Different HostKeys for different hostnames or IPs in the same sshd?..

[Mikhail T.]

Hello!

Like many organizations, we have "disaster-recovery" location, where separate 
servers are running ready to take up important services should the primary 
location fail.

Some of the services provided involve accepting files over scp (and sftp), and 
here is the problem... The primary and the secondary hosts use different 
host-keys... If the hosts were accessed as "service-primary.example.net" and 
"host-dr.example.net", this would be fine, but the users (and the automated 
tools) would have to be told, which name to use.

So, we want to use an "umbrella" DNS name "service.example.net" to control the 
destination. Under normal circumstances, it is a CNAME for the 
"service-primary.example.net", but in case of a disaster, it will be changed to 
"service-dr.example.net".

How do we configure things so that the users and the automated scripts aren't 
"freaked-out" by the key of "service.example.net" suddenly changing, when the 
DNS is changed? Other than both machines using the same  hostkey, of course...

Can sshd use a different key depending on which name it is contacted under -- 
that is, does the ssh-protocol have anything like HTTP's Host:-header? If not, 
can sshd offer a different key depending on the IP-address, that the incoming 
connection uses?

Thanks for any ideas. Yours,

    -mi