A master server is a server that AssaultCube game servers register with. It distributes a list of registered servers to the AssaultCube clients.
The default master server, often abbreviated to MS, accepts http requests at assault.cubers.net on port 28760. A default AssaultCube server sends its name, IP address and port number to the master server, in order to be added to the server list. When the menu option "Multiplayer -> Join a server" is selected in a default client, the list of registered servers is sent to the client.
Contrary to popular belief, the MS does not store or report information about client numbers, map rotations or current play states. These are all reported by the individual AssaultCube game servers.
Alternative master servers
In most cases all clients will only ever need to connect to one master server. It is, however, possible to create a new master server, compiling the binary from the MS source code. AssualtCube servers and clients can be configured to report to, or request a server list from, a different master server, in both cases using the -m switch.
It is possible to prevent servers from registering with the MS, and to prevent clients from receiving the server list, using the Central blacklist.