gpasswd - administer /etc/group and /etc/gshadow
gpasswd [option] group
The gpasswd command is used to administer /etc/group, and /etc/gshadow. Every group can have administrators, members and a password.
System administrators can use the -A option to define group administrator(s) and the -M option to define members. They have all rights of group administrators and members.
gpasswd called by a group administrator with a group name only prompts for the new password of the group.
If a password is set the members can still use newgrp(1) without a password, and non-members must supply the password.
Group passwords are an inherent security problem since more than one person is permitted to know the password. However, groups are a useful tool for permitting co-operation between different users.
Except for the -A and -M options, the options cannot be combined.
The options which apply to the gpasswd command are:
-a, --add user
-d, --delete user
-Q, --root CHROOT_DIR
-A, --administrators user,...
-M, --members user,...
This tool only operates on the /etc/group and /etc/gshadow files. Thus you cannot change any NIS or LDAP group. This must be performed on the corresponding server.
The following configuration variables in /etc/login.defs change the behavior of this tool:
It can take one of these values: DES (default), MD5, SHA256, SHA512. MD5 and DES should not be used for new hashes, see crypt(5) for recommendations.
Note: this parameter overrides the MD5_CRYPT_ENAB variable.
The default value is 0, meaning that there are no limits in the number of members in a group.
This feature (split group) permits to limit the length of lines in the group file. This is useful to make sure that lines for NIS groups are not larger than 1024 characters.
If you need to enforce such limit, you can use 25.
Note: split groups may not be supported by all tools (even in the Shadow toolsuite). You should not use this variable unless you really need it.
This variable is superseded by the ENCRYPT_METHOD variable or by any command line option used to configure the encryption algorithm.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)
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), 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.