I just made a big mistake. Made a change to the sshd_config on a production server that locked me out. My heart skipped a beat when I lost connection and was presented with
brandons_mbp:~ bleon$ ssh domain
ssh: connect to host domain port 22: Connection refused
This is on one of my clients web servers. But I was able to revert the changes. It was a little complicated but it worked.
I knew the exact lines that needed to be removed just getting to them without connecting to the server was the trick. This is why running on EC2 saved me. I stopped the running instance for the web server and disconnected the volume from the instance.
I fired up a temporary CentOS instance (any *nix instance should work, CentOS just happens to be my favorite) and attached the volume to the new instance as a secondary disk. This allowed me to SSH into the new instance and browse the filesystem of the server I locked myself out of. Now using vi or your favorite editor edit the sshd_config file, shutdown the server, disconnect the volume and re attache it to your web-server instance.
I started the webserver instance and crossed my fingers, and when I tried to SSH in again I was successfull.
It would also be a good idea to create a snapshot of your volume before attempting any of this to be safe. And to test any changes your going to make that could potentially lock you out on a system you have physical access to.
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