From time to time you might encounter a failure message about SSH and host identification. SSH remembers the fingerprint of keys of other hosts it connects to. It stores these keys in a file, so that if the fingerprint changes you will hear about it. This can happen with some DHCP addresses. For example, HostA has a DHCP address of 192.168.1.102. You change HostA’s IP address, to a static address, and the lease for 192.168.1.102 expires. Then you bring up HostB and it gets the DHCP address of 192.168.1.102. When you go to ssh to 192.168.1.102, you get an error. That’s because SSH recognizes that it’s a different host altogether. This helps prevent in MIM attacks, or IP spoofing. In this case we know what’s going on, so it’s safe to remove the old fingerprint for HostA and reconnect to HostB subsequently storing it’s fingerprint.

# ssh 192.168.1.102
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@    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 ECDSA key sent by the remote host is
34:6e:16:28:90:21:bd:6a:80:e4:97:41:85:ef:4a:ad.
Please contact your system administrator.
Add correct host key in /root/.ssh/known_hosts to get rid of this message.
Offending ECDSA key in /root/.ssh/known_hosts:15
ECDSA host key for 192.168.1.102 has changed and you have requested strict checking.
Host key verification failed.
#

One way to fix this is to use vi to edit the known_hosts file. Use the :15 command to navigate to line 15 and use dd keystroke to remove this entry, then :wq to save and quit vi.

Another alternative is to use sed to remove line 15:

# sed -i '15d' /root/.ssh/known_hosts
#

Also, the ssh-keygen utility comes with this built in:

# ssh-keygen -f '/root/.ssh/known_hosts' -R 192.168.1.102
/root/.ssh/known_hosts updated.
Original contents retained as /root/.ssh/known_hosts.old
# 
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