table of contents
| CHPASSWD(8) | System Management Commands | CHPASSWD(8) |
NAME¶
chpasswd - update passwords in batch mode
SYNOPSIS¶
chpasswd [options]
DESCRIPTION¶
The chpasswd command reads a list of user name and password pairs from standard input and uses this information to update a group of existing users. Each line is of the format:
user_name:password
By default the passwords must be supplied in clear-text, and are encrypted by chpasswd. Also the password age will be updated, if present.
By default, passwords are encrypted by PAM, but (even if not recommended) you can select a different encryption method with the -e, -m, or -c options.
Except when PAM is used to encrypt the passwords, chpasswd first updates all the passwords in memory, and then commits all the changes to disk if no errors occurred for any user.
When PAM is used to encrypt the passwords (and update the passwords in the system database) then if a password cannot be updated chpasswd continues updating the passwords of the next users, and will return an error code on exit.
This command is intended to be used in a large system environment where many accounts are created at a single time.
OPTIONS¶
The options which apply to the chpasswd command are:
-c, --crypt-method METHOD
The available methods are DES, MD5, SHA256, SHA512, YESCRYPT and NONE if your libc supports these methods.
By default, PAM is used to encrypt the passwords.
-e, --encrypted
-h, --help
-m, --md5
-R, --root CHROOT_DIR
-P, --prefix PREFIX_DIR
-s, --sha-rounds ROUNDS
You can only use this option with crypt method: SHA256 SHA512 YESCRYPT
By default, the number of rounds for SHA256 or SHA512 is defined by the SHA_CRYPT_MIN_ROUNDS and SHA_CRYPT_MAX_ROUNDS variables in /etc/login.defs.
A minimal value of 1000 and a maximal value of 999,999,999 will be enforced for SHA256 and SHA512. The default number of rounds is 5000.
By default, the number of rounds for YESCRYPT is defined by the YESCRYPT_COST_FACTOR in /etc/login.defs.
A minimal value of 1 and a maximal value of 11 will be enforced for YESCRYPT. The default number of rounds is 5.
CAVEATS¶
Remember to set permissions or umask to prevent readability of unencrypted files by other users.
CONFIGURATION¶
The following configuration variables in /etc/login.defs change the behavior of this tool:
SHA_CRYPT_MIN_ROUNDS (number), SHA_CRYPT_MAX_ROUNDS (number)
With a lot of rounds, it is more difficult to brute force the password. But note also that more CPU resources will be needed to authenticate users.
If not specified, the libc will choose the default number of rounds (5000), which is orders of magnitude too low for modern hardware.
The values must be inside the 1000-999,999,999 range.
If only one of the SHA_CRYPT_MIN_ROUNDS or SHA_CRYPT_MAX_ROUNDS values is set, then this value will be used.
If SHA_CRYPT_MIN_ROUNDS > SHA_CRYPT_MAX_ROUNDS, the highest value will be used.
Note: This only affect the generation of group passwords. The generation of user passwords is done by PAM and subject to the PAM configuration. It is recommended to set this variable consistently with the PAM configuration.
YESCRYPT_COST_FACTOR (number)
With a high cost factor, it is more difficult to brute force the password. But note also that more CPU resources will be needed to authenticate users.
The value must be inside the 1-11 range.
Note: This only affect the generation of group passwords. The generation of user passwords is done by PAM and subject to the PAM configuration. It is recommended to set this variable consistently with the PAM configuration.
FILES¶
/etc/passwd
/etc/shadow
/etc/login.defs
/etc/pam.d/chpasswd
SEE ALSO¶
| 08/05/2024 | shadow-utils 4.16.0 |