Subject: Re: [X2Go-Dev] Bug#241: Changed host key cannot be updated

From: Mike Gabriel <[email-masked]>
Date: Fri, 21 Jun 2013 10:20:49 +0200

Hi Heinrich,

On So 16 Jun 2013 14:36:32 CEST Heinrich Schuchardt wrote:

> Dear maintainer,
>
> from time to time the SSH key used for identification by a X2GO  
> server may change.
>
> When trying to connect the server a pop up is shown:
>
> "Anmeldung fehlgeschlagen"
> "Host-Key des Servers hat sich geändert Er lautet jetzt:
> 00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00
> Aus Sicherheitsgründen wird die Verbindung abgebrochen"
>
> The user is left puzzled with what he should do next.
>
> There is no indication in which file there is a problem, e.g.
> ~/.ssh/known_hosts
> or
> %APPDATA%\ssh\known_hosts
>
> There is no indication which entry in this file is corrupted.
>
> Deleting file known_hosts is a bad idea because it may contain the  
> keys for dozens of validated servers.
>
> There are examples of more informative output, e.g. from command  
> line program ssh:
>
> @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
> @    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @
> @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
> IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
> Someone could be eavesdropping on you right now (man-in-the-middle attack)!
> It is also possible that a host key has just been changed.
> The fingerprint for the RSA key sent by the remote host is
> 00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00.
> Please contact your system administrator.
> Add correct host key in /home/user/.ssh/known_hosts to get rid of  
> this message.
> Offending RSA key in /home/user/.ssh/known_hosts:1
> RSA host key for 10.0.0.5 has changed and you have requested strict checking.
> Host key verification failed.
>
> Here I can identify the filename: /home/user/.ssh/known_hosts
> and the line of the the entry: 1
>
> Manual editing of known_hosts is now possible but not too good an  
> idea because it is error prone.
>
> A good solution is what you see in PuTTY. A warning pop up is shown  
> and you get the choice to update file known_hosts.
>
> Best regards

The above surely is a good point to discuss first before implementing.

Obviously, such a replace-host-key button would improve usability in  
case host key changes occur.

However, if someone captured DNS and replaced my X2Go server by an  
agressive X2Go server, I (as developer) surely want to protect the  
user from simply klicking ,,Yeah, ok man... replace that host key...  
and can we go on then please...''.

The SSH-unexperienced user (i.e. probably nearly everyone in the  
windows world) will then just simply click ,,replace host key''.

So, for me this kind of replace-host-key dialog should at least have a  
double confirmation check dialog: Are you sure to replace... -> Are  
you really sure???. That kind of thing.

Heinrich: if you could come up with a patch for this issue, it would  
surely speed up an inclusion of your requested feature.

@all: comments, opinions on such a new feature?

Mike

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