Re: Additional AD Comment: draft-ietf-secsh-publickey-subsystem and garbage

[Nicolas Williams <Nicolas.Williams%sun.com@localhost>]

On Thu, Aug 31, 2006 at 08:52:03AM -0400, Sam Hartman wrote:
> I'd like to draw your attention to a particularly annoying  part of RFC 4254:
> 
>       This last form executes a predefined subsystem.  It is expected that
>       these will include a general file transfer mechanism, and possibly
>       other features.  Implementations may also allow configuring more such
>       mechanisms.  As the user's shell is usually used to execute the
>       subsystem, it is advisable for the subsystem protocol to have a
>       "magic cookie" at the beginning of the protocol transaction to
>       distinguish it from arbitrary output generated by shell
>       initialization scripts, etc.  This spurious output from the shell may
>       be filtered out either at the server or at the client.
> 
> 
> In order to guarantee interoperability, your subsystem needs to be
> able to filter out leading garbage and clients MUST do so.

The text you quote says "advisable" and "may."

It is typically a misconfiguration to have user shells output anything
when there is no tty, and sysadmins bloody well know this -- users
don't though, which is why it's advisable to have this protocol feature,
but it certainly isn't REQUIRED by the text you quote.

Nico
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