RHCE first day

 at  Single execution of routine work

crontab  Routine work performed in a loop

1.at Configure in 5 Hours later, , take "This is a at task" write in root Home directory at_test In file

 

2.crontab To configure , Every Saturday , Sunday 9 Half past six ,14 spot Remind me to go RHCE course write in root Home directory cron_test In file

 

3. On two Linux Upper Department chrony service ： requirement ： The first one synchronizes the time from Alibaba cloud server , The second synchronizes the time from the first

Chrony The configuration file is /etc/chrony.conf

# Use pool.ntp.org The public server in the project . With server open , In theory, you can add as many time servers as you want .

server 0.centos.pool.ntp.org iburst
server 1.centos.pool.ntp.org iburst
server 2.centos.pool.ntp.org iburst

# Appoint NTP Client address , To allow or deny connection to the machine acting as the clock server
# Allow NTP client access from local network.
#allow 192.168.0.0/16

 

# View time synchronization status
timedatectl status

see ntp_servers

chronyc sources -v

  Synchronize the first time

 

 

 

4. What is symmetric encryption , What is asymmetric encryption , What are the encryption algorithms of symmetric encryption , What encryption algorithms are needed for asymmetric encryption

  Symmetric encryption refers to Encryption and decryption use the same secret key , So it's called symmetric encryption . Symmetric encryption has only one secret key , As the private key .

Asymmetric encryption refers to Yes. ： Encryption and decryption use different keys , As a public key , The other as a private key . Public key encrypted information , Only the private key can decrypt . Private key encrypted information , Only public key can decrypt .

Symmetric encryption algorithm ：DES、3DES、DESX、Blowfish、IDEA、RC4、RC5、RC6 and AES

Asymmetric encryption algorithm ：RSA、ECC（ For mobile devices ）、Diffie-Hellman、El Gamal、DSA（ For digital signature ）.