46

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?

11
  • Why wouldn't it be secure for your use case? Jun 16 '17 at 10:39
  • StrictHostKeyChecking=no is vulnerable to MITM. Is ssh-keyscan secure against MITM? Jun 16 '17 at 10:48
  • I fail to understand why I'm overly worried about somebody impersonating a stranger I've never met whom I'm trusting enough to write code I'm about to download and run... Jun 16 '17 at 11:23
  • 2
    Because this is my source code in a private repo on github, and I don't want a MITM (e.g.) introducing malicious changes when I push commits. That's just one example. Jun 16 '17 at 11:28
  • I (for better or worse) choose to trust github. I don't choose to trust every random network link between me and them. Jun 16 '17 at 11:28
41

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.

7
  • Why is the order of step 1 and 2 important? Dec 31 '20 at 10:46
  • 2
    What about if ! ssh-keygen -F github.com; then?
    – x-yuri
    Apr 1 at 22:06
  • @DanielGoldberg - if you check on the web server before you take a copy of the key then an attacker may realise you did the check. If they do that then they can turn off their man in the middle attack before you detect it. You will be safe this time, but they will keep trying until, some time in future, you forget to check.
    – Michael
    Sep 13 at 17:21
  • @x-yuri that only works if you already connected to the host and you trust the connection you made before. If in doubt, follow the above procedure anyway.
    – Michael
    Sep 13 at 17:22
  • I mean, you do grep github.com ~/.ssh/known_hosts. Why not use a more specialized tool (ssh-keygen) that does just that, but also knows the format of the file?
    – x-yuri
    Sep 14 at 3:13
30

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

4
  • 1
    What is the difference between hashed/unhashed? I know what hashing is, just not why it is applied in this context. Feb 14 '19 at 10:22
  • 5
    Hashing your entries allows you to hide information about the hosts you are used to connect to. Check the accepted answer at security.stackexchange.com/questions/56268/…
    – Wee
    Feb 15 '19 at 11:16
  • 8
    This doesn't answer the question. Using ssh-keyscan is subject to a man in the middle attack which means that the key you store could be a key belonging to the attacker trying to break into your system. Please see my answer for a way to check things.
    – Michael
    Jun 24 '19 at 7:26
  • Att. Getting attention for answers that have aways been incorrect - "the two top answers are actually insecure" Jun 24 '19 at 15:40
5

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.

6
  • I've got the keys like you said, how do I 'add them to my script'? Feb 14 '19 at 10:19
  • Either as a separate file or as a string in your script.
    – Jakuje
    Feb 14 '19 at 11:35
  • 2
    This answer is dangerous because hidden in the "verify them manually" is a whole difficult process. I've added an answer below which atempts to explain how to do that and use the results safely.
    – Michael
    Jun 18 '19 at 15:55
  • 1
    Att. Getting attention for answers that have aways been incorrect - "the two top answers are actually insecure" Jun 24 '19 at 15:40
  • 1
    @RogerLipscombe I agree the answer could be interpreted securely - "verify them manually" isn't explained, but if you understand it to mean "following the instructions in the ssh manual page" then that would probably be fine. The risk here is that someone might not understand that and could, for example, try insecure things like checking the key after logging in to the server. That's why I called this answer "dangerous" and other one "insecure" - the detail of how you check manually is the important bit. Thanks for the accept.
    – Michael
    Aug 1 '19 at 16:30
4

I've wrote simple script (add_to_known_hosts) to handle this:

It won't create duplicate entries in known_hosts, and it will check if if fingerprint matches one provided as second argument.

#!/usr/bin/env bash
# First argument should be hostname (or IP)
# Second argument should be referential fingerprint
# Example: add_to_known_hosts github.com SHA256:nThbg6kXUpJWGl7E1IGOCspRomTxdCARLviKw6E5SY8

host=$1
fingerprint=$2

ip=$(getent hosts $1 | awk '{ print $1 }')
echo $ip

keys=$(ssh-keyscan -t rsa $host $ip)

# Iterate over keys (host and ip)
while IFS= read -r key; do
    # Extract Host name (or IP)
    key_host=$(echo $key | awk '{ print $1 }')

    # Extracting fingerprint of key
    key_fingerprint=$(echo $key | ssh-keygen -lf - | awk '{ print $2 }')

    # Check that fingerprint matches one provided as second parameter
    if [[ $fingerprint != $key_fingerprint ]]; then
      echo "Fingerprint match failed: '$fingerprint' (expected) != '$key_fingerprint' (got)";
      exit 1;
    fi

    # Add key to known_hosts if it doesn't exist
    if ! grep $key_host ~/.ssh/known_hosts > /dev/null
    then
       echo "Adding fingerprint $key_fingerprint for $key_host to ~/.ssh/known_hosts"
       echo $key >> ~/.ssh/known_hosts
    fi
done <<< "$keys"
1

Automating SSH Known_Hosts fingerprint check

I have been trying to do this in Python on Jupyterhub for a little while but @Michael's answer was really helpful!

Let us be clear -- by design this step to confirm a known host should NOT be automated given susceptibility to man in the middle attacks (more info here, but it should be obvious why this is a concern).

Manually checking SSH RSA Fingerprint for GitHub.com with Python & Bash

The insecure workaround -o "StrictHostKeyChecking no" was alright for testing but glad to have an alternative.

  • Notice I use ! bang in Jupyter to invoke bash command for ssh-keyscan & cat
# MANUALLY GET TRUSTED RSA FINGERPRINT FOR GITHUB
# https://docs.github.com/en/github/authenticating-to-github/githubs-ssh-key-fingerprints

rsa_pubkey_fingerprint = 'SHA256:nThbg6kXUpJWGl7E1IGOCspRomTxdCARLviKw6E5SY8'

# Get the host rsa info for github, write to a file
# pipe to get the RSA FingerPrint, and finally write that fingerprint to file

!ssh-keyscan -t rsa github.com | tee ./github_ssh_test | ssh-keygen -lf - >> ./fingerprint_rsa

Assert Fingerprints match to confirm known hosts

You can now use a python assertion to ensure authenticity of our trusted RSA fingerprint vs the Scanned-over-your-internet RSA fingerprint:

assert(rsa_pubkey_fingerprint == open("./fingerprint_rsa", "r").read().split()[1])

# if the assertion passes and the trusted matches the scanned
# it'll write to your known_hosts file!

!cat ./github_ssh_test >> $HOME/.ssh/known_hosts

# you are now good to test your SSH connection
!ssh -T git@github.com

Thanks and a much better workaround to ignoring the strict check!

0

To generate the known_hosts file, after generating your private and public keys and copied the public key to site.com, you can do

ssh -T account@site.com

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.