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Attempting to SSH into my server, I got the "SOMEONE DOING SOMETHING NASTY" message. First time. The message states, "It is also possible that a host key has just been changed."

I am not clear about the meaning of "just".

Does it mean "has recently changed" or does it mean "no big deal, the key has changed".

Either way, I want to know WHY would the key change? I am not aware of anything I did that would cause it to change.

My IP address has not changed. I am working with this server on various ports and everything is normal.

All of the questions about this problem recommend to remove the old key in my known_hosts file, and I have done that.

Is this dramatic error message meant to be basically ignored, or could there really be someone doing something nasty?

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    The host key does not change on its own, so if you have not changed it then you should certainly investigate that urgently. Commented Dec 19, 2020 at 20:02
  • @MichaelHomer What can I do to investigate? So far, I have not re-ssh'ed but used Webmin to check if there are any new or unfamiliar users (no) or any unfamiliar processes running. What else can I do?
    – Ken
    Commented Dec 19, 2020 at 20:41
  • @MichaelHomer I just SSH'ed to this server from a different local machine (my laptop). No error, no changed key. Does this indicate that the local machine where I got the error may be compromised?
    – Ken
    Commented Dec 19, 2020 at 21:01
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    If you had previously connected from that other machine (and cached the host key fingerprint), it suggests that the known_hosts file on the original machine may have changed since you last successfully connected from it. That could be bitrot, an accident, or a deliberate edit. It could also be a network issue or a number of other causes, and there's not much information here to narrow that down. If either the original machine or the server were compromised this would be a sign that it was fairly inept, while MITM is fairly unlikely, so some non-malicious change is more probable. Commented Dec 19, 2020 at 22:19

1 Answer 1

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SSH keys change under the following conditions:

  1. Someone intentionally re-generated the SSH keys
  2. SSH service (sshd) was uninstalled and its configuration purged and it was later re-installed
  3. Server OS was re-installed (rebuilt)
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  • I have done none of those things. This machine (a DigitalOcean droplet running Ubuntu) is identical to another one and I keep both up-to-date. The other one has not exhibited this problem. If there was an update to SSH in the last few days - don't recall one - could that have triggered this?
    – Ken
    Commented Dec 19, 2020 at 20:48
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    4. You're actually connecting to a different host than you thought. Commented Dec 19, 2020 at 20:53
  • Do you mean, when I go through SSH? I have services running on my machine that are performing normally. I can access that machine through SFTP or Webmin, etc I see @MichaelHomer you are adding a fourth option...
    – Ken
    Commented Dec 19, 2020 at 20:53
  • I don't know what is happening, I was just completing the list. The point of the host key check is to detect MITM, so other services appearing to function normally is not a major data point as those can be proxied (though SFTP is still over SSH, of course). Commented Dec 19, 2020 at 20:59
  • Thank you. I am confused though. Am I worried about the server or my local machine?
    – Ken
    Commented Dec 19, 2020 at 21:05

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