RTC , A real-time clock , hardware clock
System clock , known as a software clock, that is maintained by the kernel and its initial
value is based on the real-time clock. Once the system is booted and the system clock is
initialized, the system clock is completely independent of the real-time clock.
timedatectl , hwclock , date .
separate different net into different zones.
/usr/share , /usr/lib ,
Pluggable Authentication Modules
SMTP , POP/IMAP
SMTP used for transfer from client to server and server 2 server .
POP/IMAP used for retrieve email from mail server .
- Mail Transport Agent
- Mail Delivery Agent
- Mail User Agent is synonymous with an email client application
--Write by Marcustar，关关雎鸠，在河之洲