I do a lot of work on EC2, where I ssh into a few instances I use for specific purposes. Each time I did this I'd get a prompt like:

$ ssh_ec2nf
The authenticity of host 'ec2-54-224-39-217.compute-1.amazonaws.com
(54.224.39.217)' can't be established.
ED25519 key fingerprint is SHA256:...
This host key is known by the following other names/addresses:
    ~/.ssh/known_hosts:591: ec2-18-208-226-191.compute-1.amazonaws.com
    ~/.ssh/known_hosts:594: ec2-54-162-24-54.compute-1.amazonaws.com
    ~/.ssh/known_hosts:595: ec2-54-92-171-153.compute-1.amazonaws.com
    ~/.ssh/known_hosts:596: ec2-3-88-72-156.compute-1.amazonaws.com
    ~/.ssh/known_hosts:598: ec2-3-82-12-101.compute-1.amazonaws.com
    ~/.ssh/known_hosts:600: ec2-3-94-81-150.compute-1.amazonaws.com
    ~/.ssh/known_hosts:601: ec2-18-234-179-96.compute-1.amazonaws.com
    ~/.ssh/known_hosts:602: ec2-18-232-154-156.compute-1.amazonaws.com
    (185 additional names omitted)
Are you sure you want to continue connecting (yes/no/[fingerprint])?

The issue is that each time I start my instance it gets a new hostname (which is just derived from the IP) and so SSH's trust on first use doesn't work properly.

Checking that "185 additional names omitted" is about the number I'd expect to see is ok, but not great. And it delays login.

I figured out how to fix this today:

1. Edit ~/.ssh/known_hosts to add an entry for each EC2 host I use under my alias for it. So I have c2-44-222-215-215.compute-1.amazonaws.com ssh-ed25519 AAAA... and I duplicate that to add ec2nf ssh-ed25519 AAAA... etc.

2. Modify my ec2 ssh script to set HostKeyAlias: ssh -o "StrictHostKeyChecking=yes" -o "HostKeyAlias=ec2nf" ...

More secure and more convenient!

(What got me to fix this was an interaction with my auto-shutdown script, where if I did start_ec2nf && sleep 20 && ssh_ec2nf but then went and did something else for a minute or two the machine would often turn itself off before I came back and got around to saying yes.)