Linux From Scratch - Version 6.1

Chapter 6. Installing Basic System Software

6.54. Shadow-4.0.9

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

Approximate build time: 0.4 SBU
Required disk space: 13.7 MB
Installation depends on: Bash, Binutils, Bison, Coreutils, Diffutils, GCC, Gettext, Glibc, Grep, Make, and Sed

6.54.1. Installation of Shadow

Prepare Shadow for compilation: 

./configure --libdir=/lib --enable-shared

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

sed -i 's/groups$(EXEEXT) //' src/Makefile
sed -i '/groups/d' man/Makefile

Compile the package: 

make

Install the package: 

make install

Shadow uses two files to configure authentication settings for the system. Install these two configuration files: 

cp etc/{limits,login.access} /etc

Instead of using the default crypt method, use the more secure MD5 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. Both of these can be accomplished by changing the relevant configuration file while copying it to its destination: 

sed -e's@#MD5_CRYPT_ENAB.no@MD5_CRYPT_ENAB yes@' \
    -e 's@/var/spool/mail@/var/mail@' \
    etc/login.defs.linux > /etc/login.defs

Move a misplaced program to its proper location: 

mv /usr/bin/passwd /bin

Move Shadow's libraries to more appropriate locations: 

mv /lib/libshadow.*a /usr/lib
rm /lib/libshadow.so
ln -sf ../../lib/libshadow.so.0 /usr/lib/libshadow.so

The -D option of the useradd program requires the /etc/default directory for it to work properly: 

mkdir /etc/default

6.54.2. 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

Under normal circumstances, passwords will not have been created yet. However, if returning to this section later to enable shadowing, reset any current user passwords with the passwd command or any group passwords with the gpasswd command.

6.54.3. Setting the root password

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

passwd root

6.54.4. Contents of Shadow

Installed programs: chage, chfn, chpasswd, chsh, expiry, faillog, gpasswd, groupadd, groupdel, groupmod, groups, grpck, grpconv, grpunconv, lastlog, login, logoutd, mkpasswd, newgrp, newusers, passwd, pwck, pwconv, pwunconv, sg (link to newgrp), useradd, userdel, usermod, vigr (link to vipw), and vipw
Installed libraries: libshadow.[a,so]

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

chpasswd

Used to update the passwords of an entire series of user accounts

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

groupmod

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

groups

Reports the groups of which the given users are members

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

mkpasswd

Generates random passwords

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

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

libshadow

Contains functions used by most programs in this package