In case this is any use to anyone else –
I’ve had AWS instances running for a few years. Today I went to create another one for something and infuriatingly, couldn’t connect to it over ssh at all: ssh just kept timing out.
I found a few links to do with groups, but the default group created for me in the (much improved) wizard seemed to be fine for incoming ssh connections. I then found a bunch of links about ip tables and the like, and after an extremely frustrating hour got it to work. Essentially (and without having done much research on this), AWS seems to have moved to a Virtual Private Cloud architecture, which existing users like me seem to have been moved into by AWS creating a default VPC for me, but one which somehow didn’t allow incoming connections.
I’m really not sure precisely why it worked, but it happened shortly after I created a new VPC and then created an instance using that. My VPC has DNS resolusion, DNS hostnames and “classic link”.
So: vague, but maybe it’ll help someone.
Amazon Linux AMI Instance Type Matrix “This table shows which flavors of the Amazon Linux AMI are recommended on each Amazon EC2 instance type.”