How to setup virtual users for sftp (sshd) with access to a specific sub directory?

Question

I try to setup virtual users for sftp with sshd and/or pam.

My folder structure looks like this:

/
|-- var
|   +-- www
|       +-- site1
|           +-- folder_1
|           +-- folder_2
|       +-- site2
|           +-- folder_1
|           +-- folder_2

Each site dose have a Linux user (e.g. site1, site2) with the same group (site1, site2) where the home directory is e.g. /var/www/site1. The umask is 0022 and all files and (sub)folders for e.g. site1 have UID=site1 anf GID=site1 and this should not be changed.

I was able to setup sshd so that each Linux user gets his home directory as sftp root folder is e.g. /var/www/site1 for user site1. The config in /etc/ssh/sshd_config looks something like this
(only last few lines, the rest is default configuration):

UsePAM yes
Subsystem sftp internal-sftp
Match User *,!root
    ChrootDirectory %h
    ForceCommand internal-sftp -u 0022
    AllowTcpForwarding no

Now I want to add a Virtual User (no new Linux user or group if not needed) where the sftp root folder is e.g. /var/www/site1/folder1 for a sftp user e.g. site1_folder1. This user should be a child of the user site1 and use the same UID and GID but only have access to his subdirectory.

How do I have to change my configuration, and where can I add Virtual User?

I need this on Ubuntu 16.04 LTS, LAMP Server

Answer 1

ProFTPd allows you to create virtual FTP/SFTP users and map them to a real user's uid and gid.

Answer 2

No such thing as a virtual user, anyone connecting will need a user account, and you can chroot jail them in their respective home directory.