Re: Clusters, known_hosts, host keys, and "REMOTE HOST IDENTIFICATION HAS CHANGED"

I suspect the problem is that the application specific IP addresses move from host/server to server, and since the Host Key is tied to the host/server rather than IP address - the SSH clients get worried.

If you have control over your user's SSH clients, then you could implement something like #4 below. If you don't have control over your user's SSH clients, you're hosed and have to run separate instances of sshd bound individually to each IP address - the management headache you're trying to avoid.

I would suggest as an enhancement to #4 that the SSH client be allowed to define a named pool (of the server hosts) and a list of "swimmers" (awful name) that would accept any of the host keys for the pool. That way, you could have distinct, multiple pools. Each pool could be administered separately, and a "swimmer" could only be in a single pool at a time.

I don't see a good way around this that isn't a management nightmare. Either you're managing sshd bound to an IP address rather than a host, or you're managing some client configuration. At least, by managing the sshd's, you're not binding yourself to any particular version of the ssh client and can take advantage of any bug fixes or other enhancements that might occur in the client.

On Sep 18, 2009, at 1:08 PM, H. Kurth Bemis wrote:

On Thu, 2009-09-17 at 16:53 -0700, Steve Bonds wrote:
SSH List-dwellers:

I'm using OpenSSH in an environment with lots of clusters. These
clusters have IP addresses which are associated with a particular
application rather than with a particular host. Oftentimes
(especially for file transfers) it's helpful to ssh/scp to the IP
address associated with the application rather than the one associated
with the host. However, given that each host has its own host key, we
frequently get:


Which of course panics the user the first time they see it, and causes
them to ignore it the second time onward-- neither of which are
desired behaviors...

I've thought about several solutions to this including:

1) Make all the host keys the same (hundreds of hosts, kind of
diminishes the value of a host key...)
2) Configure ssh to ignore host key changes (harder than you might
think since often new ssh clients are brought in)
3) Give each application its own dedicated ssh and host key (tricky to
set up and monitor, fairly high effort)
4) Tweak OpenSSH so that it will accept any host key from a list
(requires some programming effort, might not be a good idea)
5) Other?

What do you all think of option 4? In particular, I was thinking that
there might be a way to allow hosts on the same subnet to simply
prompt to add the additional key for the same DNS name rather than
popping up the man-in-the-middle warning. If there were multiple keys
present in known_hosts for a given hostname, any of them would be

Could this be done without weakening the host security of OpenSSH?
Should I instead just hold The Great Re-Keying and go with option 1?

I appreciate any advice.


-- Steve Bonds

Maybe the issue doesn't really involve modifying OpenSSH at all. If you
have access to the hosts, wouldn't it be possible to
pre-generate .known_hosts with all the host keys in your cluster? Then
each client would have every key in it's .known_hosts, so it wouldn't
matter which host the client was connecting to.

Then if one of the keys change, you can generate a new .known_hosts.
Users are still alerted if a key changes on it's own.

Whatever your final solution, please remember to share with the
class. :]


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