Ftp server

vsftpd

sudo apt-get install vsftpd

Configuration file: /etc/vsftpd.conf to add users: /etc/ftpusers

After making changes to configuration:

sudo service vsftpd restart

To change root directory in /etc/vsftpd.conf: modify:

local_root=/var/www/sites/$USER 

To avoid server automatic startup at boot run:

echo manual >> /etc/init/vsftpd.override

/etc/vsftpd.conf example:

listen=YES
anonymous_enable=NO
local_enable=YES
virtual_use_local_privs=YES
write_enable=YES
connect_from_port_20=YES
secure_chroot_dir=/var/run/vsftpd
pam_service_name=vsftpd
guest_enable=YES
user_sub_token=$USER
local_root=/var/www/sites/$USER
chroot_local_user=YES
hide_ids=YES

Using PAM to create virtual users

(using the configuration file example from above)

sudo apt-get install libpam-pwdfile

Create a password for each user (if you have apache installed)

htpasswd -c /etc/vsftpd/passwd bob
htpasswd /etc/vsftpd/passwd alice
....

Configure PAM to use the password file:

# Customized login using htpasswd file
auth    required pam_pwdfile.so pwdfile /etc/vsftpd/passwd
account required pam_permit.so

Extra options

If you need to have multiple logins for the same folder, such as for a client who wants each developer to have their own login, then add the following line to the /etc/vsftpd.conf file:

user_config_dir=/var/www/users 

It's easiest if each virtual user for a client has a login that starts with that clients name, so alice working for client1 would be
client1-alice. Add their username and password to the password file, and then create a file in the user
config folder /var/www/users called client1-alice with the single line

local_root=/var/www/sites/client1

If you are offering multiple logins to lots of clients then it's easiest to create a file for each client eg /var/www/users/client1 would contain the single line

local_root=/var/www/sites/client1

and then each for user for that client would just create a symbolic link instead

cd /var/www/users
ln -s client1 client1-alice

as you are less likely to make mistakes if each user points to the same file.