Multiple Authentication Databases

Dovecot supports defining multiple authentication databases, so that if the password doesn't match in the first database, it checks the next one. This can be useful if you want to easily support having both local system users in /etc/passwd and virtual users.

Currently the fallback works only with the PLAIN authentication mechanism.

Often you also want a different mail location for system and virtual users. The best way to do this would be to always have mails stored below the home directory.

• System users' mails: /home/user/Maildir
• Virtual users' mails: /var/vmail/domain/user/Maildir

This can be done by simply having both system and virtual userdbs return home directory properly (i.e. virtual users' home=/var/vmail/%d/%n) and then set mail_location = maildir:~/Maildir.

If it's not possible to have a home directory for virtual users (avoid that if possible), you can do this by pointing multiple authentication databases to system users' mail location and have the virtual userdb override it by returning mail passdb extra fields.

Example with Home Dirs

• System users' mails: /home/user/Maildir
• Virtual users' mails: /var/vmail/domain/user/Maildir

dovecot.conf

# Mail location for both system and virtual users:
mail_location = maildir:~/Maildir

# try to authenticate using SQL database first
passdb {
  driver = sql
  args = /etc/dovecot/dovecot-sql.conf.ext
}

# fallback to PAM
passdb {
  driver = pam
}

# look up users from SQL first (even if authentication was done using PAM!)
userdb {
  driver = sql
  args = /etc/dovecot/dovecot-sql.conf.ext
}

# if not found, fallback to /etc/passwd
userdb {
  driver = passwd
}

dovecot-sql.conf.ext

password_query = SELECT userid as user, password FROM users \
    WHERE userid = '%u'
user_query = SELECT uid, gid, '/var/vmail/%d/%n' as home \
    FROM users WHERE userid = '%u'