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Faisal Mahmud
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“The mind is not a vessel to be filled, but a fire to be kindled.” ― Plutarch

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How to get your host key fingerprint on an Ubuntu 14.04 server

Faisal MahmudFaisal Mahmud

What is a fingerprint?

Your SSH server has a unique fingerprint that is generated using a server's unique host key in order to identify the server to a client.

Why does this matter?

Whenever you make a connection to an SSH server You want to make sure that you are connecting to the right server.

If you get a warning where your client is saying that the host key cannot be verified or that is has changed you need to make sure to verify the fingerprint.

HOW DO I KNOW THE FINGERPRINT OF MY SERVER?

Run the command below

ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key.pub  

The expected output should be

2048 xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx /etc/ssh/ssh_host_rsa_key.pub (RSA)  

How do I make sure that I am connecting to the correct server?

If you are using a Mac or Linux machine you should be able to find the known_hosts file in ~/.ssh/known_hosts

The file should look something like

1.2.3.4 ssh-rsa Alongstringwithalphanumeric...  
2.4.6.8 ssh-rsa Anotherlongstringwithalphan...  

How do I verify the servers fingerprint?

  1. Ask the server owner what the fingerprint is
  2. Remove the corresponding line from ~/.ssh/known_hosts, ie if you are connecting to server with IP 1.2.3.4 then you need to remove the line that starts with IP 1.2.3.4 in ~/.ssh/known_hosts
  3. Run ssh youruser@1.2.3.4 and make sure that the fingerprint matches the one you got from the server owner in Step 1 above.

The output should be something like

The authenticity of host '1.2.3.4 (1.2.3.4)' can't be established.  
RSA key fingerprint is xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx:xx.  
Are you sure you want to continue connecting (yes/no)?  
Faisal Mahmud
Author

Faisal Mahmud

“The mind is not a vessel to be filled, but a fire to be kindled.” ― Plutarch

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