Linux From Scratch - Version 12.0-rc1

Chapter 8. Installing Basic System Software

8.26. Shadow-4.13

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

Approximate build time: 0.1 SBU
Required disk space: 46 MB

8.26.1. Installation of Shadow

[Note]

Note

If you would like to enforce the use of strong passwords, refer to https://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. Also, prevent the installation of manual pages that were already installed in Section 8.3, “Man-pages-6.05.01”: 

sed -i 's/groups$(EXEEXT) //' src/Makefile.in
find man -name Makefile.in -exec sed -i 's/groups\.1 / /'   {} \;
find man -name Makefile.in -exec sed -i 's/getspnam\.3 / /' {} \;
find man -name Makefile.in -exec sed -i 's/passwd\.5 / /'   {} \;

Instead of using the default crypt method, use the much more secure YESCRYPT 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. And, remove /bin and /sbin from the PATH, since they are simply symlinks to their counterparts in /usr.

[Note]

Note

If you wish to include /bin and/or /sbin in the PATH for some reason, modify the PATH in .bashrc after LFS has been built. 

sed -e 's:#ENCRYPT_METHOD DES:ENCRYPT_METHOD YESCRYPT:' \
    -e 's:/var/spool/mail:/var/mail:'                   \
    -e '/PATH=/{s@/sbin:@@;s@/bin:@@}'                  \
    -i etc/login.defs
[Note]

Note

If you chose to build Shadow with Cracklib support, issue this command: 

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

Prepare Shadow for compilation: 

touch /usr/bin/passwd
./configure --sysconfdir=/etc   \
            --disable-static    \
            --with-{b,yes}crypt \
            --with-group-name-max-length=32

The meaning of the new configuration options:

touch /usr/bin/passwd

The file /usr/bin/passwd needs to exist because its location is hardcoded in some programs; if it does not already exist, the installation script will create it in the wrong place.

--with-{b,yes}crypt

The shell expands this to two switches, --with-bcrypt and --with-yescrypt. They allow shadow to use the Bcrypt and Yescrypt algorithms implemented by Libxcrypt for hashing passwords. These algorithms are more secure (in particular, much more resistant to GPU-based attacks) than the traditional SHA algorithms.

--with-group-name-max-length=32

The longest permissible user name is 32 characters. Make the maximum length of a group name the same.

Compile the package: 

make

This package does not come with a test suite.

Install the package: 

make exec_prefix=/usr install
make -C man install-man

8.26.2. 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 you use 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 must 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 default configuration for the useradd utility needs some explanation. First, the default action for the useradd utility is to create the user and a group with the same name as the user. By default the user ID (UID) and group ID (GID) numbers will begin at 1000. This means if you don't pass extra parameters to useradd, each user will be a member of a unique group on the system. If this behavior is undesirable, you'll need to pass either the -g or -N parameter to useradd, or else change the setting of USERGROUPS_ENAB in /etc/login.defs. See useradd(8) for more information.

Second, to change the default parameters, the file /etc/default/useradd must be created and tailored to suit your particular needs. Create it with: 

mkdir -p /etc/default
useradd -D --gid 999

/etc/default/useradd parameter explanations

GROUP=999

This parameter sets the beginning of the group numbers used in the /etc/group file. The particular value 999 comes from the --gid parameter above. You may set it to any desired value. 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. Note also that if you don't have a group with an ID equal to this number on your system, then the first time you use useradd without the -g parameter, an error message will be generated—useradd: unknown GID 999, even though the account has been created correctly. That is why we created the group users with this group ID in Section 7.6, “Creating Essential Files and Symlinks”.

CREATE_MAIL_SPOOL=yes

This parameter causes useradd to create a mailbox file for each new user. useradd will assign the group ownership of this file to the mail group with 0660 permissions. If you would rather not create these files, issue the following command: 

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

8.26.3. Setting the Root Password

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

passwd root

8.26.4. Contents of Shadow

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

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, and to reset the failure count

getsubids

Is used to list the subordinate id ranges for a user

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

newgidmap

Is used to set the gid mapping of a user namespace

newgrp

Is used to change the current GID during a login session

newuidmap

Is used to set the uid mapping of a user namespace

newusers

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

nologin

Displays a message saying an account is not available; it is designed to be used as the default shell for disabled 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 specified 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

libsubid

library to handle subordinate id ranges for users and groups