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

[Sam Hartman <hartmans-ietf%mit.edu@localhost>]

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 spec doesn't currently do this.