Ash: /usr/libexec/sftp-server: not found when using scp

[hucste]

Hi.
My WRT router: OpenWrt 21.02.2, r16495-bf0c965af0
(a Xiaomi Redmi AC2100)

Before attempting the sysupgrade to 21.02.3, I backup my system, by:
sysupgrade -b /tmp/backup-${HOSTNAME}-$(date +%F).tar.gz

Since my station, on Devuan Ceres, I use scp command to retrieve this backup, like:

$ scp router:/tmp/backup-***-2022-04-21.tar.gz .
ash: /usr/libexec/sftp-server: not found
scp: Connection closed

Result: I cant get this file. Why?
(On the previous 21.02.1, I dont had this problem)

[lleachii]

@hucste

ash: /usr/libexec/sftp-server: not found

@hucste

Why?

Install the software on your computer.

[colo]

This is a consequence of your client machine using a very recent OpenSSH release (9.0 - check https://www.openssh.com/txt/release-9.0 for more info), which changes the scp program to use the SFTP protocol under the hood, which vanilla OpenWrt/dropbear installations do not support. To work around the problem on the client side, use the new -O (that is an uppercase letter "o") switch when invoking scp, which will cause it to fall back to the legacy behavior.

[hucste]

@colo: right!

$ scp -O router:/tmp/backup-***-2022-04-21.tar.gz $(pwd)
backup-***-2022-04-21.tar.gz         

Thanks!
(now, I go to continue the upgrade — I'm happy)