CHPASSWD

Section: System Management Commands (8)
Updated: 07/24/2009
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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 supplied password must be in clear-text, and is encrypted by chpasswd. Also the password age will be updated, if present.

The default encryption algorithm can be defined for the system with the ENCRYPT_METHOD variable of /etc/login.defs, and can be overwiten with the -e, -m, or -c options.

chpasswd first update the password in memory, and then commit all the changes to disk if no errors occured for any users.

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

: Use the specified method to encrypt the passwords.
The available methods are DES, MD5, NONE, and SHA256 or SHA512 if your libc support these methods.

-e, --encrypted

: Supplied passwords are in encrypted form.

-h, --help

: Display help message and exit.

-m, --md5

: Use MD5 encryption instead of DES when the supplied passwords are not encrypted.

-s, --sha-rounds

Use the specified number of rounds to encrypt the passwords.

The value 0 means that the system will choose the default number of rounds for the crypt method (5000).

A minimal value of 1000 and a maximal value of 999,999,999 will be enforced.

You can only use this option with the SHA256 or SHA512 crypt method.

By default, the number of rounds is defined by the SHA_CRYPT_MIN_ROUNDS and SHA_CRYPT_MAX_ROUNDS variables in /etc/login.defs.

CAVEATS

Remember to set permissions or umask to prevent readability of unencrypted files by other users.

You should make sure the passwords and the encryption method respect the system's password policy.

CONFIGURATION

The following configuration variables in /etc/login.defs change the behavior of this tool:

ENCRYPT_METHOD (string)

This defines the system default encryption algorithm for encrypting passwords (if no algorithm are specified on the command line).

It can take one of these values:

: * DES (default)

: * MD5

: * SHA256

: * SHA512

: Note: this parameter overrides the MD5_CRYPT_ENAB variable.

MD5_CRYPT_ENAB (boolean)

Indicate if passwords must be encrypted using the MD5-based algorithm. If set to yes, new passwords will be encrypted using the MD5-based algorithm compatible with the one used by recent releases of FreeBSD. It supports passwords of unlimited length and longer salt strings. Set to no if you need to copy encrypted passwords to other systems which don't understand the new algorithm. Default is no.

This variable is superceded by the ENCRYPT_METHOD variable or by any command line option used to configure the encryption algorithm.

This variable is deprecated. You should use ENCRYPT_METHOD.

SHA_CRYPT_MIN_ROUNDS (number), SHA_CRYPT_MAX_ROUNDS (number)

When ENCRYPT_METHOD is set to SHA256 or SHA512, this defines the number of SHA rounds used by the encryption algorithm by default (when the number of rounds is not specified on the command line).

With a lot of rounds, it is more difficult to brute forcing 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).

The values must be inside the 1000-999999999 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.

FILES

/etc/passwd

: User account information.

/etc/shadow

: Secure user account information.

/etc/login.defs

: Shadow password suite configuration.