Hi Matt,
As root, type "hostname <new hostname>", and the hostname will be changed
immediately. To make this change permanent across reboots, edit the file
'/etc/rc.conf', and add the following line:
hostname="new.host.name"
The '/etc/rc.conf' file is described here:
http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/configtuning-core-configuration.html
-Andy.
来源：https://lists.freebsd.org/pipermail/freebsd-questions/2006-April/119355.html
su
vim /etc/rc.conf
修改hostname
vim /etc/hosts
替换原有的hostname为新的hostname
hostname new-hostname #再执行这条命令，这样就不用reboot，也能生效了。
https://www.cyberciti.biz/faq/howot-freebsd-change-hostname-without-reboot/