"Add correct host key in known_hosts" / multiple ssh host keys per hostname?

 Remove that the entry from known_hosts using:

ssh-keygen -R *ip_address_or_hostname*

This will remove the problematic IP or hostname from known_hosts file and try to connect again.

how to avoid being asked "Enter passphrase for key " when I'm doing ssh operation on a remote host?

 On Mac, add UseKeyChain to ~/.ssh/config

nano ~/.ssh/config

and add the following

Host *

    UseKeychain yes

Make SSH connection live for long

ssh -o ServerAliveInterval=60 ubuntu@ip

Fixing “WARNING: UNPROTECTED PRIVATE KEY FILE!” on Linux / OS X Mac

If you are getting this error then you probably reset the permissions on your hidden .ssh directory in your user folder, and your keys aren’t going to work anymore. It’s very important that these files not be writable by just anybody with a login to the box, so openssh will give you an error if you try to use them.

The full error message:

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@         WARNING: UNPROTECTED PRIVATE KEY FILE!          @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
Permissions 0744 for '/home/geek/.ssh/id_rsa' are too open.
It is recommended that your private key files are NOT accessible by others.
This private key will be ignored.
bad permissions: ignore key: /home/geek/.ssh/id_rsa

To fix this, you’ll need to reset the permissions back to default:

sudo chmod 600 ~/.ssh/id_rsa
sudo chmod 600 ~/.ssh/id_rsa.pub

If you are getting another error:

Are you sure you want to continue connecting (yes/no)? yes
Failed to add the host to the list of known hosts (/home/geek/.ssh/known_hosts).

This means that the permissions on that file are also set incorrectly, and can be adjusted with this:

sudo chmod 644 ~/.ssh/known_hosts

Finally, you may need to adjust the directory permissions as well:

sudo chmod 755 ~/.ssh

This should get you back up and running.

GIT + How to change or remove Passpharse for a GIT SSH Key

>> ssh-keygen -f ~/.ssh/id_rsa -p

>> Enter old passphrase: 
>> Key has comment '/home/gsekhar/.ssh/id_castlighthealth_rsa'>>   

Enter new passphrase (empty for no passphrase): >>  

Enter same passphrase again: >> Your identification has been saved with the 

new passphrase.