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Re: [Fab-user] EC2 host keys

From: Jeff Forcier
Subject: Re: [Fab-user] EC2 host keys
Date: Sat, 9 May 2009 15:53:37 -0400

HI Pat,

Yea, I get that part, I think the salient question here was confirming
that these "new servers" are sitting behind the same IP and/or
hostname, which is why from SSH's point of view, they appear to be the
same server with a totally new host key. (I wonder if this is
something that the EC2 folks have considered? Seems like a pretty odd
problem to introduce.) Is that the case? Mostly just curious at this
point, I've not used EC2 before.

Anyway, I've made the tweaks I mentioned, and attempted to document
this general area since there were no prior docs:
http://docs.fabfile.org/usage.html#ssh-behavior . Do let me know if
I've misconstrued anything.

I think your suggestion of further delving into Paramiko's host key
list functionality may be a good one -- let me know if you come up
with anything useful. For now, anyway, I guess it's good that this
discussion resulted in more docs and making Fabric's SSH behavior more
open/configurable :) (and finding a pre-existing bug or two, to boot.)


On Sat, May 9, 2009 at 3:39 PM, Patrick J McNerthney
<address@hidden> wrote:
> Jeff,
> Many of Christian's suggestions could be leveraged and more importantly
> probably *should* be leverage, when the server involved has been configured
> into a known state and should always be of that known state.  One could then
> launch multiple instances of these servers, all sharing the same ssh keys.
> The wrinkle in my scenario is getting to that known configured state.  To do
> this, I am launching a brand new, generic Linux (technically, in my case an
> Ubuntu 8.10 Server) EC2 instance.  This is pretty much exactly like
> installing Linux on a new piece of hardware for the first time.  The ssh
> installation on this new EC2 Linux instance initializes itself, which means
> that is generates a clean set of ssh keys as it's identity.
> I was looking over the paramiko source and noticed that one can have a
> "local host file".  I think that maybe I can do something with that to have
> my cake and eat it to.  What is needed is the explicit clearing of a
> particular server's key only at the time of EC2 instance creation.  Once the
> instance is up and running, then I do in fact want to enforce the host key
> validation on all subsequent accesses of that server.
> I'll work with that and see where I get.
> Short term, that makes sense to make it an explicit override to ignore the
> system host file, so that it is not the default behavior.
> Thanks,
> Pat
> Jeff Forcier wrote:
>> Christian raises some good points; I had been wondering the same
>> thing, but was willing to overlook "how things should work" in favor
>> of "how things apparently work, in some cases" since I tend to prefer
>> pragmatism over dogma.
>> However, I was just writing up some docs about this aspect of
>> Paramiko/Fabric, and realized something that should have struck me
>> before: the recent change means that man-in-the-middle attacks will
>> now *not be detected by default* and that makes me pretty
>> uncomfortable.
>> I think I will split this out into two distinct settings, because it
>> really is two distinct problems being solved (adding new/unknown, but
>> valid, hosts, versus detection of changed host keys for known hosts).
>> Thus, Pat would still be able to control the loading of host keys, but
>> the default behavior will still be honoring the security spirit of
>> SSH.
>> Thoughts?
>> -Jeff
>> On Sat, May 9, 2009 at 2:31 PM, Christian Vest Hansen
>> <address@hidden> wrote:
>>> On Sat, May 9, 2009 at 7:42 PM, Patrick J McNerthney
>>> <address@hidden> wrote:
>>>> Jeff,
>>>> Thanks for the response.
>>>> Sorry, I did use IP address in my original message, but probably should
>>>> have
>>>> used DNS name.  It actually does not matter, the problem exists for
>>>> both.
>>>>  You are correct that I could disable the additional checking of IP
>>>> address
>>>> conflicts, but I have the same exact problem with the DNS name.
>>>> The problem is that there is an entry in my known_hosts file for a
>>>> particular DNS name that has a key that does not match the DNS name
>>>> being
>>>> used.  This is caused by the recycling of both DNS names and IP address
>>>> by
>>>> Amazon's EC2.
>>> Out of curiosity: do the servers get new IPs while they are running,
>>> or do the recycling only happen when servers are rebooted?
>>> Also, is there a way to partition the servers? I mean, you may have
>>> two kinds of servers, like front-end and back-end, and then you say
>>> all front-ends are in this IP range and all back-ends are in that
>>> range. Is that possible?
>>> Where this all leads is; I'm wondering if it might make sense if all
>>> servers of a kind shared the same key pair, so it wouldn't make any
>>> difference if they swapped IP addresses amongst one another.
>>> This way, I think, the servers could swap IPs all they want and we
>>> could still connect to them in a manner that is both reliable and
>>> resistant to DNS cache poisoning (once their fingerprints are in
>>> known_hosts, that is).
>>>> To replicate, I did the following:
>>>> 1.  removed both my known_hosts file and my .ssh config file.
>>>> 2.  ssh'ed into ServerA, then exited.  There are now two lines in the
>>>> known_hosts file, one for the dns name and one for the ip address.
>>>> 3.  ssh'ed into ServerB, then exited.  There are now four lines in the
>>>> known_hosts file.
>>>> 4.  modified the known_hosts file so that ServerA's entries contains
>>>> ServerB's key.
>>>> 5.  attempted to ssh to ServerA and was rejected because the host key
>>>> has
>>>> changed.
>>>> 6.  attempted to ssh to ServerA with "StrictHostKeyChecking no", this
>>>> worked.
>>>> So it is with this setup, where the known_hosts file contains the wrong
>>>> key
>>>> for a server, I am unable to use Fabric against such a server.  What I
>>>> want
>>>> to do is replicate the effect of "StrictHostKeyChecking no" in Fabric.
>>>> In this scenario, this is a case of an invalid server key, not a missing
>>>> server key.  The missing host key policy never gets called.  This is why
>>>> the
>>>> env.reject_unknown_keys currently has no effect.
>>>> Any clearer?
>>>> Pat
>>>> Jeff Forcier wrote:
>>>>> Hi Pat,
>>>>> First, I think this partially falls under an existing TODO item, which
>>>>> I plan to have in place for 1.0 and hopefully 0.9: honoring any
>>>>> .ssh/config options that we have functionality for. In this case,
>>>>> Fabric would probably check your StrictHostKeyChecking option and use
>>>>> that to override the default value of env.reject_unknown_keys.
>>>>> Secondly, I've read over your use case and I may be missing something:
>>>>> as things currently are, why isn't setting env.reject_unknown_keys to
>>>>> False good enough? Simply loading the host key list is not what drives
>>>>> the reject/don't-reject decision: that's driven by the policy given to
>>>>> set_missing_host_key_policy.
>>>>> I just double checked this by tweaking a (static) IP in my known_hosts
>>>>> file so that it was incorrect, then ensuring my fabfile had
>>>>> env.reject_unknown_keys = False. When connecting to that server, the
>>>>> "new" IP was added as a new entry to my known_hosts and the connection
>>>>> was created without issue.
>>>>> Is the problem that you don't want the new IPs added to your host list,
>>>>> or
>>>>> what?
>>>>> Best,
>>>>> Jeff
>>>>> P.S. In checking "man ssh_config" I found that there's an even more
>>>>> specific setting, CheckHostIP, which sounds like it fits your
>>>>> situation better than StrictHostKeyChecking. Unfortunately, Paramiko
>>>>> doesn't appear to support that level of granularity, so we're out of
>>>>> luck with that for now. Wanted to mention it anyways, though, in case
>>>>> you weren't aware of it, for non-Fabric use.
>>>>> On Sat, May 9, 2009 at 11:10 AM, Patrick J McNerthney
>>>>> <address@hidden> wrote:
>>>>>> I have an issue with Amazon EC2 instances where ssh host keys have
>>>>>> been
>>>>>> saved in .ssh/known_hosts but are incompatible with an EC2 instance ip
>>>>>> address.  This occurs when the ip address has been reassigned to a new
>>>>>> EC2
>>>>>> instance.  So the basic sequence of events are:
>>>>>> o Start an EC2 instance which is assigned an ip address.
>>>>>> o ssh to that ip address and that server's ssh key is associated with
>>>>>> that
>>>>>> ip address in the known_hosts file.
>>>>>> o Terminate that EC2 instance.
>>>>>> o A new EC2 instance is started and it happens to get assigned the
>>>>>> same
>>>>>> ip
>>>>>> address.
>>>>>> At this point, if I first ssh to it, I have ssh configured with
>>>>>> StrictHostKeyChecking set to no, so ssh will emit a warning about this
>>>>>> ip
>>>>>> address having a new key, but still allows me to continue.
>>>>>> However, if at this point I try to use Fabric to execute some
>>>>>> commands,
>>>>>> it
>>>>>> always will fail.  This is because the SSHClient.load_system_host_keys
>>>>>> is
>>>>>> always called, causing the connection to fail if there is an
>>>>>> incompatibility
>>>>>> between the ip address and the server key.
>>>>>> I have addressed this in my own fork here:
>>>>>> http://github.com/iciclespider/fabric/commit/08ad1c491e5643990c2a35e865784d2b61aa742f
>>>>>> What this does is replace this:
>>>>>>  client.load_system_host_keys()
>>>>>>  if not env.reject_unknown_keys:
>>>>>>      client.set_missing_host_key_policy(ssh.AutoAddPolicy())
>>>>>> with this:
>>>>>>  if env.reject_unknown_keys:
>>>>>>     client.load_system_host_keys()
>>>>>>  else:
>>>>>>      client.set_missing_host_key_policy(ssh.AutoAddPolicy())
>>>>>> I also considered using another env setting value to control this, but
>>>>>> my
>>>>>> conclusion that this behavior is in fact in line with the implied
>>>>>> behavior
>>>>>> of the "reject_unknown_keys" name.  In other words, the list of known
>>>>>> keys
>>>>>> should only be loaded if the intention is to reject those keys that
>>>>>> are
>>>>>> not
>>>>>> known.
>>>>>> Pat McNerthney
>>>>>> ClearPoint Metrics, Inc.
>>>>>> _______________________________________________
>>>>>> Fab-user mailing list
>>>>>> address@hidden
>>>>>> http://lists.nongnu.org/mailman/listinfo/fab-user
>>>> _______________________________________________
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>>> --
>>> Venlig hilsen / Kind regards,
>>> Christian Vest Hansen.

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