Avoiding the remote host identification changed warning ssh'ing to localhost

If you run several virtual machines under VirtualBox and use port forwarding (Settings > Network > Advanced > Port Forwarding) to map its port 22 to a localhost port so that you can ssh to it, you will end up with several remote hosts all accessed from localhost but at different ports. In such a situation, if you have an older ssh client, you might end up with this well known warning when you try to connect:

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@    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @
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IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!

The reason is that older ssh clients only store the domain name or the IP address of the server in the ~/.ssh/known_hosts file. So there can only be one host key for localhost and when you connect to a different port there is a key mismatch. Newer ssh clients store the port number also and don't have this problem. You can fix this temporarily by deleting the host key entry but when you connect to another server it happens again.

I've seen solutions that suggest suppressing the host key check. There is another way. All addresses in the 127.0.0.0/8 subnet are localhost so you can use distinct IP addresses for each VM. For example in ~/.ssh/config you could have something like this:

Host centos64

Hostname localhost

        Port 2201

Host debian64

Hostname 127.0.0.2

        Port 2202

This way there are distinct host key entries for the two VMs and you will not get the dreaded clash and warning.

Once again, this is only necessary when using an older ssh client.