Securely add a host (e.g. GitHub) to the SSH known_hosts file

How can I add a host key to the SSH known_hosts file securely?

I'm setting up a development machine, and I want to (e.g.) prevent git from prompting when I clone a repository from github.com using SSH.

I know that I can use StrictHostKeyChecking=no (e.g. this answer), but that's not secure.

So far, I've found...

1. GitHub publishes their SSH key fingerprints at GitHub's SSH key fingerprints

2. I can use ssh-keyscan to get the host key for github.com.

How do I combine these facts? Given a prepopulated list of fingerprints, how do I verify that the output of ssh-keyscan can be added to the known_hosts file?

I guess I'm asking the following:

How do I get the fingerprint for a key returned by ssh-keyscan?

Let's assume that I've already been MITM-ed for SSH, but that I can trust the GitHub HTTPS page (because it has a valid certificate chain).

That means that I've got some (suspect) SSH host keys (from ssh-keyscan) and some (trusted) key fingerprints. How do I verify one against the other?

Related: how do I hash the host portion of the output from ssh-keyscan? Or can I mix hashed/unhashed hosts in known_hosts?

The most important part of "securely" adding a key to the known_hosts file is to get the key fingerprint from the server administrator. The key fingerprint should look something like this:

2048 SHA256:nThbg6kXUpJWGl7E1IGOCspRomTxdCARLviKw6E5SY8 github.com (RSA)

In the case of GitHub, normally we can't talk directly to an administrator. However, they put the key on their web pages so we can recover the information from there.

Manual key installation

1) Take a copy of the key from the server and get its fingerprint. N.B.: Do this before checking the fingerprint.

$ ssh-keyscan -t rsa github.com | tee github-key-temp | ssh-keygen -lf -
# github.com:22 SSH-2.0-babeld-f3847d63
2048 SHA256:nThbg6kXUpJWGl7E1IGOCspRomTxdCARLviKw6E5SY8 github.com (RSA)

2) Get a copy of the key fingerprint from the server administrator - in this case navigate to the page with the information on github.com

1. Go to github.com
2. Go to the help page (on the menu on the right if logged in; at the bottom of the homepage otherwise).
3. In the Getting Started section go to Connecting to GitHub with SSH
4. Go to Testing your SSH connection
5. Copy the SHA256 fingerprint from that page into your text editor for later use.

3) Compare the keys from the two sources

By placing them directly one above the other in a text editor, it is easy to see if something has changed

2048 SHA256:nThbg6kXUpJWGl7E1IGOCspRomTxdCARLviKw6E5SY8 github.com (RSA) #key recovered from github website
2048 SHA256:nThbg6kXUpJ3Gl7E1InsaspRomtxdcArLviKaEsTGY8 github.com (RSA) #key recovered with keyscan

(Note that the second key has been manipulated, but it looks quite similar to the original - if something like this happens you are under serious attack and should contact a trusted security expert.)

If the keys are different abort the procedure and get in touch with a security expert

4) If the keys compare correctly then you should install the key you already downloaded

cat github-key-temp >> ~/.ssh/known_hosts

Or to install for all users on a system (as root):

cat github-key-temp >> /etc/ssh/ssh_known_hosts

Automated key installation

If you need to add a key during a build process then you should follow steps 1-3 of the manual process above.

Having done that, examine the contents of your github-key-temp file and make a script to add those contents to your known hosts file.

if ! grep github.com ~/.ssh/known_hosts > /dev/null
then
     echo "github.com ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAq2A7hRGmdnm9tUDbO9IDSwBK6TbQa+PXYPCPy6rbTrTtw7PHkccKrpp0yVhp5HdEIcKr6pLlVDBfOLX9QUsyCOV0wzfjIJNlGEYsdlLJizHhbn2mUjvSAHQqZETYP81eFzLQNnPHt4EVVUh7VfDESU84KezmD5QlWpXLmvU31/yMf+Se8xhHTvKSCZIFImWwoG6mbUoWf9nzpIoaSjB+weqqUUmpaaasXVal72J+UX2B+2RPW3RcT0eOzQgqlJL3RKrTJvdsjE3JEAvGq3lGHSZXy28G3skua2SmVi/w4yCE6gbODqnTWlg7+wC604ydGXA8VJiS5ap43JXiUFFAaQ==" >> ~/.ssh/known_hosts
fi

You should now get rid of any ssh commands which have StrictHostKeyChecking disabled.

You can mix hashed/unhashed entries in your known_hosts file.

So if you want to add github key, you can just do :

ssh-keyscan github.com >> ~/.ssh/known_hosts

If you want it hashed, add -H

ssh-keyscan -H github.com >> ~/.ssh/known_hosts

The easiest way is to manually fetch the keys using ssh-keyscan, verify them manually:

$ ssh-keyscan -t rsa github.com | ssh-keygen -lf -
# github.com:22 SSH-2.0-libssh-0.7.0
2048 SHA256:nThbg6kXUpJWGl7E1IGOCspRomTxdCARLviKw6E5SY8 github.com (RSA)

And add them to your script, which will then carry the "authoritative" public key.