How to reset forgotten root password for Linux KVM qcow2 image/vm

in Categories , , , last updated February 10, 2018

I have Debian Linux VM running on KVM. I think I forgotten the password for the root account and I am no longer able to run ‘su -‘ command. How do I reset the password for the root account for KVM VM which is in qcow2 format?

You can modify images with guestfish. It is a shell and command-line tool for examining and editing virtual machine filesystems. It uses libguestfs and exposes all of the functionality of the guestfs API. This page shows how to use guestfish to change the root account password.

How to install guestfish

If you are using CentOS/RHEL use yum command:
$ sudo yum install libguestfs-tools
Fedora Linux user run dnf command:
$ sudo dnf install libguestfs-tools
Debian/Ubuntu Linux user run apt command/apt-get command:
$ sudo apt install libguestfs-tools

Step 1 – Shutdown guest VM

Run the following virsh command:
# virsh list
Sample outputs:

 Id    Name                           State
----------------------------------------------------
 2     debian9-vm1                    running

To shutdown the VM named debian9-vm1:
# virsh shutdown 2
OR
# virsh shutdown debian9-vm1
Sample outputs:

Domain debian9-vm1 is being shutdown

Step 2 – Find location of KVM VM image

Type the following command to get location of qcow2 image:
# virsh dumpxml debian9-vm1 | grep 'source file'
Sample outputs:

<source file='/var/lib/libvirt/images/debian9-vm1.qcow2'/>

Step 3 – Reset/change the root password using guestfish

First generate new root user account password by typing the following command:
# openssl passwd -1 mySecretRootAccountPasswordHere
$1$M1bf5Y3T$p2CYEz8vlUD2R.fXydTLt.

Please copy $1$M1bf5Y3T$p2CYEz8vlUD2R.fXydTLt. password. You need to use this one in next few steps.

How to reset forgotten root password for Linux KVM vm

Let us start the procedure by running the following guestfish command:
# guestfish --rw -a /var/lib/libvirt/images/debian9-vm1.qcow2
You will see a prompt as follows:

><fs>

To launch the backend either type ‘run’ or ‘launch’ command:

><fs> launch

To list partitions type:

><fs> list-filesystems

Now mount whatever disk you found it. For example, I found /dev/sda1, so I run the following mount command:

><fs> mount /dev/sda1 /

Edit the /etc/shadow file using a text editor such as vi command or emacs command:

><fs> vi /etc/shadow

How to reset forgotten root password for Linux KVM qcow2 image vm
Find root account entry and delete encrypted password. From:

root:$6$FU5Nl9oxxxx:17572:0:99999:7:::

To (replace as follows from above openssl command):

root:$1$M1bf5Y3T$p2CYEz8vlUD2R.fXydTLt.:17572:0:99999:7:::

Save and close the file in vi/vim. Run sync command:

><fs> flush

Finally, quit guestfish:

><fs> quit

Step 4 – Start VM

It is time to test your new root password. So start the VM:
# virsh start debian9-vm1
Sample outputs:

Domain debian9-vm1 started

You can now login using console or ssh:
# ssh vivek@debian9-vm1
$ su -

OR use console command:
# virsh list
# virsh console debian9-vm1

Login as root user using ssh or vish console command

A note about virt-customize command

If you find above method difficult try the following simple command:
# virsh shutdown debian9-vm1
# virt-customize -a /var/lib/libvirt/images/debian9-vm1.qcow2 --root-password password:NewRootUserPasswordHere --uninstall cloud-init

Sample outputs:
virt-customize reset password
Start the VM and test new root password with ssh/console command:
# virsh start debian9-vm1
For more info read man page of guestfish:
$ man guestfish
$ man virt-customize

See also

This entry is 4 of 4 in the Linux KVM Cloud Computing/VM Tutorial series. Keep reading the rest of the series:
  1. How to install KVM on CentOS 7 / RHEL 7 Headless Server
  2. How to install KVM on Ubuntu 16.04 LTS Headless Server
  3. How to install KVM server on Debian Linux 9 Headless Server
  4. How to reset root password for Linux KVM VM

Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin, DevOps engineer, and a trainer for the Linux operating system/Unix shell scripting. Get the latest tutorials on SysAdmin, Linux/Unix and open source topics via RSS/XML feed or weekly email newsletter.

Share this on (or read 0 comments/add one below):

Start the discussion at www.nixcraft.com