As many of you probably do, I am not using my ISP DNS because of very low performance, and I am using Google's primary and secondary DNS (8.8.8.8 and 8.8.4.4) which are good speed-wise but, knowing Google's ethics, probably not so much privacy-wise.
So my question is: if somebody uses Google's DNS, do extensions like AdBlock Plus, Ghostery etc. become immediately useless, since you are specifically sending Google your browsing information?
No. Of course not.
The privacy-specific purpose of adblock, ghostery, etc., is to prevent your identity on one site from being easily associated with your identity on another site, by the use of cookies, page widgets, referrer identification, etc. Whether or not this protects your actual real-world privacy in any meaningful way is seriously suspect, especially given the fact that people are more than willing to do things like log in to Spotify using their Facebook account, completely undermining the whole point of these plugins.
The privacy-specific threat of DNS is a bit more general an nonspecific: A person at IP address X visited both Facebook and Spotify early Friday evening. It doesn't really tell you a whole heap unless you actually have Facebook's and Spotify's logs to try to do some correlation. And of course if you have that, then the DNS information is redundant and completely unnecessary.
But more importantly, the real threat here is your ISP, not Google. Your ISP traditionally runs your DNS servers, and can view, intercept, and modify your DNS queries and responses no matter whom you send them to. But your ISP also is in a position to log all of your traffic patterns; what sites you visit and when, how much data you transfer from each, and if you're not encrypting everything, then the full contents of .. everything.
And in real-world cases that actually mattered, Google has consistently taken the site of protecting the privacy of its users, while many ISPs have consistently betrayed the trust of their users.
In other words, you're taking extraordinary action to protect yourself from Google even though Google is typically trustworthy, while you're not taking any action to protect yourself from your ISP, even though your ISP is typically not trustworthy.
Using Google DNS doesn't make privacy protection tools useless.
To begin with, Google is far from the only advertising network around (although it is by far the biggest). Google DNS won't help other companies than Google track you at all.
Secondly, there is only a limited amount of data that gets sent to the DNS server every time.
Once a DNS request gets sent to the DNS server, the DNS server has the following information:
Your IP address requested the IP address associated with the domain name at time on date.
For example:
203.0.113.77 requested the IP address associated with www.example.com at 00:00:00 UTC on 1 January 1970.
If you are behind NAT, the DNS server will not be able to distinguish you from other users on your network.
So how can Google use this information? Of course, they could use it in ways that do not affect your privacy, such as getting an impression which web sites are popular globally.
However, Google does keep a record of your visits of every web site that displays their advertisements / trackers / widgets and how you interact with these.
If you repeatedly visit a web site with their advertisements on it shortly after a DNS request for that domain is sent, they can then associate your visits to those domain names with that record. They may then be able to associate other DNS requests from your IP address with that record.
Furthermore, although to my knowledge Google doesn't do this at the moment, they can associate the record of your actions on web sites with their ads and trackers with any Google profile you may be logged in to at the moment, so they are able to associate those visits to your name (and maybe the home address you have set in Google Maps).
If they know you visit certain web sites, they may be able to refine the selection of the ads served to you based on the content of certain sites. But domain names are not complete URLs. They only know you visited travel.example.com, not that you visited http://travel.example.com/holidays/Somewhere. Therefore, the data they get from your DNS are much less precise than what they could have gotten from tracking code on the page itself.
It depends on what you expect an adblocker to do for you. I see two things here which they usually try to reach
Speedup browsing
This is the main reason for me to use adblockers. The amount of unneccessary pages you usually visit automatically is immense and some of them are quite slow, and so they slowdown the overall browsing experience a lot. This will not suffer from using Google's DNS servers.
Avoid user-tracking
I know many people who consider this the main achievement of adblockers. I do not agree completely with that, as there are always many methods to track a user. However, adblockers do a good job to get rid of the most anoying tracking sites. There is a loss of privacy if you use Google's DNS servers, but that's just my opinion. Others may see this differently. But I would not go so far to say it makes using adblockers useless.
I do not know, why your provider gives answers so slowly, but I often found, that the bottleneck is the router. So you may try to use your providers DNS servers directly instead of letting the router forward the requests.
No, they do not become useless if you're using Google's Public DNS. DNS just gives domain names you're connecting to with no context, and you're indistinguishable from anyone else on your network or who, at one time or another, had the same IP address as you. Even if Google logs 100% of DNS queries, they can't use it to identify anyone without incredible effort that just isn't worth it on their part (or anyone else's).
