Deprecation of scp protocol and improving sftp client

[Colin Watson]

On Tue, Jun 16, 2020 at 06:41:09PM +0200, Jakub Jelen wrote:
>  * Change scp to use sftp internally

As an existence proof, pscp (from PuTTY) does exactly this; it tries the
sftp protocol and falls back to the scp protocol if that fails, and it
has -scp or -sftp options to force one or the other.  I've long meant to
get round to putting something similar for OpenSSH, but never got far
enough to really be worth mentioning.  (Of course it would still need to
retain scp "source" and "sink" modes if invoked with -f or -t, to retain
compatibility, since those are used on the server when an scp client
connects; but there's no particular obstacle to that.)

>  * Modify sshd to use some compatibility "scpd" to support old clients

This should be unnecessary.  When an scp client connects to an scp
server, it passes the -f (source) or -t (sink) flag as appropriate;
neither is part of the documented user-facing interface to scp.  I don't
see any particular reason why scp (the program) couldn't continue to
speak the scp protocol when invoked with -f/-t, but speak the sftp
protocol when invoked in the normal way.

-- 
Colin Watson (he/him)                              [cjwatson at debian.org]