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I've written this small utility script:

for h in $SERVER_LIST; do ssh $h "uptime"; done

When a new server is added to $SERVER_LIST, the script is stopped with:

The authenticity of host 'blah.blah.blah (10.10.10.10)' can't be established.
RSA key fingerprint is a4:d9:a4:d9:a4:d9a4:d9:a4:d9a4:d9a4:d9a4:d9a4:d9a4:d9.
Are you sure you want to continue connecting (yes/no)?

I've tried yes:

for h in $SERVER_LIST; do yes | ssh $h "uptime"; done

with no luck.

Is there a way to parametrize ssh to automatically accept any new key?

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1  
Lekensteyn's answer is excellent and correct, but I just wanted to note that since ssh is expecting "yes" and yes outputs "y", you might have had better luck with for h in $SERVER_LIST; do yes yes | ssh $h "uptime"; done (note the extra yes, which tells yes what to say instead of "y"). –  chazomaticus Apr 18 '12 at 17:54
    
+1 Good to know - thanks! –  Adam Matan Apr 19 '12 at 7:26

3 Answers 3

up vote 51 down vote accepted

Use the StrictHostKeyChecking option, for example:

ssh -oStrictHostKeyChecking=no $h uptime

This option can also be added to ~/.ssh/config, e.g.:

Host somehost
    Hostname 10.0.0.1
    StrictHostKeyChecking no

Note that when the host keys have changed, you'll get a warning, even with this option:

$ ssh -oStrictHostKeyChecking=no somehost uptime
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that a host key has just been changed.
The fingerprint for the RSA key sent by the remote host is
31:6f:2a:d5:76:c3:1e:74:f7:73:2f:96:16:12:e0:d8.
Please contact your system administrator.
Add correct host key in /home/peter/.ssh/known_hosts to get rid of this message.
Offending RSA key in /home/peter/.ssh/known_hosts:24
  remove with: ssh-keygen -f "/home/peter/.ssh/known_hosts" -R 10.0.0.1
Password authentication is disabled to avoid man-in-the-middle attacks.
Keyboard-interactive authentication is disabled to avoid man-in-the-middle attacks.
ash: uptime: not found

If your hosts are not often reinstalled, you could make this less secure (but more convenient for often-changing host keys) with the -oUserKnownHostsFile=/dev/null option. This discards all received host keys so it'll never generate the warning.

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Great answer, thanks! –  Adam Matan Apr 18 '12 at 10:11

You can use the following command to add the fingerprint for a server to your known_hosts

ssh-keyscan -H <ip-address> >> ~/.ssh/known_hosts
ssh-keyscan -H <hostname> >> ~/.ssh/known_hosts

NOTE: Replace < ip-address > and < hostname > with the IP and dns name of the server you want to add.

The only issue with this is that you will end up with some servers in your known_hosts twice. Its not really a big deal, just mentionn. To ensure there are no duplicates, you could remove all servers first by running the following first:

ssh-keygen -R <ip-address>
ssh-keygen -R <hostname>

So you could run:

for h in $SERVER_LIST; do
    ip=$(dig +short $h)
    ssh-keygen -R $h
    ssh-keygen -R $ip
    ssh-keyscan -H $ip >> ~/.ssh/known_hosts
    ssh-keyscan -H $h >> ~/.ssh/known_hosts
done

Once thing to keep in mind when removing just to re-add, you are essentially removing the security of verifying the fingerprint. So you would definitely not want to run this script before each execution of your utility script.

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I'm a bit late with this response, but the sensible way would be to do a ssh-keyscan on the new machine before you run the uptime gathering.

ssh-keyscan  <newhost> >> ~/.ssh/known_hosts

Disabling the sanity check for convenience sake sounds like a bad plan, even if you think you're totally in control of the environment.

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