Interop problem with old dropbear and new openssh

[Graham Cobb]

On Friday 30 September 2011 22:45:03 Graham Cobb wrote:
> However, recently I upgraded one of my desktops and I can no longer connect
> to the router.  Dropbear on the router is exiting with:
> 
> exit before auth: bad buf_getwriteptr
> 
> Looking at the output from ssh -vv, the problem occurs right after
> 
> debug1: sending SSH2_MSG_KEXDH_INIT
> debug1: expecting SSH2_MSG_KEXDH_REPLY
> 
> The desktop is running "SSH-2.0-OpenSSH_5.9p1 Debian-1" (it failed with 5.8
> as well).  However, another desktop still running "SSH-2.0-OpenSSH_5.5p1
> Debian-6" still works fine.  The config files are identical and I can't
> find anything in the changelog between those versions which looks like it
> would affect the content of that message.

Just to close off this thread, in case anyone finds it in the future.  The 
problem is definitely a dropbear bug.  It is broken in dropbear 0.44 but works 
with dropbear 0.53.1.  I found the OpenSSH code changes which trigger the bug 
and there are no option settings which can workround the dropbear problem.

Thanks to Damien for his help.

Graham