PI-Hole (https://pi-hole.net/) is quite a useful AD blocker acting as a DNS blackhole. This makes it ideal 
as a parental filter as well. To be able to use it like this, the integration of a parental blacklist is necessary.
In contrary to the AD blocklists used by PI-Hole, parental blacklists usually have different categories to choose
from. This makes some changes to the default PI-Hole config necessary.
Since I am in Germany, Shalla's blacklist is a good choice for a parental filter list. However, the scripts contained in
this git should be modifiable to other blacklists as well. YMMV.

This HowTo assumes PI-Hole to be installed with lighttpd under /var/www/html and pihole config files under /etc/pihole.
All commands to be executed as root.

* Set up a directory /var/www/shalla owned by www-data.www-data  (this is where the category and listfiles reside)
* Put getshallagroups.sh in /root/bin, set up a cronjob to let it run daily (e.g. 3AM, this fetches the available categories)
* Put getshalla.sh in /root/bin, set up a cronjob to let it run every fife minutes (checks for changes and rebuilds filter)
* Put external.conf to /etc/lighttpd
* Create a snakeoil certificate: openssl req -x509 -newkey rsa:4096 -keyout key.pem -out /etc/lighttpd/cert.pem -days 365 -nodes
* mkdir /etc/lighttpd/.htpasswd/
* Copy hash.sh to /etc/lighttpd [1]
* Run /etc/lighttpd/hash.sh admin Blocklists mypassword >>/etc/lighttpd/.htpasswd/lighttpd-htdigest.user
* mkdir /var/www/html/lists, /var/www/html/errors
* Put index.cgi to /var/www/html/lists, chmod to 755
* Put index.html to /var/www/html/errors
* Look for a nice 403 or whatever image and put it to /var/www/html/errors, changing index.html accordingly
* Restart lighttpd
* Go to https://hostname:8082/admin/, log in and go to Settings -> PI Hole's black lists
  Add a new list https://localhost/shalla.txt, save and update
* Test

So what's this all about ?
* The admin interface is now only accessible via https://hostname:8082/admin/
* The standard behaviour for blocked sites has changed: Group/reason is not displayed. Instead a nice pic shows up
* Blocked https sites are now displaying a blocking page as well
* Blocked categories/sites and whitelisted sites are now managed by https://hostname:8082/lists/index.cgi
* Access to the list page is password protected -unfortunately not yet coupled to PI-Hole's auth though
* Available categories are fetched once a day
* Blocklists, black- and whitelists are checked every five minutes for changes. In case of any change, the filter lists 
  are rebuilt and put into pihole
* White- and blacklists should NOT be maintained at the normal PI-Hole interface

Security Note:
Since the default config of PI-Hole is inherently unsafe, my modifications are as well. DANGER WILL ROBINSON !
Make sure to put reasonable firewall-rules in place. Some local IPtables-rules are a nice touch too.

Any errors ? Something missing ? Suggested improvements ? Drop me a note !

References: 
[1] https://jacobsalmela.com/2014/05/25/password-protect-a-lighttpd-web-server-on-a-raspberry-pi-using-mod-auth/