OpenSSH 6.4 connection to Cisco 6506 routers/switches fails

[Loganaden Velvindron]

On Tue, Dec 24, 2013 at 12:52 AM,  <mikep at noc.utoronto.ca> wrote:
> On Wed, 13 Nov 2013, Loganaden Velvindron wrote:
>
>> On Wed, Nov 13, 2013 at 2:05 AM, Darren Tucker <dtucker at zip.com.au> wrote:
>>>
>>> On Tue, Nov 12, 2013 at 4:40 PM, <mikep at noc.utoronto.ca> wrote:
>>>
>>>> Just upgraded to OpenSSH_6.4 with OpenSSL 1.0.1e and libz.so.1.2.8.
>>>> Now some (but not all) Cisco router logins hang:
>>>>
>>>> debug1: sending SSH2_MSG_KEXDH_INIT
>>>> debug1: expecting SSH2_MSG_KEXDH_REPLY
>>>>  [hangs]
>>>>
>>>
>>> Suggestions in approximate order of likelihood.
>>>  - the additional KexAlgorithms exceed some static buffer in the Cisco.
>>>  Try:
>>> "KexAlgorithms
>>> diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1"
>>>  - you have some kind of path MTU problem and the extra traffic from the
>>> additional algorithms pushes you past some packet boundary.  Check the
>>> "send-q" column on client and the equivalent on the server and see if
>>> they're non-zero and non-decreasing).
>>
>>
>> Shouldn't Mike open a ticket at CISCO so that they start fixing the
>> software on their side as well ?
>
>
> Sorry to have taken so long to get back to you about this - your suggestion
> about "KexAlgorithms" caused me to test a lot of combinations to find what
> will work. It turns out the Cisco SSH server only supports a limited set of
> ciphers (this is documented sort-of by Cisco, and is displayed when you try
> to force a non-supported cipher).

That's short-sighted coming from them.

I have tested and I have the same problem with the latest snapshot. This is very
annoying.

Do you have a ticket number where I can also chip in ?


>
> This in turn seems to limit the key exchange mechanisms that will work.
>
> Forcing a cipher with '-c' also appears to force something in the Kex for
> OpenSSH; I can't find anything about Kex in any Cisco docs.
>
> I have created a special section of the 'ssh_config' file for those devices
> with these options, and all seems to be working fine:
>
> Ciphers aes128-cbc,3des-cbc,aes192-cbc,aes256-cbc
> KexAlgorithms
> diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchan
> ge-sha1,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1
>
> Thank you for the help!
>
>
>>>> Originally I had 'Cipher blowfish' set in '/etc/ssh/ssh_config', but
>>>> removing that makes no difference.
>>>
>>>
>>> That's because Cipher affects only Protocol 1 (which was some time in the
>>> past the only version at least some Cisco devices spoke).
>>>
>>>> However, forcing '-c 3des' does
>>>> allow it to work (even though '3des' is supposed to be the default):
>>>
>>>
>>> 3des is the default Cipher Protocol 1.  Protocol 2 takes a list (Ciphers)
>>> and its default is
>>>
>>>                 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
>>>                 aes128-gcm at openssh.com,aes256-gcm at openssh.com,
>>>                 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
>>>                 aes256-cbc,arcfour
>>>
>>> the -c option overrides both.
>>>
>>> --
>>> Darren Tucker (dtucker at zip.com.au)
>>> GPG key 8FF4FA69 / D9A3 86E9 7EEE AF4B B2D4  37C9 C982 80C7 8FF4 FA69
>>>     Good judgement comes with experience. Unfortunately, the experience
>>> usually comes from bad judgement.
>>> _______________________________________________
>>> openssh-unix-dev mailing list
>>> openssh-unix-dev at mindrot.org
>>> https://lists.mindrot.org/mailman/listinfo/openssh-unix-dev
>
>
> Mike
> --
> Mike Peterson                            Information Security Analyst -
> Audit
> E-mail: mikep at noc.utoronto.ca                WWW:
> http://www.noc.utoronto.ca/
> Tel: 416-978-5230                                           Fax:
> 416-978-6620



-- 
This message is strictly personal and the opinions expressed do not
represent those of my employers, either past or present.