Invalid modulus length

[jjx]

Today, when trying to ssh a cisco router i got this error:

 PDT: SSH2 1: input: padlength 7 bytes
 PDT: SSH2 1: SSH2_MSG_KEXINIT received
 PDT: SSH2:kex: client->server enc:aes128-cbc mac:hmac-md5
 PDT: SSH2:kex: server->client enc:aes128-cbc mac:hmac-md5
 PDT: SSH2 1: ssh_receive: 24 bytes received
 PDT: SSH2 1: input: total packet length of 24 bytes
 PDT: SSH2 1: partial packet length(block size)8 bytes,needed 16 bytes,
               maclen 0
PDT: SSH2 1: input: padlength 6 bytes
PDT: SSH2 1: SSH2_MSG_KEX_DH_GEX_REQUEST received
PDT: SSH2 1:  Invalid modulus length

The above are received from cisco ssh debugs
ssh version is 2

To help troubleshoot:
putty was working (ssh v2)
Changing to ssh v1 it works with SecureCRT
Key size: 1024, 2048 or 512 didnt change behavior

Thanks

[rtb]

Hi jjx,

The easiest way to resolve this issue, is to move the 'diffie-hellman' option to the top of the list in 'Options/Session Options.../Connection/SSH2/Key exchange'.

Does this resolve the issue experienced when connecting to the Cisco device?

[tnygren]

Hi JJX,

Our developers also now have a resolution for this issue in a pre-release version of SecureCRT 6.1 that will not require any changes to the session.

If you would like to test this version, please send me and reference this forum thread (3216).

[yqyj]

I got the same issue and error message. Removing diffie-hellman doesn't fix the issue.

[rtb]

Hi yqyj,

I am sorry to hear of the difficulty you are experiencing when attempting to connect with a Cisco device.

What Cisco IOS are you using?

What version of SecureCRT are you using?

[gkershaw]

We were able to overcome the "Invalid Modules" error by inserting

ip tacacs source-interface Loopback0

on our router...

Hope that helps!

[bgagnon]

Hi gkershaw,

Thank you for the suggestion.

yqyj:

You also might want to read this forum post if you are still experiencing issues when connecting to a Cisco device.