I installed openssh on my new Ubuntu14.04 server, turned on ssh, and enjoyed it after I finished installation. Every thing was fine; however, after just one night, I was blocked when using putty as usual. I got the message: connection refused.
I rebooted the server, and I were again able to connect my server through ssh. But, after half a day, it happened again: my connection through ssh is refused.
I thought some important settings might be accidentally modified, so I re-installed Ubuntu 14.04 on the server. It worked at that moment but failed at the next day.
I wondered if there was anything wrong on the firewall, but the firewall manager said that another server, which is one of the new servers in our lab, under the same firewall policy does not have this problem, so there should be some problems on our machine.
There is no physical problem on internet, because the engineer showed me that he can ping this server through port 80.
This is confusing because the problem exists even though I re-installed the OS. There is no physical problem on the internet, and the firewall policy does not block other servers from my IP. Are there any ssh variables or system settings that I should check?