shadow - shadow password file
/etc/shadow is an access-restricted ASCII system file that stores users’ hashed passwords and related information. The shadow file can be used in conjunction with other shadow sources, including the NIS maps passwd.byname and passwd.byuid or password data stored on an LDAP server. Programs use the getspnam(3C) routines to access this information. Shell scripts use the getent(8) command to access this information.
Unlike the /etc/passwd file, /etc/shadow does not have general read permission.
The fields for each user entry are separated by colons. Each user is separated from the next by a newline. Each entry in the shadow file is a single line of the form:
username:password :lastchg:min: max:warn:inactive: expire:flag
The fields are defined as follows:
The user’s login name (UID).
A cryptographically hashed password for the user generated by crypt(3C) or pwhash(1), a lock string to indicate that the login is not accessible, or no string, which shows that there is no password for the login.
The lock string is defined as *LK* in the first four characters of the password field if the account was manually locked, or *AL* if the account was automatically locked due to the number of authentication failures reaching the configured maximum allowed. See policy.conf(5) and user_attr(5).
The number of days between January 1, 1970, and the date that the password was last modified. The lastchg value is a decimal number, as interpreted by strtol(3C).
The minimum number of days required between password changes. This field must be set to 0 or above to enable password aging.
The maximum number of days the password is valid.
The number of days before password expires that the user is warned.
The number of days of inactivity allowed for that user. This is counted on a per-machine basis; the information about the last login is taken from the machine’s lastlog file.
An absolute date expressed as the number of days since the UNIX Epoch (January 1, 1970). When this number is reached the login can no longer be used. For example, an expire value of 17410 specifies a login expiration of September 1, 2017.
Reserved. May be set to arbitrary values. Traditionally, the low order for bits are a failed login count.
The bits in the remainder may or may not be zero. They may be used at any time for any other purposes.
A value of −1 for min, max, or warn disables password aging.
The encrypted password consists of at most CRYPT_MAXCIPHERTEXTLEN characters chosen from a 64-character alphabet (., /, 0–9, A–Z, a–z). Two additional special characters: the dollar sign ($) and the comma (,), can also be used and are defined in crypt(3C).
To make system administration manageable, /etc/shadow entries should appear in exactly the same order as /etc/passwd entries.
Values for the various time-related fields are interpreted as Coordinated Universal Time (UTC).
The authorizations, as defined in user_attr(5), which are required to modify the various shadow fields are as follows:
Shadow password file
Name-service switch configuration file
Time of last login
See attributes(7) for descriptions of the following attributes:
login(1), passwd(1), pwhash(1), strtol(3C), crypt(3C), crypt_gensalt(3C), getspnam(3C), putspent(3C), pam_chauthtok(3PAM), usermgr-1(3rad), nsswitch.conf(5), passwd(5), attributes(7), pam_unix_account(7), pam_unix_auth(7), useradm(8), useradd(8), userdel(8), usermod(8)
If password aging is turned on in any name service the passwd: line in the /etc/nsswitch.conf file must have a format specified in the nsswitch.conf(5) man page.
If the /etc/nsswitch.conf passwd policy is not in one of the supported formats, logins will not be allowed upon password expiration, because the software does not know how to handle password updates under these conditions. See nsswitch.conf(5) for additional information.