7.6. Creating Essential Files and Symlinks

#### Linux From Scratch - Version r13.0-131

### Chapter 7. Entering Chroot and Building Additional Temporary Tools

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# 7.6. Creating Essential Files and Symlinks

Historically, Linux maintained a list of the mounted file systems in the file `/etc/mtab`. Modern kernels maintain this list internally and expose it to the user via the `/proc` filesystem. To satisfy utilities that expect to find `/etc/mtab`, create the following symbolic link:

    ln -sv /proc/self/mounts /etc/mtab

Create a basic `/etc/hosts` file to be referenced in some test suites, and in one of Perl's configuration files as well:

    cat > /etc/hosts << EOF
    127.0.0.1  localhost $(hostname)
    ::1        localhost
    EOF

In order for user `root` to be able to login and for the name “root” to be recognized, there must be relevant entries in the `/etc/passwd` and `/etc/group` files.

Create the `/etc/passwd` file by running the following command:

    cat > /etc/passwd << "EOF"
    root:x:0:0:root:/root:/bin/bash
    bin:x:1:1:bin:/dev/null:/usr/bin/false
    daemon:x:6:6:Daemon User:/dev/null:/usr/bin/false
    messagebus:x:18:18:D-Bus Message Daemon User:/run/dbus:/usr/bin/false
    uuidd:x:80:80:UUID Generation Daemon User:/dev/null:/usr/bin/false
    nobody:x:65534:65534:Unprivileged User:/dev/null:/usr/bin/false
    EOF

The actual password for `root` will be set later.

Create the `/etc/group` file by running the following command:

    cat > /etc/group << "EOF"
    root:x:0:
    bin:x:1:daemon
    sys:x:2:
    kmem:x:3:
    tape:x:4:
    tty:x:5:
    daemon:x:6:
    floppy:x:7:
    disk:x:8:
    lp:x:9:
    dialout:x:10:
    audio:x:11:
    video:x:12:
    utmp:x:13:
    clock:x:14:
    cdrom:x:15:
    adm:x:16:
    messagebus:x:18:
    input:x:24:
    mail:x:34:
    kvm:x:61:
    uuidd:x:80:
    wheel:x:97:
    users:x:999:
    nogroup:x:65534:
    EOF

The created groups are not part of any standard—they are groups decided on in part by the requirements of the Udev configuration in Chapter 9, and in part by common conventions employed by a number of existing Linux distributions. In addition, some test suites rely on specific users or groups. The Linux Standard Base (LSB, available at https://refspecs.linuxfoundation.org/lsb.shtml) only recommends that, besides the group `root` with a Group ID (GID) of 0, a group `bin` with a GID of 1 be present. The GID of 5 is widely used for the `tty` group, and the number 5 is also used in `/etc/fstab` for the `devpts` filesystem. All other group names and GIDs can be chosen freely by the system administrator since well-written programs do not depend on GID numbers, but rather use the group's name.

The ID 65534 is used by the kernel for NFS and separate user namespaces for unmapped users and groups (those exist on the NFS server or the parent user namespace, but “do not exist” on the local machine or in the separate namespace). We assign `nobody` and `nogroup` to avoid an unnamed ID. But other distros may treat this ID differently, so any portable program should not depend on this assignment.

Some tests in Chapter 8 need a regular user. We add this user here and delete this account at the end of that chapter.

    echo "tester:x:101:101::/home/tester:/bin/bash" >> /etc/passwd
    echo "tester:x:101:" >> /etc/group
    install -o tester -d /home/tester

To remove the “I have no name!” prompt, start a new shell. Since the `/etc/passwd` and `/etc/group` files have been created, user name and group name resolution will now work:

    exec /usr/bin/bash --login

The login, agetty, and init programs (and others) use a number of log files to record information such as who was logged into the system and when. However, these programs will not write to the log files if they do not already exist. Initialize the log files and give them proper permissions:

    touch /var/log/{btmp,lastlog,faillog,wtmp}
    chgrp -v utmp /var/log/lastlog
    chmod -v 664  /var/log/lastlog
    chmod -v 600  /var/log/btmp

The `/var/log/wtmp` file records all logins and logouts. The `/var/log/lastlog` file records when each user last logged in. The `/var/log/faillog` file records failed login attempts. The `/var/log/btmp` file records the bad login attempts.

### Note

The `/run/utmp` file records the users that are currently logged in. This file is created dynamically in the boot scripts.

### Note

The `utmp`, `wtmp`, `btmp`, and `lastlog` files use 32-bit integers for timestamps and they'll be fundamentally broken after year 2038. Many packages have stopped using them and other packages are going to stop using them. It is probably best to consider them deprecated.

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