Admins Handbook

Due to an error in proftpd, you might need an virtual group.

/etc/proftpd/proftpd.conf:

GroupOwner www-data
Umask 002 003

AllowOverwrite on
AuthGroupFile /etc/proftpd/ftpd.group

/etc/proftpd/ftpd.group
www-data:x:33:testuser
Same ID and name as the local user.