how to avoid ssh asking permission?

We are attempting to speed up the installation of oracle nodes for RAC installation.
this requires that we get ssh installed and configured so that it doesn’t prompt for a password.

The problem is:
On first usage, we are prompted for

RSA key fingerprint is 96:a9:23:5c:cc:d1:0a:d4:70:22:93:e9:9e:1e:74:2f.
Are you sure you want to continue connecting (yes/no)? yes

Is there a way to avoid that or are we doomed to connect at least once on every server from every server manually?

Answers:

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Method 1

Update December 2019:

As Chris Adams pointed out below, there has been a fairly significant change to Openssh in the 6.5 years since this answer was written, and there is a new option that is much safer than the original advice below:

* ssh(1): expand the StrictHostKeyChecking option with two new
   settings. The first "accept-new" will automatically accept
   hitherto-unseen keys but will refuse connections for changed or
   invalid hostkeys. This is a safer subset of the current behaviour
   of StrictHostKeyChecking=no. The second setting "off", is a synonym
   for the current behaviour of StrictHostKeyChecking=no: accept new
   host keys, and continue connection for hosts with incorrect
   hostkeys. A future release will change the meaning of
   StrictHostKeyChecking=no to the behaviour of "accept-new". bz#2400

So instead of setting StrictHostKeyChecking no in your ssh_config file, set StrictHostKeyChecking accept-new.

Set StrictHostKeyChecking no in your /etc/ssh/ssh_config file, where it will be a global option used by every user on the server. Or set it in your ~/.ssh/config file, where it will be the default for only the current user. Or you can use it on the command line:

ssh -o StrictHostKeyChecking=no -l "$user" "$host"

Here’s an explanation of how this works from man ssh_config
(or see this more current version):

StrictHostKeyChecking

If this flag is set to “yes”, ssh will never automatically add
host keys to the $HOME/.ssh/known_hosts file, and refuses to
connect to hosts whose host key has changed. 
This provides maximum protection
against trojan horse attacks, however, can be
annoying when the /etc/ssh/ssh_known_hosts file is
poorly maintained,
or connections to new hosts are frequently made.  This
option forces the user to manually add all new hosts.  If this
flag is set to “no”, ssh will automatically add new host keys to
the user known hosts files.  If this flag is set to “ask”, new
host keys will be added to the user known host files only after
the user has confirmed that is what they really want to do, and
ssh will refuse to connect to hosts whose host key has changed. 
The host keys of known hosts will be verified automatically in
all cases.  The argument must be “yes”, “no” or “ask”.  The
default is “ask”.

Method 2

ssh-keyscan – Gather ssh public keys

If you already know the list of hosts you will connect to, you can just issue:

ssh-keyscan host1 host2 host3 host4

You can give the -H option to have it hash the results like ssh defaults to now

Also you can give -t keytype were keytype is dsa, rsa, or ecdsa if you have a preference as to which type of key to grab instead of the default.

Once you have run ssh-keyscan it will have pre-populated your known-hosts file and you won’t have ssh asking you for permission to add a new key.

Method 3

Ignore Host

Ignore the HostKeyChecking. For this I use e.g.:

ssh -oStrictHostKeyChecking=no -oUserKnownHostsFile=/dev/null <a href="https://getridbug.com/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="037670667143667b626e736f662d6d6677">[email protected]</a>

Add Host

Add the host’s/server’s fingerprint to .ssh/known_hosts prior to your first connect. This is the safer way.

Method 4

You can add the fingerprint to each server’s known_hosts. For a single user:

cat ~/.ssh/known_hosts
echo "$SERVER,$PORT ssh-rsa $SERVER_KEY_FINGERPRINT" >> ~/.ssh/known_hosts

Method 5

Execute the following snippet before trying.

mkdir -p ~/.ssh     
echo "Host *" > ~/.ssh/config     
echo " StrictHostKeyChecking no" >> ~/.ssh/config

ps: Strictly not for production servers, beware of ManInMiddle

Method 6

I like Tim’s answer for one off type things however, if this is a host you intend on connecting to on a regular basis I would create an entry in your ~/.ssh/config (create it if the file does not exist).

# this example shows wildcard for IP
# you can even use more than one wildcard 10.0.*.* for example
Host 192.168.56.*
    StrictHostKeyChecking no
    UserKnownHostsFile /dev/null
# you can even alias it, which is really useful when scp'ing/rsyncing foo:/path/to/remote
Host foo
    HostName foo-long-192-10-135-55.hostname.not-going-to-remember.doh
    StrictHostKeyChecking no
    UserKnownHostsFile /dev/null

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0