Alestic.com - A Personal AWS Blog by Eric Hammond

The ssh protocol uses two different keys to keep you secure:

1. The user ssh key is the one we normally think of. This authenticates us to the remote host, proving that we are who we say we are and allowing us to log in.

2. The ssh host key gets less attention, but is also important. This authenticates the remote host to our local computer and proves that the ssh session is encrypted so that nobody can be listening in.

Every time you see a prompt like the following, ssh is checking the host key and asking you to make sure that your session is going to be encrypted securely.

The authenticity of host 'ec2-...' can't be established.
ECDSA key fingerprint is ca:79:72:ea:23:94:5e:f5:f0:b8:c0:5a:17:8c:6f:a8.
Are you sure you want to continue connecting (yes/no)? 

If you answer “yes” without verifying that the remote ssh host key fingerprint is the same, then you are basically saying:

I don’t need this ssh session encrypted. It’s fine for any man-in-the-middle to intercept the communication.

Ouch! (But a lot of people do this.)

Note: If you have a line like the following in your ssh config file, then you are automatically answering “yes” to this prompt for every ssh connection.

# DON'T DO THIS!
StrictHostKeyChecking false

Care about security

Since you do care about security and privacy, you want to verify that you are talking to the right server using encryption and that no man-in-the-middle can intercept your session.

There are a couple approaches you can take to check the fingerprint for a new Amazon EC2 instance. The first is to wait for the console output to be available from the instance, retrieve it, and verify that the ssh host key fingerprint in the console output is the same as the one which is being presented to you in the prompt.

Scott Moser has written a blog post describing how to verify ssh keys on EC2 instances. It’s worth reading so that you understand the principles and the official way to do this.

The rest of this article is going to present a different approach that lets you in to your new instance quickly and securely.

Passing ssh host key to new EC2 instance

Instead of letting the new EC2 instance generate its own ssh host key and waiting for it to communicate the fingerprint through the EC2 console output, we can generate the new ssh host key on our local system and pass it to the new instance.

Using this approach, we already know the public side of the ssh key so we don’t have to wait for it to become available through the console (which can take minutes).

Generate a new ssh host key for the new EC2 instance.

tmpdir=$(mktemp -d /tmp/ssh-host-key.XXXXXX)
keyfile=$tmpdir/ssh_host_ecdsa_key
ssh-keygen -q -t ecdsa -N "" -C "" -f $keyfile

Create the user-data script that will set the ssh host key.

userdatafile=$tmpdir/set-ssh-host-key.user-data
cat <<EOF >$userdatafile
#!/bin/bash -xeu
cat <<EOKEY >/etc/ssh/ssh_host_ecdsa_key
$(cat $keyfile)
EOKEY
cat <<EOKEY >/etc/ssh/ssh_host_ecdsa_key.pub
$(cat $keyfile.pub)
EOKEY
EOF

Run an EC2 instance, say Ubuntu 11.10 Oneiric, passing in the user-data script. Make a note of the new instance id.

ec2-run-instances --key $USER --user-data-file $userdatafile ami-4dad7424
instanceid=i-...

Wait for the instance to get a public DNS name and make a note of it.

ec2-describe-instances $instanceid
host=ec2-...compute-1.amazonaws.com

Add new public ssh host key to our local ssh known_hosts after removing any leftover key (e.g., from previous EC2 instance at same IP address).

knownhosts=$HOME/.ssh/known_hosts
ssh-keygen -R $host -f $knownhosts
ssh-keygen -R $(dig +short $host) -f $knownhosts
(
  echo -n "$host "; cat $keyfile.pub
  echo -n "$(dig +short $host) "; cat $keyfile.pub
) >> $knownhosts

When the instance starts running and the user-data script has executed, you can ssh in to the server without being prompted to verify the fingerprint

ssh ubuntu@$host

Don’t forget to clean up and to terminate your test instance.

rm -rf $tmpdir
ec2-terminate-instances $instanceid

Caveat

There is one big drawback in the above sample implementation of this approach. We have placed secret information (the private ssh host key) into the EC2 user-data, which I generally recommend against.

Any user who can log in to the instance or who can cause the instance to request a URL and get the output, can retrieve the user-data. You might think this is unlikely to happen, but I’d rather avoid or minimize unnecessary risk.

In a production implementation of this approach, I would take steps like the following:

1. Upload the new ssh host key to S3 in a private object.

2. Generate an authenticated URL to the S3 object and have that URL expire in, say, 10 minutes.

3. In the user-data script, download the ssh host key with the authenticated, expiring S3 URL.

Now, there is a short window of exposure and you don’t have to worry about protecting the user-data after the URL has expired.