Subject: Re: [PATCH] Follow RFC4253 section 11.4

Re: [PATCH] Follow RFC4253 section 11.4

From: Peter Stuge <>
Date: Thu, 1 Mar 2012 22:39:00 +0100

Henrik Nordström wrote:
> The SSH_MSG_UNIMPLEMENTED is in response to the packet with seqno 4
> (fifth packet), which is the keep-alive sent before authentication.
> Note that SSH_MSG_UNIMPLEMENTED should be sent in response to
> unimplemented messages even if want_reply == 0. want_reply is a
> message specific flag, not part of transport.

I don't know. I think SSH_MSG_UNIMPLEMENTED would only be acceptable
if the server does not implement SSH_MSG_GLOBAL_REQUEST at all.
(It might not, it's only used for setup and cancelling of

An easy way out for libssh2 could be to set want reply = 1 for
keepalive and require either SSH_MSG_UNIMPLEMENTED,
if the global request is sent after something else has been sent, but
before response has been received, there is still a problem. This
might already be managed within libssh2 though, that keepalives are
never sent in the middle of another request.

I don't think an SSH client can allow itself to send a global request
with want reply = 0 if it is not already sure that the server
supports that particular request, unless the client adds some
elaborate buffering of global requests it has sent and tries to track
which ones have been answered by the server. Not what we want.

I think want reply = 1 for keepalive would be fine, and isolates the
problem to that part of the code.

Received on 2012-03-01