The Warning

bash
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@    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @
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IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!

This error means the server's host key doesn't match what's stored in your known_hosts file.

When This Is Expected

The key changes legitimately when:

  • You rebuilt the server
  • The server was reinstalled
  • You're connecting to a different server at the same IP/hostname
  • Cloud infrastructure was replaced

When This Is Suspicious

Be concerned if:

  • You didn't change anything on the server
  • Multiple people report this issue
  • The server is production and shouldn't have changed

The Quick Fix

If you're sure the key changed legitimately, remove the old entry:

bash
ssh-keygen -R server.example.com
# Or by IP
ssh-keygen -R 192.168.1.100

Then reconnect. SSH will ask you to verify the new key:

bash
ssh user@server.example.com
# Type "yes" to accept the new key

The Safer Approach

If you're not sure, verify the new key with the server administrator or check it from the server console:

bash
# On the server
ssh-keygen -l -f /etc/ssh/ssh_host_ed25519_key.pub
# Or
cat /etc/ssh/ssh_host_ed25519_key.pub

Compare this fingerprint with what SSH shows when you reconnect.

Edit known_hosts Manually

To remove a specific line from ~/.ssh/known_hosts:

```bash # Find the line number grep -n "server.example.com" ~/.ssh/known_hosts # Output: 42 server.example.com ssh-ed25519 AAAA...

# Delete line 42 sed -i '42d' ~/.ssh/known_hosts ```

Preventing Future Issues

For cloud servers that might be rebuilt, consider:

  1. 1.Using host aliases - Add to ~/.ssh/config:
bash
Host myserver
  HostName 192.168.1.100
  User admin
  UserKnownKnownHostsFile ~/.ssh/known_hosts_myservers
  1. 1.Disabling strict checking (less secure, use only for throwaway servers):
bash
Host temp-server
  StrictHostKeyChecking no
  UserKnownHostsFile /dev/null
  1. 1.Persisting host keys - If you control the server, back up /etc/ssh/ssh_host_* and restore after rebuild.