5.3. Creating the passwd, group, and lastlog Files

A proper Linux system maintains a list of the mounted file systems in the file /etc/mtab. With the way our embedded system is designed, we will be using a symlink to /proc/mounts:

ln -svf ../proc/mounts ${CLFS}/targetfs/etc/mtab

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.

cat > ${CLFS}/targetfs/etc/passwd << "EOF"
root::0:0:root:/root:/bin/ash
EOF

The actual password for root (the “::” used here is just a placeholder and allow you to login with no password) will be set later.

Additional optional users you may want to add:

bin:x:1:1:bin:/bin:/bin/false

Can be useful for compatibility with legacy applications.

daemon:x:2:6:daemon:/sbin:/bin/false

It is often recommended to use an unprivileged User ID/Group ID for daemons in order to limit their access to the system.

adm:x:3:16:adm:/var/adm:/bin/false

Was used for programs that performed administrative tasks.

lp:x:10:9:lp:/var/spool/lp:/bin/false

Used by programs for printing.

mail:x:30:30:mail:/var/mail:/bin/false

Often used by email programs.

news:x:31:31:news:/var/spool/news:/bin/false

Often used for network news servers.

uucp:x:32:32:uucp:/var/spool/uucp:/bin/false

Often used for Unix-to-Unix Copy of files from one server to the next

operator:x:50:0:operator:/root:/bin/ash

Often used to allow system operators to access the system.

postmaster:x:51:30:postmaster:/var/spool/mail:/bin/false

Generally used as an account that receives all the information of troubles with the mail server.

nobody:x:65534:65534:nobody:/:/bin/false

Used by NFS.

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

cat > ${CLFS}/targetfs/etc/group << "EOF"
root:x:0:
bin:x:1:
sys:x:2:
kmem:x:3:
tty:x:4:
tape: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:
EOF

Additional optional groups you may want to add

adm:x:16:root,adm,daemon

All users in this group are allowed to do administrative tasks

console:x:17:

This group has direct access to the console

cdrw:x:18:

This group is allowed to use the CDRW drive

mail:x:30:mail

Used by MTAs (Mail Transport Agents)

news:x:31:news

Used by Network News Servers

uucp:x:32:uucp

Used by the Unix-to-Unix copy users

users:x:100:

The default GID used by shadow for new users

nogroup:x:65533:

This is a default group used by some programs that do not require a group

nobody:x:65534:

This is used by NFS

The created groups are not part of any standard—they are groups decided on in part by the requirements of BusyBox later in this chapter, and in part by common convention employed by a number of existing Linux distributions. The Linux Standard Base (LSB, available at http://www.linuxbase.org) recommends only that, besides the group root with a Group ID (GID) of 0, a group bin with a GID of 1 be present. 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 login, agetty, and init programs (and others) use the lastlog file to record information such as who was logged into the system and when. However, these programs will not write to the lastlog file if it does not already exist. Initialize the lastlog file and give it proper permissions:

touch ${CLFS}/targetfs/var/log/lastlog
chmod -v 664 ${CLFS}/targetfs/var/log/lastlog