In computer networking, specifically Internet Relay Chat (IRC), netsplit is a term used to describe the disconnection of a given node from the previously established network or between two nodes. As an IRC network is an undirected acyclic graph, a split between any two nodes splits the entire network into two pieces. Consider the graphic to the right, which represents the computer network.
Each line represents an established connection between the two nodes; each node represents either a server or a user on the network. Therefore, the server C is connected directly to A, which is also connected to B and D. If a disruption in the connection between C and A occurs, the connection may in result be terminated. This can occur either by a socket producing an error, or by excessive lag in which the far server A anticipates this case (which is called a timeout). When the connection between A and C is severed, users that were connected to other servers which are no longer reachable on the network appear to quit. For example, if user Sara is connected to server A, user Bob is connected to server B, and user Joe is connected to C, and C splits (disconnects) from A, it will appear to Joe as if Sara and Bob both quit (disconnected from the network), and it will appear to both Sara and Bob that Joe quit. This happens because the servers in which they are connected are informed of the change in the network status, and update their local information accordingly to display the change. Later, server C may relink (reconnect) to a server on the network and the users who appeared to have quit will rejoin; the process of sending this updated information to all servers on the network is called a netburst (or sync). Occasionally some users will attempt to use netsplits to gain access to passworded channels. If none of the channel users were on server C, a user could join a private channel and later gain access when the servers relink. This is commonly known as riding the split. Another typical netsplit-oriented IRC attack is Nickname Colliding. In this attack, a user on a split segment of the network would change nicknames to that of a user on the other side of the split network. Upon reconnection, the network would disconnect both users due to the fact that only one nickname may be in use at one time. Modern IRC server software has largely eliminated this method, but servers using older software may still be vulnerable. For more information, see IRC takeover.
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What does a netsplit look like?
Below is an example of typical netsplit. When two servers split, a user sees this as large number of users quitting. After the servers are reconnected, it is seen as the users rejoining again.
On regular IRC clients
* usera has quit (a.irc.net b.irc.net) * userb has quit (a.irc.net b.irc.net) * userc has quit (a.irc.net b.irc.net) * usera has joined #channel * userb has joined #channel * userc has joined #channel
On command-line client irssi
00:00 -!- Netsplit a.irc.net <-> b.irc.net quits: usera, userb, userc 00:00 -!- Netsplit over, joins: usera, userb, userc
To a user on a private network
* usera has quit (*.net *.split) * userb has quit (*.net *.split) * userc has quit (*.net *.split) * usera has joined #channel * userb has joined #channel * userc has joined #channel
