ssh host keys on cloned virtual machines

Demi Marie Obenour demiobenour at
Sun Feb 26 04:13:53 AEDT 2023

On 2/25/23 07:50, Nico Kadel-Garcia wrote:
> On Fri, Feb 24, 2023 at 10:01 AM Jochen Bern <Jochen.Bern at> wrote:
>> On 24.02.23 12:58, Keine Eile wrote:
>>> does any one of you have a best practice on renewing ssh host keys on
>>> cloned machines?
>>> I have a customer who never thought about that, while cloning all VMs
>>> from one template. Now all machines have the exact same host key.
>>> My approach would be to store a machines MAC address(es). Then when
>>> starting the sshd.service, check if this MAC has changed. If so, remove
>>> all host keys, let sshd create new ones.
>> Strictly speaking, *if* you have an interest to make sure that *every*
>> VM gets unique host keypairs, then you should implement a cleanup
>> routine that takes care of "everything"¹ that matters to you.
> These vagaries are why many environments simply disable the validation
> of hostkeys in their .ssh/config settings and move on to work that is
> of some more effective use to their workplace. I've encountered,
> several times, when sites relied on extensive use of SSH key managed
> git access and shattered their deployment systems when the git server
> got moved and hostkeys were either incorrectly migrated or the IP was
> a re-used IP of a previously accessed SSH target. Hilarity ensued.
> This kind of hand-tuning of every deployment rapidly becomes a waste
> of admin time and serves little purpose without very tight control of
> the "known_hosts", which can be overridden by local users anyway.

Are SSH host certificates the solution?
Demi Marie Obenour (she/her/hers)
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