is SFTPv4 support possible?

[Damien Miller]

On Mon, 27 Sep 2021, Mike Frysinger wrote:

> would SFTPv4 support be acceptable in OpenSSH's server & client ?
> 
> SFTPv4 supports higher stat fidelity & user/group names (not just numeric):
> https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-04#section-10.1
> 
> iiuc, some of the newer features are a bit contentious, like the NFS ACL models
> (which only get worse it seems with SFTPv5 & SFTPv6).  from a quick scan, it
> seems these only show up in the file attrs operations, and the server could
> just not return any (never set SSH_FILEXFER_ATTR_ACL).  if the client were to
> set file attrs and they have SSH_FILEXFER_ATTR_ACL set, then the server could
> also ignore it, or have it return SSH_FX_OP_UNSUPPORTED.
> 
> the other feature that looks a bit dicey is opening files in text mode.  the
> caveats the spec sprinkles around makes it sound like it too would be easier
> to simply return SSH_FX_OP_UNSUPPORTED.
> 
> requiring filenames be UTF-8 sounds fine ... the code already treats paths as
> a series of bytes (effectively ignoring the encoding), so can just keep doing
> that.  if the client sends invalid UTF-8, that's a client bug.  if the server
> has a path that is invalid UTF-8, then the client can decide to ignore it, or
> process it.
> 
> everything else looks pretty straight forward.

How many of these things could be implemented as extensions?

AFAIK we could use SSH_FILEXFER_ATTR_EXTENDED extensions to send
nanosecond timestamps as well as user/group names, and it should be
backwards compatible...

-d