Running EC2 instance suddenly refuses SSH connection

Question

I\'ve set up the EC2 instance couple days ago and even last night I was able to SSH to it with no problems. Today morning, I can\'t ssh to it. Port 22 is already open in the

Answer 1

This happened to me on a Red Hat EC2 instance because these two lines were being automatically appended the end of the /etc/ssh/sshd_config file every time I launched my instance:

PermitRootLogin without-password
UseDNS no

One of these append operations was done without a line break, so the tail of the sshd_config file looked like:

PermitRootLogin without-password
UseDNS noPermitRootLogin without-password
UseDNS no

That caused sshd to fail to start on the next launch. I think this was caused by the bug reported here: https://bugzilla.redhat.com/show_bug.cgi?id=956531 The solution was to remove all the duplicate entries at the bottom of the sshd_config file, and add extra line breaks at the end.

Answer 2

Looks like sshd might have stopped for some reason. Is the instance EBS backed? if thats the case, try shutting it down and starting it back up. That should solve the problem.

Also, are you able to ssh from AWS web console? They have a java plugin there to ssh into the instance.

Answer 3

In my case, the volume was out of space and a service was failing to start. I used the AWS tutorial (from Sherzod's post) to mount it on a good EC2 instance and clean it up and remove the service from startup before remounting it and verifying that things worked.

Answer 4

Had the same issue, but sys logs had this:

Starting sshd: /var/empty/sshd must be owned by root and not group or world-writable. [FAILED]

Used the same steps described above to detach volume and attach to connectable instance. Then used:

sudo chmod 755 /var/empty/sshd

sudo chown root:root /var/empty/sshd

(https://support.microsoft.com/en-us/help/4092816/ssh-fails-because-var-empty-sshd-is-not-owned-by-root-and-is-not-group)

Then detached and reattached to original EC2 Instance and could now access via ssh.

Answer 5

For those of you who came across this post because you are unable to SSH into your EC2 instance after a reboot, this is cross-posted to a similar question at serverfault:

From the AWS Developer Forum post on this topic:

Try stopping the broken instance, detaching the EBS volume, and attaching it as a secondary volume to another instance. Once you've mounted the broken volume somewhere on the other instance, check the /etc/sshd_config file (near the bottom). I had a few RHEL instances where Yum scrogged the sshd_config inserting duplicate lines at the bottom that caused sshd to fail on startup because of syntax errors.

Once you've fixed it, just unmount the volume, detach, reattach to your other instance and fire it back up again.

Let's break this down, with links to the AWS documentation:

1. Stop the broken instance and detach the EBS (root) volume by going into the EC2 Management Console, clicking on "Elastic Block Store" > "Volumes", the right-clicking on the volume associated with the instance you stopped.
2. Start a new instance in the same region and of the same OS as the broken instance then attach the original EBS root volume as a secondary volume to your new instance. The commands in step 4 below assume you mount the volume to a folder called "data".
3. Once you've mounted the broken volume somewhere on the other instance,
4. check the "/etc/sshd_config" file for the duplicate entries by issuing these commands:
   • cd /etc/ssh
   • sudo nano sshd_config
   • ctrl-v a bunch of times to get to the bottom of the file
   • ctrl-k all the lines at the bottom mentioning "PermitRootLogin without-password" and "UseDNS no"
   • ctrl-x and Y to save and exit the edited file
5. @Telegard points out (in his comment) that we've only fixed the symptom. We can fix the cause by commenting out the 3 related lines in the "/etc/rc.local" file. So:
   • cd /etc
   • sudo nano rc.local
   • look for the "PermitRootLogin..." lines and delete them
   • ctrl-x and Y to save and exit the edited file
6. Once you've fixed it, just unmount the volume,
7. detach by going into the EC2 Management Console, clicking on "Elastic Block Store" > "Volumes", the right-clicking on the volume associated with the instance you stopped,
8. reattach to your other instance and
9. fire it back up again.

Answer 6

Go to your AWS management console > select instance > right click and select "Get System Logs" This will list what went wrong.