Linux: how to recover root password

In Linux, if you forget the root password, you can recover it in this way:

1. Reboot the machine, at grub menu, edit the boot entry for Linux
2. press 'e' to to edit the entry,
   append " single" at the end of the kernel line
   alternatively, you can also append " init=/bin/sh" at the end of the kernel line
3. press 'b' to boot Linux into single user mode
4. now you will be able to log in the system without entering password 
5. type 'passwd' to reset root password

Use PAM to enforce Linux password complexity

It's always an audit requirement to have a system not too short and not so easy to be guessed.
To enforce the password length, we can use /etc/login.defs

PASS_MIN_LEN      8

next time when user changes password, anything shorter than 8 characters will be rejected.

To enforce the password complexity, we have to make sure it consists of  uppercase, lowercase, special characters, and digits. This can be easily done through the use of PAM.

$ man pam_cracklib

lcredit=N

(N >= 0) This is the maximum credit for having lower case letters in the new password.
(N < 0) This is the minimum number of lower case letters that must be met for a new password.

So to force at least 1 lowercase character in the password, we should use negative number, lcredit=-1
To enforce a password having at least 4 lower cases, 2 upper cases, 1 special character, and 1 digit, we can update the /etc/pam.d/system-auth

password requisite pam_cracklib.so dcredit=-1 ucredit=-2 lcredit=-4 ocredit=-1