Default DNS Upstream

[PromoFaux]

What does this PR aim to accomplish?:

See https://discourse.pi-hole.net/t/how-to-modify-upstream-server-only-via-gui-on-v6/70943/13?u=promofaux

Lower the barrier to entry/reduce confusion by setting DNS upstreams to 8.8.8.8;8.8.4.4 if we detect that it has not already been set by either the environment variable or config file. Falling back to a default rather than hard-exiting is probably more user friendly, on reflection.

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[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/how-to-modify-upstream-server-only-via-gui-on-v6/70943/20

[yubiuser]

Why not only remove the exit 1 line and keep the upstream unset + print the warning?

[PromoFaux]

Because ultimately nobody will read the warning.

I originally put the exit 1 to make it extremely obvious that something needed configuring, but if by defaulting it does not need to be configured immediately.

If we don't default and also don't exit, the startup checks wont always work (e.g in the case of the container having itself as a resolver - either via compose/run or via the host's settings)

[Gontier-Julien]

Might i suggest on using maybe Cloudflare Dns (1.1.1.1 & 1.0.0.1) or Quad9 Dns (9.9.9.9 & 149.112.112.112)?

My reasoning behind this is that for Cloudflare is the fastest dns out here, while Quad9 offer better privacy but slower.
Both have a privacy policies meanwhile Google Dns as none as far as i'm aware.

Providing a faster and/or more private dns upstream would be better i think for some default.

Also Cloudflare dns i widely know so i don't think many people would mind + having as faster (even slightly) dns is always a better user experience.

[PromoFaux]

Google DNS just happens to be the one that's at the top of the list, so someone setting up a bare metal installation would arrive in exactly the same place if they just mashed enter throughout the installer.

Even in setting a default, this is by no means an opinionated default - it's just what happens to be the current default (and personally the one I fall back to out of habit). Users are still free to change to whichever one they want to use. The same could be said of the Steven Black blocklist that's included by default. We're not forcing it on anyone, it just prevents Pi-hole from not working out of the box - which can put newer/unfamiliar users off.