proftpd

For testing proftpd sftp module

apt install proftpd-basic openssh-client openssh-server

Add a local user user

adduser user

Add the following snippet for sftp under /etc/proftpd/conf.d/sftp.conf

<IfModule mod_sftp.c>
    SFTPEngine on
    Port 2222
    SFTPLog /var/log/proftpd/sftp.log

    SFTPHostKey /etc/ssh/ssh_host_rsa_key

    SFTPAuthMethods publickey password

    SFTPAuthorizedUserKeys file:/etc/proftpd/authorized_keys/%u

    RequireValidShell off

    SFTPCompression delayed
</IfModule>

restart proftpd

systemctl restart proftpd

Create a RFC4716 key for user user (log as user and if needed create a key by ssh-keygen)

ssh-keygen -e

copy this key to /etc/proftpd/authorized_keys/user

test sftp by

sftp -vvvvv -P 2222 user@localhost