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Fixing Files on the Root EBS Volume of an EC2 Instance

2015-03-25

You can examine and edit files on the root EBS volume on an EC2 instance even if you are in what you considered a disastrous situation like:

    • You lost your ssh key or forgot your password
    • You made a mistake editing the /etc/sudoers file and can no longer gain root access withsudo to fix it
    • Your long running instance is hung for some reason, cannot be contacted, and fails to boot properly
    • You need to recover files off of the instance but cannot get to it

On a physical computer sitting at your desk, you could simply boot the system with a CD or USB stick, mount the hard drive, check out and fix the files, then reboot the computer to be back in business.

A remote EC2 instance, however, seems distant and inaccessible when you are in one of these situations. Fortunately, AWS provides us with the power and flexibility to be able to recover a system like this, provided that we are running EBS boot instances and not instance-store.

The approach on EC2 is somewhat similar to the physical solution, but we’re going to move and mount the faulty “hard drive” (root EBS volume) to a different instance, fix it, then move it back.

In some situations, it might simply be easier to start a new EC2 instance and throw away the bad one, but if you really want to fix your files, here is the approach that has worked for many:

Set Up

Identify the original instance (A) and the root EBS volume that contains the broken root file system with the files you want to view and edit.

instance_a=i-XXXXXXXX

root_device=$(aws ec2 describe-instances   --instance-ids $instance_a   --output text   --query 'Reservations[*].Instances[*].RootDeviceName')
volume=$(aws ec2 describe-instances   --instance-ids $instance_a   --output text   --query 'Reservations[*].Instances[*].BlockDeviceMappings[?DeviceName==`'$root_device'`].[Ebs.VolumeId]')

Identify the second EC2 instance (B) that you will use to fix the files on the original EBS volume. This instance must be running in the same availability zone as instance A so that it can have the EBS volume attached to it. If you don’t have an instance already running, start a temporary one.

instance_b=i-YYYYYYYY

Stop (do not terminate) the broken instance A, wait for it to come to a complete stop, detach the root EBS volume from the instance, wait for it to be detached, then attach the volume to instance B on an unused device.

aws ec2 stop-instances --instance-ids $instance_a
aws ec2 detach-volume --volume-id $volume --output text --query 'State'
aws ec2 attach-volume  --volume-id $volume --instance-id $instance_b --device /dev/sdj

ssh to instance B and mount the volume so that you can access its file system.

ssh [instance b]
sudo mkdir -m 000 /vol-a
sudo mount /dev/xvdj /vol-a

Note: On older kernels, you may need to mount /dev/sdj instead of /dev/xvdj inside the instance.

Fix It

At this point your entire root file system from instance A is available for viewing and editing under /vol-a on instance B. For example, you may want to:

    • Put the correct ssh keys in /vol-a/home/ubuntu/.ssh/authorized_keys
    • Edit and fix /vol-a/etc/sudoers
    • Look for error messages in /vol-a/var/log/syslog
    • Copy important files out of /vol-a/

Note: The uids on the two instances may not be identical, so take care if you are creating, editing, or copying files that belong to non-root users. For example, your mysql user on instance A may have the same UID as your postfix user on instance B which could cause problems if you chown files with one name and then move the volume back to A.

Wrap Up

After you are done and you are happy with the files under /vol-a, unmount the file system (still on instance-B):

sudo umount /vol-a
sudo rmdir /vol-a

Now, back on your system with ec2-api-tools, continue moving the EBS volume back to its home on the original instance A and start the instance again:

aws ec2 detach-volume --volume-id $volume --output text --query 'State'
aws ec2 attach-volume  --volume-id $volume --instance-id $instance_a --device $root_device
aws ec2 start-instances --instance-ids $instance_a

Hopefully, you fixed the problem, instance A comes up just fine, and you can accomplish what you originally set out to do. If not, you may need to continue repeating these steps until you have it working.

Note: If it was not in a VPC, instance A may come up with a different public and private IP address than before. If you had an Elastic IP address assigned to instance A when you stopped it, you’ll need to re-associate it after starting it up again.

Remember! If your instance B was temporarily started just for this process, don’t forget to terminate it now.

mrluo

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