During an IRC split (when two servers on the same IRC network are desyncronized), it's sometimes possible for two users to join the IRC network with the same nick, by logging in to disconnected servers. Since a server can't tell what nicks are occupied on the other server, the situation will persist for some time -- but once the servers reestablish connection, the matter would need to be solved.

The solution of a typical IRC server to this dilemma would be to disconnect both users sharing a nick (by issuing a /kill command).

The technique was abused by malicious users to disconnect other legitimate users and bots from IRC.

There are two types of nick collisions:
  • The first one is the one described by Toastie and the most common. It happens when two clients are using the same nickname but are on servers being separated by an IRC Split. These days, it is somewhat difficult to collide someone like this, since the splitted server will not allow anybody to take a nickname that was on another server before the netsplit, for a long period of time (I think it's 45 minutes on IRCNet :?).
    This technique is most of the times used by 31337 IRC Warriors, in conjuction with Server-Opping, to takeover channels..
  • The second type of nick collision is called 'Lag Collide'. It is done by using two clients on different servers. The first client sits on a fast server and monitors nick changes and sometimes joins. The second one sits on a lagged server (such as irc.stealth.net on IRCNet) and changes nickname everytime the first client sees a nick change. Since the lagged server does not see the nick change on the fast server as soon as the client does, it will accept the nickchange of the malicious user. Then, as soon as the lagged server sees the nick change from the fast one, it will kill the malicious user (second client) AND the unsuspecting user (the one that changed nickname first). This kind of collision cannot really be used to takeover a channel (except if there are bots that change nickname every x seconds..), but is very annoying and difficult to stop.
A nick collision looks like this (on IRCNet):
*** Mon\GNAM (~Monchan@italian.warez.system.n3tw0rk.wox.org) Quit (Killed (ircd.stealth.net (Mon\GNAM[*.de] != irc-1.stealth.net[@206.252.192.195])))

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