https://pi-hole.net/

pihole on Synology NAS Docker - initial install
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See also: https://kevq.uk/how-to-setup-pi-hole-on-a-synology-nas/
Quick Notes:
Package Center - docker - install
Docker app - Registry - Search - pihole - doubl-click on pihole (official Pi-hole Docker image from pi-hole.net) - Choose: Latest (default)
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Synology File Station - docker - create "pihole" - in new "pihole" create another "pihole" and "dnsmasq.d"
Filestructer:
docker/pihole/pihole
docker/pihole/dnsmasq.d
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Docker app - Image - double-click the pihole image (container config wizard should start)
Container Name: pihole or synologypihole or such
Advanced Settings - check autho-restart
Volume tab - add two folders:
Folder				Mount Path
docker/pihole/dnsmasq.d		/etc/dnsmasq.d
docker/pihole/pihole		/etc/pihole
Network tab - check "Use the same network as Docker Host"
Environment tab - add following variables:
Variable		Value
WEBPASSWORD		(web login pihole password)
DNSMASQ_LISTENING	local
WEB_PORT		8080
Environment tab - edit ServerIP to be the IP of Synology IP address (defaults to 0.0.0.0 but need to set to actual IP (Example: 192.168.1.5))
Apply
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Navigate to web interface to login with above set password and use:
http://192.168.1.5:8080/admin
Set preferred DNS provider from web interface:
Settings - DNS - Example: Cloudflare - Save
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Now, change DNS settings on your router to point to above IP address.  Set any manually configured DNS entries on other devices also to point to pihole IP above.  It is best to have two piholes setup for redundancy.

To upgrade Synology NAS Docker pihole:
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Docker app - Registry - Search - pihole - doubl-click on pihole (official Pi-hole Docker image from pi-hole.net) - Choose: Latest (default)
Docker app - Container - click on your pihole container - Action - Stop - Action - Delete
Docker app - Image - double-click the pihole image (container config wizard should start)
Container Name: pihole or synologypihole or such
Advanced Settings - check autho-restart
Volume tab - add two folders:
Folder				Mount Path
docker/pihole/dnsmasq.d		/etc/dnsmasq.d
docker/pihole/pihole		/etc/pihole
Network tab - check "Use the same network as Docker Host"
Environment tab - add following variables:
Variable		Value
WEBPASSWORD		(web login pihole password)
DNSMASQ_LISTENING	local
WEB_PORT		8080
Environment tab - edit ServerIP to be the IP of Synology IP address (defaults to 0.0.0.0 but need to set to actual IP (Example: 192.168.1.5))
Apply
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Navigate to web interface to login with above set password and use:
http://192.168.1.5:8080/admin