A. /etc/passwd file stores essential information, which is required during login i.e. user account information.
/etc/passwd is a text file, that contains a list of the system’s accounts, giving for each account some useful information like user ID, group ID, home directory, shell, etc. It should have general read permission as many utilities, like ls use it to map user IDs to user names, but write access only for the superuser (root).
Understanding fields in /etc/passwd
The /etc/passwd contains one entry per line for each user (or user account) of the system. All fields are separated by a colon (:) symbol. Total seven fields as follows.
Generally, passwd file entry looks as follows (click to enlarge image):
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/etc/passwd is only used for local users only. To see list of all users, enter:
$ cat /etc/passwd
To search for a username called tom, enter:
$ grep tom /etc/passwd
/etc/passwd file permission
The permission on the /etc/passwd file should be read only to users (-rw-r–r–) and the owner must be root:
$ ls -l /etc/passwd
Output:
-rw-r--r-- 1 root root 2659 Sep 17 01:46 /etc/passwd
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 contains one entry per line for each user listed in /etc/passwd file. Generally, shadow file entry looks as follows (click to enlarge image):
| 1. User name : It is your login name 2. Password: It your encrypted password. The password should be minimum 6-8 characters long including special characters/digits 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 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.
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