Allow multiple upstream DNS servers (incl. IPv6)

[DL6ER]

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How familiar are you with the codebase?: 10

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• Allow multiple upstream DNS servers
  

• Allow IPv6 addresses in the exclude filter

After reading the dnsmasq code a bit more I figured out that this will not drastically increase the used bandwidth (as I though initially). dnsmasq will send queries to any of the upstream servers that are enabled and tries to favor servers that are known to be up (favor = the fastest to respond). Only if this favorite server fails to answer, dnsmasq will reach out to the others. It will occasionally send a query to all of them to identify if the favored server is still the best choice.

Core changes: pi-hole/pi-hole#1135

This template was created based on the work of udemy-dl.

[AzureMarker]

Approved

[darkrain-nl]

Just to give an idea how much each server gets to do...
I run a config like this for some time already, 6 or 7 months, without any problems.
So good thing to add.

[DL6ER]

With all 10 enabled. Level3 is slightly preferred by dnsmasq it seems. Still, the time interval in between which dnsmasq check for the fastes is quote short and the "favored" server changed frequently in my setup.

[david-kracht]

An option to use the all-servers flag of dnsmasq would be nice. From my point of view it changes the way the servers is accessed. With the flag, there seems to be parallel requests, with the first reply being used for forwarding. This might be a option to control number of dns requests vs. minimum latency. Maybe worth a small feature request ?!

Just my 2 ct.

Edit: The chart seems to be devided in a local part and equi-divided parts for each server with the mentioned flag.