Fixing Files on the Root EBS Volume of an EC2 Instance

| 8 Comments

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 with sudo 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 volume that contains the broken root EBS volume with the files you want to view and edit.

instance_a=i-XXXXXXXX

volume=$(ec2-describe-instances $instance_a |
  egrep '^BLOCKDEVICE./dev/sda1' | cut -f3)

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 the broken instance A (waiting for it to come to a complete stop), detach the root EBS volume from the instance (waiting for it to be detached), then attach the volume to instance B on an unused device.

ec2-stop-instances $instance_a
ec2-detach-volume $volume
ec2-attach-volume --instance $instance_b --device /dev/sdj $volume

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

ssh ...instance b...

sudo mkdir -p 000 /vol-a
sudo mount /dev/sdj /vol-a

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 it’s home on the original instance A and start the instance again:

ec2-detach-volume $volume
ec2-attach-volume --instance $instance_a --device /dev/sda1 $volume
ec2-start-instances $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 you had an Elastic IP address assigned to instance A when you stopped it, you’ll need to reassociate 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.

8 Comments

Hi
Would like to know whether this can cause change in private/internal i-p address of original instance particularly when the volume in question is a root volume. What can be done if there is a requirement of internal i-p to be not changed? for eg some software license is host i-p based.

thanks

hpk. warrier :

Stopping and starting a standard instance is likely to give you a different internal IP address. An Elastic IP address lets you keep a specific external IP address. Starting an instance in VPC should allow you to assign and keep a specific internal IP address.

Hi Eric

I'm facing a problem here.I launched new instance with new key pair. In new instance I mounted the volume of original instance to dir /vol-old. I rename the authorized_keys file in /vol-old/home/ubuntu/.ssh/authorized_keys_bkup. Then I copied the authorized_keys file from /home/ubuntu/.ssh/authorized_keys to /vol-old/home/ubuntu/.ssh/authorized_keys .

Then i detached the volume and re-attached to original instance. However when I try to access original instance with new key details, i get "Network error: connection refused".

Any suggestions or hint to check further?
thanks

hpk.warrier:

"connection refused" is completely different from "Permission denied". You haven't even made a connection to the ssh server, so it doesn't matter what your authorized_keys file contains. Here's and article where I point out information you'll want to provide when asking for help with EC2 connection problems: http://alestic.com/2009/08/ec2-connectivity

Thank you so much for this. You just saved my bacon.

AItOawmpKNo2PRpo9MqhFQIqhH83X385iVSgyq0:

I don't usually approve and post comments that just say "thanks" but I like bacon, so you're welcome.

Help, I lost the reference to $volume by accident during "Fix It", and now I can't properly do "Wrap Up". Also, attempting to get a new reference with the help of ec2-describe-instances doesn't work anymore. How do I get a reference to the volume now?

AItOawl9bAPQBgcgkXqJOaMOqX9q0V0xpxBnm-o:

Use this to see what volume os attached to the /dev/sdj on the second instance:

ec2-describe-instances $instance_b

Then follow the instructions to move it to the first instance and restart.

Leave a comment

Ubuntu AMIs

Ubuntu AMIs for EC2:


More Entries

EC2 create-image Does Not Fully "Stop" The Instance
The EC2 create-image API/command/console action is a convenient trigger to create an AMI from a running (or stopped) EBS boot instance. It takes a snapshot of the instance’s EBS volume(s)…
Finding the Region for an AWS Resource ID
use concurrent AWS command line requests to search the world for your instance, image, volume, snapshot, … Background Amazon EC2 and many other AWS services are divided up into various…
Changing The Default "ubuntu" Username On New EC2 Instances
configure your own ssh username in user-data The official Ubuntu AMIs create a default user with the username ubuntu which is used for the initial ssh access, i.e.: ssh ubuntu@<HOST>…
Default ssh Usernames For Connecting To EC2 Instances
Each AMI publisher on EC2 decides what user (or users) should have ssh access enabled by default and what ssh credentials should allow you to gain access as that user.…
New c3.* Instance Types on Amazon EC2 - Nice!
Worth switching. Amazon shared that the new c3.* instance types have been in high demand on EC2 since they were released. I finally had a minute to take a look…
Query EC2 Account Limits with AWS API
Here’s a useful tip mentioned in one of the sessions at AWS re:Invent this year. There is a little known API call that lets you query some of the EC2…
Using aws-cli --query Option To Simplify Output
My favorite session at AWS re:Invent was James Saryerwinnie’s clear, concise, and informative tour of the aws-cli (command line interface), which according to GitHub logs he is enhancing like crazy.…
Reset S3 Object Timestamp for Bucket Lifecycle Expiration
use aws-cli to extend expiration and restart the delete or archive countdown on objects in an S3 bucket Background S3 buckets allow you to specify lifecycle rules that tell AWS…
Installing aws-cli, the New AWS Command Line Tool
consistent control over more AWS services with aws-cli, a single, powerful command line tool from Amazon Readers of this tech blog know that I am a fan of the power…
Using An AWS CloudFormation Stack To Allow "-" Instead Of "+" In Gmail Email Addresses
Launch a CloudFormation template to set up a stack of AWS resources to fill a simple need: Supporting Gmail addresses with “-” instead of “+” separating the user name from…
New Options In ec2-expire-snapshots v0.11
The ec2-expire-snapshots program can be used to expire EBS snapshots in Amazon EC2 on a regular schedule that you define. It can be used as a companion to ec2-consistent-snapshot or…
Replacing a CloudFront Distribution to "Invalidate" All Objects
I was chatting with Kevin Boyd (aka Beryllium) on the ##aws Freenode IRC channel about the challenge of invalidating a large number of CloudFront objects (35,000) due to a problem…
Email Alerts for AWS Billing Alarms
using CloudWatch and SNS to send yourself email messages when AWS costs accrue past limits you define The Amazon documentation describes how to use the AWS console to monitor your…
Cost of Transitioning S3 Objects to Glacier
how I was surprised by a large AWS charge and how to calculate the break-even point Glacier Archival of S3 Objects Amazon recently introduced a fantastic new feature where S3…
Running Ubuntu on Amazon EC2 in Sydney, Australia
Amazon has announced a new AWS region in Sydney, Australia with the name ap-southeast-2. The official Ubuntu AMI lookup pages (1, 2) don’t seem to be showing the new location…
Save Money by Giving Away Unused Heavy Utilization Reserved Instances
You may be able to save on future EC2 expenses by selling an unused Reserved Instance for less than its true value or even $0.01, provided it is in the…
Installing AWS Command Line Tools from Amazon Downloads
When you need an AWS command line toolset not provided by Ubuntu packages, you can download the tools directly from Amazon and install them locally. In a previous article I…
Convert Running EC2 Instance to EBS-Optimized Instance with Provisioned IOPS EBS Volumes
Amazon just announced two related features for getting super-fast, consistent performance with EBS volumes: (1) Provisioned IOPS EBS volumes, and (2) EBS-Optimized Instances. Starting new instances and EBS volumes with…
Which EC2 Availability Zone is Affected by an Outage?
Did you know that Amazon includes status messages about the health of availability zones in the output of the ec2-describe-availability-zones command, the associated API call, and the AWS console? Right…
Installing AWS Command Line Tools Using Ubuntu Packages
See also: Installing AWS Command Line Tools from Amazon Downloads Here are the steps for installing the AWS command line tools that are currently available as Ubuntu packages. These include:…
Ubuntu Developer Summit, May 2012 (Oakland)
I will be attending the Ubuntu Developer Summit (UDS) next week in Oakland, CA. ┬áThis event brings people from around the world together in one place every six months to…
Uploading Known ssh Host Key in EC2 user-data Script
The ssh protocol uses two different keys to keep you secure: The user ssh key is the one we normally think of. This authenticates us to the remote host, proving…
Seeding Torrents with Amazon S3 and s3cmd on Ubuntu
Amazon Web Services is such a huge, complex service with so many products and features that sometimes very simple but powerful features fall through the cracks when you’re reading the…
CloudCamp
There are a number of CloudCamp events coming up in cities around the world. These are free events, organized around the various concepts, technologies, and services that fall under the…
Use the Same Architecture (64-bit) on All EC2 Instance Types
A few hours ago, Amazon AWS announced that all EC2 instance types can now run 64-bit AMIs. Though t1.micro, m1.small, and c1.medium will continue to also support 32-bit AMIs, it…
ec2-consistent-snapshot on GitHub and v0.43 Released
The source for ec2-conssitent-snapshot has historically been available here: ec2-consistent-snapshot on Launchpad.net using Bazaar For your convenience, it is now also available here: ec2-consistent-snapshot on GitHub using Git You are…
You Should Use EBS Boot Instances on Amazon EC2
EBS boot vs. instance-store If you are just getting started with Amazon EC2, then use EBS boot instances and stop reading this article. Forget that you ever heard about instance-store…
Retrieve Public ssh Key From EC2
A serverfault poster had a problem that I thought was a cool challenge. I had so much fun coming up with this answer, I figured I’d share it here as…
Running EC2 Instances on a Recurring Schedule with Auto Scaling
Do you want to run short jobs on Amazon EC2 on a recurring schedule, but don’t want to pay for an instance running all the time? Would you like to…
AWS Virtual MFA and the Google Authenticator for Android
Amazon just announced that the AWS MFA (multi-factor authentication) now supports virtual or software MFA devices in addition to the physical hardware MFA devices like the one that’s been taking…