Что пишется в файле hostname?

Every GNU/Linux system has a hostname, whether you like it or not. A system's hostname usually has a corresponding entry in the domain name system (DNS), but it doesn't necessarily have to. The hostname is the name that programs such as mail and web servers use to identify the system that they are running on. Many of these programs will issue warnings, or in some cases completely fail to run if the hostname is not set correctly.