Section: hostname (5)
The /etc/hostname file configures the name of the local system that is set during boot using the sethostname?(2) system call. It should contain a single newline-terminated hostname string. The hostname may be a free-form string up to 64 characters in length; however, it is recommended that it consists only of 7-bit ASCII lower-case characters and no spaces or dots, and limits itself to the format allowed for DNS domain name labels, even though this is not a strict requirement.
Depending on the operating system, other configuration files might be checked for configuration of the hostname as well, however only as fallback.