Saturday, January 31, 2015

RHEL7: Access a virtual machine’s console.

Standard procedure

With KVM, to access the virtual machine’s console under X Window, type:

# virt-manager

If you aren’t under X Window, there is another way to access a virtual machine’s console: you can go through a serial console.
On the virtual machine, add ‘console=ttyS0‘ at the end of the kernel lines in the /boot/grub2/grub.cfg file:

# grubby --update-kernel=ALL --args="console=ttyS0"

Now, reboot the virtual machine:
# reboot

With KVM, connect to the virtual machine’s console (here
# virsh console 
Connected to domain
Escape character is ^]

Red Hat Enterprise Linux Server 7.0 (Maipo)
Kernel 3.10.0-121.el7.x86_64 on an x86_64

vm login:

Emergency procedure

Sometimes you have lost all links to your virtual machine (error in the /etc/fstab file, ssh configuration, etc) and, as you didn’t set up any virtual console, you are in real trouble. There is still a solution!
Connect to the physical host and shut down your virtual machine (here called

# virsh destroy

Define where the virtual machine image file is located (by default in the /var/lib/libvirt/images directory with a name like
# virsh dumpxml | grep "source file="

Map your virtual machine image file into the host environment (-a for add and -v for verbose):
# kpartx -av /var/lib/libvirt/images/ 
add map loop0p1 (253:2): 0 1024000 linear /dev/loop0 2048
add map loop0p2 (253:3): 0 10240000 linear /dev/loop0 1026048

From the previous display, you know that you’ve got two partitions (in fact /boot and /, distinguishable
by their respective size).
You need to mount the /boot partition to be able to change the grub configuration:
# mount /dev/mapper/loop0p1 /mnt

Then, edit the /mnt/grub2/grub.cfg file and add console=ttyS0 at the end of every line containing /vmlinuz (the linux kernel).
Unmount the partition:
# umount /mnt

Unmap the virtual machine image file (-d for delete and -v for verbose):
# kpartx -dv /var/lib/libvirt/images/
del devmap : loop0p2
del devmap : loop0p1
loop deleted : /dev/loop0

Restart your virtual machine:
# virsh start
Domain started

Connect to your virtual machine console:
# virsh console
Connected to domain
Escape character is ^]

CentOS Linux 7 (Core)
Kernel 3.10.0-123.el7.x86_64 on an x86_64

vm login: 
This procedure works for RHEL 6/CentOS 6 and RHEL 7/CentOS 7.

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