Removing entries from the known_hosts

One often encounters the following error message, particularly in situations where the machine owning the target IP address has changed:


user@host-machine:~$ ssh user@target_IP
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@ 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 the RSA host key has just been changed.
The fingerprint for the RSA key sent by the remote host is
XX:XX:XX.
Please contact your system administrator.
Add correct host key in /home/user/.ssh/known_hosts to get rid of this message.
Offending key in /home/user/.ssh/known_hosts:4
RSA host key for target_IP has changed and you have requested strict checking.
Host key verification failed.


The issue can be resolved by using the ssh-keygen command with the -R option to remove and update the known_hosts file. Simply do:


user@host-machine:~$ ssh-keygen -R target_IP -f /home/user/.ssh/known_hosts
/home/user/.ssh/known_hosts updated.
Original contents retained as /home/user/.ssh/known_hosts.old
user@host-machine:~$ ssh user@target_IP
The authenticity of host 'target_IP (target_IP)' can't be established.
RSA key fingerprint is XX:XX:XX.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added 'target_IP' (RSA) to the list of known hosts.
user@target_IP's password:
Linux XenOpen 2.6.32.21-xen #2 SMP Mon Sep 27 10:23:06 EDT 2010 x86_64 GNU/Linux
Ubuntu 10.04.1 LTS



Translating from Names to IPs

To store the translation from a machine name to its IP go to /etc/hosts under Ubuntu Linux. You would need to sudo to change the file. Add stuff like:

192.X.X.X Xen1

Doing this will make it easier to ssh, sftp, etc. as you don't need to remember the IP addresses of the machines and more easily memorizable mnemonics can be used instead

For example now you could do:

ssh xen1

and your machine would know where you want to go !