Interop problem with old dropbear and new openssh

Damien Miller djm at
Sat Oct 1 11:53:18 EST 2011

please include a full debug trace from the client and, if possible, from
the server.

On Fri, 30 Sep 2011, Graham Cobb wrote:

> Hi,
> I have a router running an old version of OpenWRT with an old version of 
> dropbear (Dropbear sshd v0.44test3).  It has been working for many years and I 
> ssh in from my desktop systems (running Debian Testing) with no problem.
> 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.
> Has anyone seen this, or does anyone have any suggestions for possible 
> workrounds?
> Graham
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