Understanding /etc/shadow file

The /etc/shadow file stores actual password in encrypted format (more like the hash of the password) for user’s account with additional properties related to user password. Basically, it stores secure user account information. All fields are separated by a colon (:) symbol. It contains one entry per line for each user listed in /etc/passwd file. Generally, shadow file entry looks as follows.

/etc/shadow file fields

(Fig.01: /etc/shadow file fields)

1. Username : It is your login name.
2. Password : It is your encrypted password. The password should be minimum 8-12 characters long including special characters, digits, lower case alphabetic and more. Usually password format is set to $id$salt$hashed, The $id is the algorithm used On GNU/Linux as follows:
   1. $1$ is MD5
   2. $2a$ is Blowfish
   3. $2y$ is Blowfish
   4. $5$ is SHA-256
   5. $6$ is SHA-512
3. Last password change (lastchanged) : Days since Jan 1, 1970 that password was last changed
4. Minimum : The minimum number of days required between password changes i.e. the number of days left before the user is allowed to change his/her password
5. Maximum : The maximum number of days the password is valid (after that user is forced to change his/her password)
6. Warn : The number of days before password is to expire that user is warned that his/her password must be changed
7. Inactive : The number of days after password expires that account is disabled
8. Expire : days since Jan 1, 1970 that account is disabled i.e. an absolute date specifying when the login may no longer be used.

The last 6 fields provides password aging and account lockout features. You need to use the chage command to setup password aging. According to man page of shadow – the password field must be filled. The encrypted password consists of 13 to 24 characters from the 64 character alphabet a through z, A through Z, 0 through 9, \. and /. Optionally it can start with a “$” character. This means the encrypted password was generated using another (not DES) algorithm. For example if it starts with “$1$” it means the MD5-based algorithm was used. Please note that a password field which starts with a exclamation mark (!) means that the password is locked. The remaining characters on the line represent the password field before the password was locked.