Re: Alternative to -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null ?

(answers assume OpenSSH, since that's what the options look like.)

Peter Valdemar Mørch (Lists) wrote:

I often know and accept that portX on serverY is not the same as it was 10 minutes ago. Therefore I don't want to use ~/.ssh/known_hosts. So
I use "ssh -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null -p portX serverY" but it is very lengthy to type and always yields this message:

Warning: Permanently added '<host>,<ip>' (RSA) to the list of known hosts

where 'list of known hosts' presumably is /dev/null.

Is there a better way to suppress using host keys? I wish there was a --no-hostkeys or similar option to do this.

If not, is there a way to avoid the above warning? It is misleading, and I prefer not to train myself to avoid warnings.

You could use LogLevel quiet.

(Yes, there are security problems when not using host keys. I know.)

I've tried to search the mailing list, but
shows some really weird results (try it!)

Further background

- We're on a LAN where our DHCP server is messed up. And corporate wisdom dictates that it isn't worth it to ensure that hosts get the same IP address at every reboot. We have to deal with it.

- We use port forwarding a lot, so port 2223 on serverX is forwarded to a particular host right now, but a different one in 10 minutes. That is reality for us.

For this one you can use "HostKeyAlias serverX" and "CheckHostIP no".

- Also, we often test fresh installations, where each test involves installation of the ssh package and hence the host keys differ from test to test.

This one is not so easy to handle automatically, since it's pretty much indistinguishable from a MITM attack.

You could use ssh-keygen -R [machine you just reinstalled] or the StrictHostKeyChecking / UserKnownHostsFile / LogLevel combination described above only for the install-test machines.

From a security standpoint an easy option to disable host keys when we
*know* they won't work is better than putting the StrictHostKeyChecking=no and UserKnownHostsFile=/dev/null in ~/.ssh/config and then teaching the eye not to see the "Warning: Permanently added..." message, isn't it?


Darren Tucker (dtucker at
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Good judgement comes with experience. Unfortunately, the experience
usually comes from bad judgement.