I am creating a bash script to provision a new server that I can deploy a web application to. One thing I always have to do is as GitHub as a known host using ssh git@github.com. How can I automate this process in a bash script, and do it in an idempotent way?


The simple way to go would be to do something like this.

ssh-keyscan remote_server >>~/.ssh/known_hosts

If this box is brand new you might also need to create the ~/.ssh directory before you run ssh-keyscan.

Keep in mind that ssh-keyscan can take an arbitrary number of hostnames. It will get all the keys it can.

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    PS - For provisioning you should be using something like puppet instead of a bash script. For puppet this could be easily handled with the sshkey resource. Also see this question for a method to manage the known_hosts en-masse serverfault.com/a/416782/984 – Zoredache Dec 14 '13 at 0:40
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    That sure sounded good to me, but after spending a few hours apiece on puppet and competitors, I scurried back to bash scripts and sanity. If those tools are intuitive, I apparently have no intuition. YMMV. – Ron Burk Apr 8 '16 at 19:32
  • Use bash. I constantly run into issues across different versions of things like puppet or ansible. We always go back to bash... 3 companies running now like this and bash is always reliable for us. – Ligemer Sep 21 '16 at 23:05

Are you trying to automate accepting the new key? If so, you could use -oStrictHostKeyChecking=no.
Doing so is a very bad idea as you're now completely wide open to man-in-the-middle attacks.

A better option would be just to manage a known_hosts file and reuse that file when you provision new servers. Stick it on github and write a simple script to download that file before sshing into github.

The strict host key checking is a good thing.

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  • Can you elaborate on the "manage known_hosts file"? I think that's what I want to do, but when I viewed the file, it's contents looked like some sort of hash/key and didn't look like something that was intended to be managed manually. – Andrew Dec 5 '13 at 2:03
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    Provision a new server, manually ssh into github like you would. Accept the host key when prompted. Log out. Copy ~/.ssh/known_hosts from that newly provisioned server somewhere else (github, web server, doesn't matter as long as you can get it). Next time you provision a server, copy that file back before sshing to github. You don't need to edit the file. – yoonix Dec 5 '13 at 2:32
  • This is better than my answer (safer). A further improvement on yoonix's answer though is to parse 'ssh-keyscan github.com' and store the returned key into ~/.ssh/known_hosts that way it isn't static in a file somewhere for you to need to update. – Sirex Dec 5 '13 at 2:45
  • That would work too, but I wouldn't consider it better. You're potentially setting yourself up for a man-in-the-middle attack if you're grabbing a new host-key every time. – yoonix Dec 5 '13 at 3:05
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    To clarify my last comment (too late to edit): Grabbing a new host-key each time you provision a host is functionally no different than setting StrictHostKeyChecking=no. With either of them you're blindly trusting whatever key gets sent each time you provision. If you think a MITM attack to be unlikely, read these two articles. Github would be a HUGE target. – yoonix Dec 5 '13 at 3:16

I'm not sure i understand the question, but i think you want to ignore the known_host prompt or avoid it entirely, in which case:

ssh -o StrictHostKeyChecking=no

or other suggestions at: http://www.joedog.org/2012/07/ssh-disable-known_hosts-prompt/

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  • I want a non-interactive way of accepting the GitHub host key (since this will happen in a bash script). – Andrew Dec 5 '13 at 2:05
  • then this'll work - it won't accept the key though, it'll ignore it entirely. Yoonix's answer is better – Sirex Dec 5 '13 at 2:44

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