Linux From Scratch - Version 7.3

Chapter 6. Installing Basic System Software

6.26. Shadow-4.1.5.1

The Shadow package contains programs for handling passwords in a secure way.

Approximate build time: 0.2 SBU
Required disk space: 42 MB

6.26.1. Installation of Shadow

[Note]

Note

If you would like to enforce the use of strong passwords, refer to http://www.linuxfromscratch.org/blfs/view/svn/postlfs/cracklib.html for installing CrackLib prior to building Shadow. Then add --with-libcrack to the configure command below.

Disable the installation of the groups program and its man pages, as Coreutils provides a better version: 

sed -i 's/groups$(EXEEXT) //' src/Makefile.in
find man -name Makefile.in -exec sed -i 's/groups\.1 / /' {} \;

Instead of using the default crypt method, use the more secure SHA-512 method of password encryption, which also allows passwords longer than 8 characters. It is also necessary to change the obsolete /var/spool/mail location for user mailboxes that Shadow uses by default to the /var/mail location used currently: 

sed -i -e 's@#ENCRYPT_METHOD DES@ENCRYPT_METHOD SHA512@' \
       -e 's@/var/spool/mail@/var/mail@' etc/login.defs
[Note]

Note

If you chose to build Shadow with Cracklib support, run the following: 

sed -i 's@DICTPATH.*@DICTPATH\t/lib/cracklib/pw_dict@' \
    etc/login.defs

Prepare Shadow for compilation: 

./configure --sysconfdir=/etc

Compile the package: 

make

This package does not come with a test suite.

Install the package: 

make install

Move a misplaced program to its proper location: 

mv -v /usr/bin/passwd /bin

6.26.2. Configuring Shadow

This package contains utilities to add, modify, and delete users and groups; set and change their passwords; and perform other administrative tasks. For a full explanation of what password shadowing means, see the doc/HOWTO file within the unpacked source tree. If using Shadow support, keep in mind that programs which need to verify passwords (display managers, FTP programs, pop3 daemons, etc.) must be Shadow-compliant. That is, they need to be able to work with shadowed passwords.

To enable shadowed passwords, run the following command: 

pwconv

To enable shadowed group passwords, run: 

grpconv

Shadow's stock configuration for the useradd utility has a few caveats that need some explanation. First, the default action for the useradd utility is to create the user and a group of the same name as the user. By default the user ID (UID) and group ID (GID) numbers will begin with 1000. This means if you don't pass parameters to useradd, each user will be a member of a unique group on the system. If this behaviour is undesirable, you'll need to pass the -g parameter to useradd. The default parameters are stored in the /etc/default/useradd file. You may need to modify two parameters in this file to suit your particular needs.

/etc/default/useradd Parameter Explanations

GROUP=1000

This parameter sets the beginning of the group numbers used in the /etc/group file. You can modify it to anything you desire. Note that useradd will never reuse a UID or GID. If the number identified in this parameter is used, it will use the next available number after this. Note also that if you don't have a group 1000 on your system the first time you use useradd without the -g parameter, you'll get a message displayed on the terminal that says: useradd: unknown GID 1000. You may disregard this message and group number 1000 will be used.

CREATE_MAIL_SPOOL=yes

This parameter causes useradd to create a mailbox file for the newly created user. useradd will make the group ownership of this file to the mail group with 0660 permissions. If you would prefer that these mailbox files are not created by useradd, issue the following command: 

sed -i 's/yes/no/' /etc/default/useradd

6.26.3. Setting the root password

Choose a password for user root and set it by running: 

passwd root

6.26.4. Contents of Shadow

Installed programs: chage, chfn, chgpasswd, chpasswd, chsh, expiry, faillog, gpasswd, groupadd, groupdel, groupmems, groupmod, grpck, grpconv, grpunconv, lastlog, login, logoutd, newgrp, newusers, nologin, passwd, pwck, pwconv, pwunconv, sg (link to newgrp), su, useradd, userdel, usermod, vigr (link to vipw), and vipw
Installed directory: /etc/default

Short Descriptions

chage

Used to change the maximum number of days between obligatory password changes

chfn

Used to change a user's full name and other information

chgpasswd

Used to update group passwords in batch mode

chpasswd

Used to update user passwords in batch mode

chsh

Used to change a user's default login shell

expiry

Checks and enforces the current password expiration policy

faillog

Is used to examine the log of login failures, to set a maximum number of failures before an account is blocked, or to reset the failure count

gpasswd

Is used to add and delete members and administrators to groups

groupadd

Creates a group with the given name

groupdel

Deletes the group with the given name

groupmems

Allows a user to administer his/her own group membership list without the requirement of super user privileges.

groupmod

Is used to modify the given group's name or GID

grpck

Verifies the integrity of the group files /etc/group and /etc/gshadow

grpconv

Creates or updates the shadow group file from the normal group file

grpunconv

Updates /etc/group from /etc/gshadow and then deletes the latter

lastlog

Reports the most recent login of all users or of a given user

login

Is used by the system to let users sign on

logoutd

Is a daemon used to enforce restrictions on log-on time and ports

newgrp

Is used to change the current GID during a login session

newusers

Is used to create or update an entire series of user accounts

nologin

Displays a message that an account is not available. Designed to be used as the default shell for accounts that have been disabled

passwd

Is used to change the password for a user or group account

pwck

Verifies the integrity of the password files /etc/passwd and /etc/shadow

pwconv

Creates or updates the shadow password file from the normal password file

pwunconv

Updates /etc/passwd from /etc/shadow and then deletes the latter

sg

Executes a given command while the user's GID is set to that of the given group

su

Runs a shell with substitute user and group IDs

useradd

Creates a new user with the given name, or updates the default new-user information

userdel

Deletes the given user account

usermod

Is used to modify the given user's login name, User Identification (UID), shell, initial group, home directory, etc.

vigr

Edits the /etc/group or /etc/gshadow files

vipw

Edits the /etc/passwd or /etc/shadow files