relayed JOIN is prefixed with amy’s full client identifier. The JOIN is also relayed back to amy, as confirmation that she successfully joined the channel.

The following messages (3, 4a, and 4b) provide amy with information about the channel. Message 3 is a RPL_TOPIC reply, providing the channel’s topic (this is a description of the channel which can be set by certain users; we’ll discuss this in detail later). Messages 4a and 4b are RPL_NAMREPLY and RPL_ENDOFNAMES replies, respectively, which tell amy what users are currently present in the channel. Notice how the doctor user has an at-sign before his nick; this indicates that doctor is a channel operator for channel #tardis. As we’ll see in the third assignment, users can have modes that give them special privileges in the server or on individual channels. For example, a channel operator is typically the only type of user that can change the channel’s topic.


Once a user has joined a channel, sending a message to the channel is essentially the same as sending a message to an individual user. The difference is that the server will relay the message to all the users in the channel, instead of just a single user. The figure above shows two messages being sent to channel #tardis. First, user doctor sends a PRIVMSG message, specifying the channel as the target (and not a nick, as we saw in “Messaging between users”). The server then relays this message to river and amy, prefixing the message with doctor’s full client identifier (messages 1, 2a, and 2b). Similarly, amy sends a message to the channel, which is relayed to doctor and river, prefixed with amy’s full client identifier (messages 3, 4a, and 4b)


Leaving a channel is accomplished with the PART message, which follows a similar pattern to joining and talking in the channel: the user wishing to leave sends a PART message, and this message is relayed to everyone in the channel so they are aware that the user has left. The server also internally removes that client from the channel, which means he will no longer receive any messages directed to that channel. The figure above shows an example of what this would look like. Notice how the PART message includes two parameters: the channel the users wants to leave, and a “parting message” (which is relayed as part of the PART message to all users in the channel).