Client Registration

When a client connects, it must REGISTER before sending other commands:

C: NICK alice
C: USER alice 0 * :Alice Smith
S: :irc.example.com 001 alice :Welcome ...

• NICK: 9-char-max nickname. Must be unique on the network.
• USER: username (typically ident-style), mode-bits, unused, real name (in trailing param).

Server replies:

• 001 RPL_WELCOME
• 002 RPL_YOURHOST
• 003 RPL_CREATED
• 004 RPL_MYINFO (server, version, modes)
• 005 RPL_ISUPPORT (capability negotiation in older form)
• ... and the MOTD (Message of the Day).

Modern: capability negotiation via IRCv3:

C: CAP LS 302
S: CAP * LS :sasl multi-prefix server-time message-tags
C: CAP REQ :sasl message-tags
S: CAP * ACK :sasl message-tags
C: AUTHENTICATE PLAIN
S: AUTHENTICATE +
C: AUTHENTICATE <base64 PLAIN credentials>
S: 903 alice :SASL authentication successful
C: CAP END
C: NICK alice
C: USER alice 0 * :Alice

CAP REQ asks for capabilities; ACK confirms. SASL provides authenticated NICK before registration completes.

Errors during registration:

• 432 ERR_ERRONEOUSNICKNAME: invalid characters.
• 433 ERR_NICKNAMEINUSE: someone else has this nick.
• 461 ERR_NEEDMOREPARAMS: command missing required args.
• 462 ERR_ALREADYREGISTRED: trying to NICK/USER twice.
• 451 ERR_NOTREGISTERED: command before registration.

After registration:

• Client can JOIN, PRIVMSG, etc.
• Client must respond to PING with PONG to stay connected.

Identity:

• nick: visible name, can change via /nick.
• user: ident, fixed for session.
• host: client's hostname (or cloak).
• Combined: nick!user@host is the userhost mask.