I have an ssh server. earlier toady, I was about to login to my server, when I got a message that said something along the lines of "Someone may be trying to do something nasty" and indicating a man in the middle attack.
I did not proceed to login. Instead, I shifted to a different network, and logged in. From the alternate network, I did not get the message about something nasty happening.
I have seen this in the past when I do something like change the login key. All the other boxes with expired keys baulk that something nasty might be happening. Until I update the keys.
This time, I have not changed anything. later, when I went to write up this question I tried to login again from the same machine that previously gave me the warning. This time, I did not get the message. I opted to not login anyhow.
For now, I do have physical access to the PC, it is running Debian 8. I forget if I have fail2ban running, but my firewall is on, and Im not using the default ssh port, only certain users (not root) can login.
how concerned should I be? what should I do in this situation?