RHEL7/CENTOS7: Interrupt the boot process in order to gain access to a system.

In RHEL 7, the procedure to get access to a system during the boot process and modify the root password has changed because of the adoption of Systemd.
RHEL 7 Root Password recovery procedure
Centos7  Root Password recovery procedure

There were several procedures floating around to recover the root password. Some were working with physical servers but not with virtual machines, some the other way around.
The following procedure works all the time.


At the beginning of the boot process, at the GRUB 2 menu, type the e key to edit.
Then, go to the kernel line (the line starting with linux16) and add the following statements at the end:
rd.break enforcing=0
Caution: The keys to press are those of a US keyboard (querty).
Note: rd.break asks for a break at an early stage of the boot process. enforcing=0 puts the system into SELinux Permissive mode.
Press Ctrl x to resume the boot process.
Then, mount the /sysroot partition as read/write:
switch_root:/# mount –o remount,rw /sysroot
Execute the chroot command on the /sysroot partition:
switch_root:/# chroot /sysroot
Change the root password:
sh-4.2# passwd root
Changing password for user root.
New passwd: mypassword
Retype new password: mypassword
passwd: all authentication token updated successfully.
sh-4.2# exit
switch_root:/# exit
Connect to your server at the console (don’t reboot now!) with the root user and the new password:
[  OK  ] Started Network Manager Script Dispatcher Service.
[  OK  ] Started Crash recovery kernel arming.
[  OK  ] Reached target Multi-User System.

CentOS Linux 7 (Core)
Kernel 3.10.0-229.14.1.el7.x86_64 on an x86_64

vm login: root
Password: mypassword
Then type:
# restorecon /etc/shadow
# reboot
If you strictly follow this procedure, you don’t need to force a SELinux relabel (# touch /.autorelabel) or load the SELinux policy (# /usr/sbin/load_policy -i).
You don’t even need to reboot at the end! In this case, type # setenforce enforcing

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