A G-line, also known as a global kill line, is a global network ban. It's similar to a K-line or kill line in that a user is banned from a network. A K-line only disconnects a user from a single server, while a G-line disconnects the user from an entire network. A user usually receives a G-line after they have received a K-line on another server and continues to abuse the network from a different server.
A G-line is an extreme measure used as a last resort when all previous attempts to discipline an abusive user have failed. This measure also means that only high-ranking global IRC (Internet Relay Chat) operators are usually permitted to set G-lines, especially on larger networks. K-lines are usually reserved for local affairs and handled by individual servers operators.
There are many reasons why a user is banned from a network by a G-line. It happens when a user violates a network policy multiple times or becomes abusive to the network and other users.
Some G-lines are in place for only a short period, which means the user has to wait for it to wear off. Other G-lines can be put into place for a longer time, and if the abuse is severe enough, a G-line can be permanent.
A G-line can be stored in the configuration file of an IRCd, but some network operators prefer to store them in the service's configuration files instead. If a G-lined user attempts to access the IRC network from which they were banned, they are automatically disconnected. Sometimes they may receive a message stating the reason for the ban, but they may also be disconnected with no notice.
Despite their similarities to K-lines, G-lines work differently from other kill lines. A G-line is set as either *@IPaddress or *@ghost, but the first option is usually preferred. When the *@IPaddress option is used, the G-line waits for the user's identity protocol to be entered when they log onto the network. If the user is G-lined, the identity protocol appears as a match on the G-line list, and the user is immediately disconnected.
If the *@ghost option is used, the server performs a reverse DNS (domain name system) lookup to determine if the user's IP address is on the G-line list. This process takes more time and may cause a delay before the ban takes effect. It is also why this method is not the preferred method. The user gets disconnected from the network but can still get onto the network for a short time.