8.30. Shadow-4.20.0

#### Linux From Scratch - Version r13.0-174-systemd

### Chapter 8. Installing Basic System Software

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# 8.30. Shadow-4.20.0

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

Approximate build time: 0.1 SBU
Required disk space: 115 MB

## 8.30.1. Installation of Shadow

### Important

If you've installed Linux-PAM, you should follow the BLFS instructions instead of this page to build, rebuild, upgrade shadow.

### Note

If you would like to enforce the use of strong passwords, install and configure Linux-PAM first. Then install and configure shadow with the PAM support. Finally install libpwquality and configure PAM to use it.

Prevent the installation of manual pages that were already installed in Section 8.3, “Man-pages-6.18”:

    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`.

### Warning

Including `/bin` and/or `/sbin` in the `PATH` variable may cause some BLFS packages fail to build, so don't do that in the `.bashrc` file or anywhere else.

    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

Now fix a problem with included headers:

    sed -i '/stdio.h/i #include <stdint.h>' lib/find_new_sub_*ids.c

Prepare Shadow for compilation:

    touch /usr/bin/passwd
    ./configure --sysconfdir=/etc   \
                --disable-static    \
                --with-{b,yes}crypt \
                --without-libbsd    \
                --disable-logind    \
                --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.

`--disable-logind`

  This option makes Shadow (specifically, the login and who programs) use the `/run/utmp` file instead of logind to track the active login sessions, as logind isn't available yet in the incomplete LFS system. But as we've discussed in Section 7.6, “Creating Essential Files and Symlinks”, the `/run/utmp` file format will be completely broken after year 2038. The LFS editors will attempt to resolve the issue before that year.

`--without-libbsd`

  Do not use the readpassphrase function from libbsd which is not in LFS. Use the internal copy instead.

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

## 8.30.2. 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

Finally, create the empty `/etc/subuid` and `/etc/subgid` files:

    touch /etc/sub{u,g}id

The content of those files will be automatically updated by some utilities provided by Shadow to allocate subordinate user and group IDs. Read the man pages subuid(5) and subgid(5) for the details about the subordinate IDs.

## 8.30.3. Setting the Root Password

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

    passwd root

## 8.30.4. Contents of Shadow

Installed programs: chage, chfn, chgpasswd, chpasswd, chsh, expiry, faillog, getsubids, gpasswd, groupadd, groupdel, groupmems, groupmod, grpck, grpconv, grpunconv, 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
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

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