CentOS 7已經不用xinetd了, 因此不會安裝啟動腳本到/etc/xinitd.d/中了, 所以找不到, 請使用Systemd:
systemctl start telnet.socket
CentOS 7 no longer uses xinetd. No will any startup script be installed onto the directory /etc/xinitd.d, casuing the not found error. Please use Systemd:
systemctl start telnet.socket
Try to follow this guide step-by-step:
https://www.unixmen.com/installing-telnet-centosrhelscientific-linux-6-7/