Amazon EC2 instance SSH RSA Fingerprint

Any time I stand up a VM in EC2 and you ssh into it for the first time, I always get this message:

The authenticity of host 'ec2-xxxxxx.compute-1.amazonaws.com (n.n.n.n)' can't be established.
RSA key fingerprint is xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx.

And like everybody else, I just say yes

Are you sure you want to continue connecting (yes/no)? yes

I understand what a fingerprint is, why it's good and all that. What I'm wondering is, years ago, since it was a physical machine I was setting up... I could check on the physical machine and validate yes, this is the fingerprint.

Is there some way in the EC2 console to independently verify "yes, this is the fingerprint"? If so, how do you find it?

Solution 1:

You can verify the fingerprint using the AWS console for instances with cloud-init.

The following is on an instance running Amazon Linux.

There is a init.d script called cloud-init:

cloud-init is the distribution-agnostic package that handles early initialization of a cloud instance.

Some of the things it configures are:

setting a default locale

setting hostname

generate ssh private keys

adding ssh keys to user's .ssh/authorized_keys so they can log in

setting up ephemeral mount points

preparing package repositories and performs a variety of at-boot customization actions based on user-data

Once you launch an instance, you can view the system log output via the AWS console without using SSH. (Which avoids your catch-22 - you can see the fingerprint before you access the instance).

You can do this by

going to your EC2 Management Console page,
clicking the "Instances" link on the sidebar,
selecting the instance you want to log into, and
navigating to Actions > Instance Settings > Get System Log.

Get System Log

If you scroll through that output, you will see something like the following:

Running cloud-init
...

cloud-init:  sshGenerating public/private rsa key pair.
Your identification has been saved in /etc/ssh/ssh_host_rsa_key.
Your public key has been saved in /etc/ssh/ssh_host_rsa_key.pub.
The key fingerprint is:
aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa [email protected]
The key's randomart image is:
+--[ RSA 2048]----+
|       aa        |
|  a    a         |
|     aa   a   a  |
|  a    aaa       |
|     aaaaa   a   |
| a   a aa        |
|aaaa   a  aa     |
|aaaaaa     a     |
| aa      a       |
+-----------------+
Generating public/private dsa key pair.
Your identification has been saved in /etc/ssh/ssh_host_dsa_key.
Your public key has been saved in /etc/ssh/ssh_host_dsa_key.pub.
The key fingerprint is:
bb:bb:bb:bb:bb:bb:bb:bb:bb:bb:bb:bb:bb:bb:bb:bb [email protected]
The key's randomart image is:
+--[ DSA 1024]----+
|                 |
|                 |
|                 |
|       b  b      |
|        b     b  |
|    b    b b     |
|  bb b bb        |
| b  bbb bb    b  |
|  bbbb   bbb     |
+-----------------+
ec2: 
ec2: #############################################################
ec2: -----BEGIN SSH HOST KEY FINGERPRINTS-----
ec2: 2048 aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa  [email protected] (RSA)
ec2: 1024 bb:bb:bb:bb:bb:bb:bb:bb:bb:bb:bb:bb:bb:bb:bb:bb  [email protected] (DSA)
ec2: -----END SSH HOST KEY FINGERPRINTS-----
ec2: #############################################################
[  OK  ]

System Log Output

Above each "randomart" image is the type of signature, such as RSA or DSA. Find the kind you are given (RSA in your case), and check the fingerprint above it.

If you wish to verify that the same key is the one on the instance (as a proof of concept, after you have SSH'd in), you can run:

ssh-keygen -lf /etc/ssh/ssh_host_rsa_key.pub
2048 aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa  [email protected] (RSA)

Alternate Approach:

In order to avoid the problem, you can use your own keys - since you generated the key, you know the fingerprint. The issue then becomes one of getting your new key onto the instance without using SSH.

Most instances use cloud-init and will support user-data. This applies to any such instance. Since you need to stop an instance in order to modify user-data, this approach requires that you either are a: launching an instance for the first time (and are setting yourself up to use known keys) or b: can stop the instance, modify the user-data, and restart the instance.

Generate a key For example, using PuttyGen on Windows:

Create a user-data script:

If you are launching a new instance, you just need to specify the keys you want to use, if you are restarting an instance that is already running, you need to have cloud-init reconfigure SSH to pull in the new keys. By default, the SSH config module of cloud-init is run once per instance, so you need to set it to run always (each boot) (This might not be ideal in some circumstances, but can be modified after you know the key, if required)

The user-data would take the following form:

#cloud-config
cloud_config_modules: #only needed if restarting an instance, omit if launching a new instance
  ...list all existing modules
  - [ssh, always]  #this is changed to always (default is once per instance)

ssh_keys:
  rsa_private: |
    -----BEGIN RSA PRIVATE KEY-----
    -----END RSA PRIVATE KEY-----

    ssh-rsa #public_key

For example:

#cloud-config
cloud_config_modules:
  - locale
  - [ssh, always]
  - set-passwords
  - mounts
  - yum-configure
  - yum-add-repo
  - package-update-upgrade-install
  - timezone
  - puppet
  - disable-ec2-metadata
  - runcmd

ssh_keys: #you can specify rsa, dsa, and ecdsa keys
  rsa_private: |
    -----BEGIN RSA PRIVATE KEY-----
    MIIEoQIBAAKCAQEAopbE8beKaKajF/SFOtntO9xt5XVZW5rlQCW6PVY1jXCq5dbj
    nEQoBGBIp6jsqLcnwYQW/tU4zXi7T0kX6NlVywiMOtjnyoOkLCX2R5OjMap3hlyj
    AO/PCKW7pE4vAHd7HyYvGW/gPezGW0WeFshp7J7dTXZdSmDquZI15rEsz07QsKWy
    /SH/rjYVObAQJN78CuU7C41LRshEeTSBM0jBSnp3jL1Ocw66qe4sV6jbcQN6QzK3
    77e+KzpUDmcxaB7plTWDSpjxVFWbY6PQcsz5d/h60wSKu90Ia9fNMHWs7cbyELhK
    VPBRs4JtWKndjtISCd5T34UnKmtTpq6g/ocrrwIBJQKCAQB7Ck/Zg/oKAZAtzcyb
    PSI7I1oVbY+68cI+Yb1e2XSiYxmLVoK7cdkYEYMXGA0KDhA/auD4MqeGvDq4ildI
    bR5UdSvZgYzQmvjHdqyJMXSUSaaPMVjCcEmlrdobeW+tU3/Ei5lDrpvb1caKQoV5
    Bl3/LB0YBovJlXNb//FwTrogVhYFexcda+DxN5a2oNSCwMosdgCP4gz+hX9zTAl9
    k/VOkLaj2h7URfsuAmwwZ+m24Bpz1r7vEtec0PraoKkBpVxBeNDPwMdosTrpGS49
    V+YRRiM8yShuRPF9mAwo62kcD7K5bToppyb6CLdCi06CTcAmQ5Sb+UwHqC/rdZI0
    wmnNAoGBAPxj6ecjM0AwrPf2TPJOtdEUHvFnc6bB23C32Yr7IWjNhij0BGG/D8Cv
    smCXDwYDH7Ss4CN/mMzG43QhfyyAz0T0BjpFmZEYqYOJAB7cwpdx4zjHzoc7WKiI
    vXPml2hdd37iVRNq6raUgDLpKfVkpY8FKcJjzFuiCXDOU1+mNxPbAoGBAKTqD/+X
    oDGsf6hkV7vgPLIXc3/BZco0l9kNkemto9RVIsr3D40bfe4PuJg3fjwFYDTq9s79
    WFR5sG/eSpNJtSGTz6LN5TQoL5xLMCIysajc+JN64w4TYCDGSEk4Xgv0X0cCgJfF
    RCedv9qObGT9/KCI/9Y/5jlZsVphNsAk4Xm9AoGBAKO2bjUP6eRymbWY1/ceTGvx
    YC3iPS3lh2u1hjCi5T0PsPf4OjGQsEWiZd3JxI5HNykWMIW6jKCBAj19guxvOlY9
    a9LFXLEkwPtfyLoSp7wuMoWyCWx5hZ3AeuNlI/CrVG36mAyYYOUiDfeCfBTLqajg
    wSQlDu9UWSaTq7OqFeNdAoGAFkkkway0yHFBrvjNle3esEhbt1F8dUVgoMp7gHFp
    KogLnuMdxvXglcrF6w5rATEorTSCN6Wx/ZPnaRAzl1z8zS+mb/JPZ+nBPqJgc1L1
    adir+EEJ7SU2gPgzSCo2OPeCfzewgzZVUXYurtT55CJSkjwG5Znurc3ZskR9BTVq
    k+kCgYARZXWS0Wyy5piq7WX7w2Hc6bVEPMCU+yJcbJ8E+F8meoQ1kXIRqIB+cAsq
    /3z3JmU19yOGms557POXgEFseMMFai3i2wTQ4mGPOM3a7yEeeKl1Zg5eHVUdVd+n
    PDzs9D+9umc6mXrRiRwJPQWo8pIKXb7SqjIA73M7H+98CBtb5w==
    -----END RSA PRIVATE KEY-----

    ssh-rsa AAAAB3NzaC1yc2EAAAABJQAAAQEAopbE8beKaKajF/SFOtntO9xt5XVZW5rlQCW6PVY1jXCq5dbjnEQoBGBIp6jsqLcnwYQW/tU4zXi7T0kX6NlVywiMOtjnyoOkLCX2R5OjMap3hlyjAO/PCKW7pE4vAHd7HyYvGW/gPezGW0WeFshp7J7dTXZdSmDquZI15rEsz07QsKWy/SH/rjYVObAQJN78CuU7C41LRshEeTSBM0jBSnp3jL1Ocw66qe4sV6jbcQN6QzK377e+KzpUDmcxaB7plTWDSpjxVFWbY6PQcsz5d/h60wSKu90Ia9fNMHWs7cbyELhKVPBRs4JtWKndjtISCd5T34UnKmtTpq6g/ocrrw== rsa-key-20140716

Stop your instance and modify the user-data:

Start the instance, and connect to it:
Note that the fingerprint shown here matches the one displayed when the key was generated.

If you are stopping the instance and have access to another instance, you can also mount the root volume and modify the keys directly without using user-data (the keys will not be overwritten since cloud-init only runs SSH config once per instance by default). An alternate approach, if you are planning ahead, is to setup cloud-init (or an init script) to log the SSH keys each boot, which increases the probability you will find them in the console log after a restart.

Amazon EC2 instance SSH RSA Fingerprint

Solution 1:

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