Basic Linux security problems

I. Linux cracked the root password. I installed centos on the VM today, but didn't prompt me to enter the password. I couldn't enter the system without the root password.
Solution: 1. Restart 2. Press e3 when the graphic interface appears, select line 3, and select kenel ........................ Press e to enter the editing mode. Add a space at the end and enter 1 such as: kernel/boot/vmlinux =/1 and press enter to exit www.2cto.com 7.) in Item 2, press B, and enter the following link: /usr/sbin/passwd enter the new root password twice.
2. I have read many articles about SSH and password-free logon. I don't think it is difficult to operate, but I can't remember how to find it every time. Now I have recorded it here, convenience in the future:
Step1. a $ ssh-keygen-t rsa one way press ENTER step2. a $ scp ~ /. Ssh/id_rsa.pub B :~ //. Ssh/a. pub // enter the password step3. B $ cat ~ /. Ssh/a. pub> ~ /. Ssh/authorized_keys success. Then, when using machine a to copy files to machine B, you do not need to enter the password. The author chen3888015