7.6. Creating Essential Files and Symlinks

Historically, Linux maintains 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 the presence of /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
systemd-journal-gateway:x:73:73:systemd Journal Gateway:/:/usr/bin/false
systemd-journal-remote:x:74:74:systemd Journal Remote:/:/usr/bin/false
systemd-journal-upload:x:75:75:systemd Journal Upload:/:/usr/bin/false
systemd-network:x:76:76:systemd Network Management:/:/usr/bin/false
systemd-resolve:x:77:77:systemd Resolver:/:/usr/bin/false
systemd-timesync:x:78:78:systemd Time Synchronization:/:/usr/bin/false
systemd-coredump:x:79:79:systemd Core Dumper:/:/usr/bin/false
uuidd:x:80:80:UUID Generation Daemon User:/dev/null:/usr/bin/false
systemd-oom:x:81:81:systemd Out Of Memory Daemon:/:/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:
usb:x:14:
cdrom:x:15:
adm:x:16:
messagebus:x:18:
systemd-journal:x:23:
input:x:24:
mail:x:34:
kvm:x:61:
systemd-journal-gateway:x:73:
systemd-journal-remote:x:74:
systemd-journal-upload:x:75:
systemd-network:x:76:
systemd-resolve:x:77:
systemd-timesync:x:78:
systemd-coredump:x:79:
uuidd:x:80:
systemd-oom:x:81:
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 convention 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 http://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 tty group, and the number 5 is also used in systemd 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 for it 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.