Encryption | RobDurdle.com

Understanding /etc/shadow file

Q. Can you explain /etc/shadow file used under Linux or UNIX?

A. /etc/shadow file stores actual password in encrypted format for user’s account with additional properties related to user password i.e. it stores secure user account information.

All fields are separated by a colon (:) symbol. It has one entry per line for each user listed in /etc/passwd file Generally, shadow file entry looks as follows:

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

User name : It is your login name
Password: It your encrypted password. The password should be minimum 6-8 characters long including special characters/digits
Last password change (lastchanged): Days since Jan 1, 1970 that password was last changed
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
Maximum: The maximum number of days the password is valid (after that user is forced to change his/her password)
Warn : The number of days before password is to expire that user is warned that his/her password must be changed
Inactive : The number of days after password expires that account is disabled
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 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.