[EDNS](https://en.m.wikipedia.org/wiki/Extension_Mechanisms_for_DNS) [Client-Subnet](https://en.wikipedia.org/wiki/EDNS_Client_Subnet) is a DNS extension letting the authoritative nameserver
_See also simpler explanation at [PrivacyGuides.org DNS Overview](https://www.privacyguides.org/en/advanced/dns-overview/#what-is-edns-client-subnet-ecs)._
- Your router does get the IPv6 subnet assignment for LAN distribution by means of [DHCPv6 Prefix Delegation](https://en.m.wikipedia.org/wiki/Prefix_delegation) which is also common on mobile networks.
If you utilize services of internet giants or content delivery networks, ECS will likely give you [the shortest distance, the lowest latency, the highest speed](https://en.m.wikipedia.org/wiki/Edge_computing) and may help with decreasing your _digital carbon footprint_.
If those matter to you, you may also like to consider [increasing your minimum TTL to around an hour in a local server](https://blog.apnic.net/2019/11/12/stop-using-ridiculously-low-dns-ttls/).
_Criticizers will ask whether changing your DNS server will save the world? No, fighting climate change takes much more, while some of it is small effortless tasks which effect cumulates. Anyway, keep reading._
> [...] we [Cloudflare] don’t pass along the EDNS subnet information. This information leaks information about a requester’s IP and, in turn, sacrifices the privacy of users. This is especially problematic as we work to encrypt more DNS traffic since the request from Resolver to Authoritative DNS is typically unencrypted. **_We’re aware of real world examples where nationstate actors have monitored EDNS subnet information to track individuals,_** which was part of the motivation for the privacy and security policies of 1.1.1.1.
> We are working with the small number of networks with a higher network/ISP density than Cloudflare (e.g., Netflix, Facebook, Google/YouTube) to come up with an EDNS IP Subnet alternative that gets them the information they need for geolocation targeting without risking user privacy and security. Those conversations have been productive and are ongoing. [...]
ECS will decrease the cost of mass surveillance as instead of having to surveill everything happening on the network, anyone between your DNS server and the authoritative nameserver can see which IP addresses access the site with a reasonable accuracy.
Additionally researchers (below) have used it to perform cache poisoning against an individual target directing them to a wrong location and with low TTL making it near impossible to audit later.
What domains do you use? What if someone far above you knew regardless of Encrypted Client-Hello?
Are the domains you use DNSSEC-signed? Do you verify DNSSEC locally? Do you use HTTPS everywhere? Do you know to not accept warnings about certificate issues? Do the other (less technical) users of your network? Would you or them be a delicious target? Do you even use GAFAM services?
Do you want the benefits of ECS with the privacy and security of not having ECS? Private ECS is a compromise solution in the middle, although not without its own issues.
Your private DNS provider will lie for you and say that your IP address is somewhere else where it will also place many others from your ISP. However what if it says you are a customer of another ISP, possibly even located in another country? It tends to have greater accuracy with IPv4 than IPv6, [see AdGuard Google Domains issue](https://adguard-dns.io/en/blog/dns-google-domains-fixed.html).
In that case you may get even worse performance than without ECS. Then again if everything works properly, you will get the benefit of ECS without the privacy impact and lessened security impact.
See the next section for testing "where you are." Consider also what is important for you if you had to pick one or two.
See also:
- [NextDNS (Medium.com): How we made DNS both fast and private with ECS](https://medium.com/nextdns/how-we-made-dns-both-fast-and-private-with-ecs-4970d70401e5)
- Note: [Cloudflare sends ECS only for `whoami.ds.akahelp.net`, nowhere else](https://developers.cloudflare.com/1.1.1.1/faq/#does-1.1.1.1-send-edns-client-subnet-header).
outside it either for work or leisure, which across continents tends to bring
round-trips overseas. Additionally private ECS (see above) tends to be bad
poor for IPv6 and for very small AS like a school, it directs to another side
of the country, but that is a very minor issue.
Meanwhile [Quad9] blocking seems almost as good in [tests like this](https://techblog.nexxwave.eu/public-dns-malware-filters-tested-in-2024/) and they give me impression [of more transparency](https://quad9.net/about/transparency-report) (as opposed to [DNS0.eu only
having a <del>Twitter</del> X account](https://twitter.com/dns0eu)). [Quad9] also has more options on whether to ECS or not (see above).
The end-users traveling outside of the EU is also solved as they [have servers all around the world](https://quad9.net/service/locations/).
Back to [DNS0.eu], while disabling private ECS is not an option, they do have other options; [default filters](https://www.dns0.eu), [no filters](https://www.dns0.eu/open), [heavier filtering (zero)](https://www.dns0.eu/zero) and [kids](https://www.dns0.eu/kids).
As the size and confusion this page induces to anyone else than me shows, I have spent too much time thinking about DNS and related matters.
- Android: while the system only gives the option between `cloudflare-dns.com` and `dns.google` (HTTP/3, see below),
web browsers are free to choose the DoH server. If the device is not expected to travel far outside the EU, DNS0.eu may be a safe choice, otherwise Quad9.
- As I have to support devices going outside of the EU, I lean towards Quad9.
- iOS (or Apple in general): same question, do the devices travel outside of the EU? Both provide configuration profiles.
- While not noticing the DNS0.eu configuration profile is difficult, [Quad9 currently hides it a bit under docs.quad9.net iOS instructions](https://docs.quad9.net/Setup_Guides/iOS/iOS_14_and_later_%28Encrypted%29/).
- Personal computers: I have reached the cursed conclusion of [using Unbound upstreams DNS0 for IPv4, Quad9 ECS for IPv6](https://gitea.blesmrt.net/mikaela/shell-things/src/branch/master/etc/unbound/unbound.conf.d/dot-dns0-quad9.conf) and [using the hosts file to point web browsers away from DNS0.eu IPv6](https://gitea.blesmrt.net/mikaela/shell-things/src/branch/master/etc/hosts/dns) using [IPv4 mapped IPv6 addresses](https://en.m.wikipedia.org/wiki/IPv6#IPv4-mapped_IPv6_addresses).
- Yes, ECS has privacy concerns, however _theoretically_ it's only a fallback if IPv4 goes down (very rare, has happened for short periods of time in my experience), but the environment also weights my decision. See above on whether to ECS or not.
- I hope to offset the risks of ECS by [not allowing TTLs below an hour](https://gitea.blesmrt.net/mikaela/shell-things/src/branch/master/etc/unbound/unbound.conf.d/min-ttl-hour.conf) so whether I have a tab open or not cannot be figured out from DNS traffic alone and somewhat relatedly [serve stale records if I must](https://gitea.blesmrt.net/mikaela/shell-things/src/branch/master/etc/unbound/unbound.conf.d/expired-stale-serving-rfc8767.conf).
- Personal servers: Personal preference, you could even use all of the DNS servers or be your own recursor. I again have small preference towards Quad9 ECS as titlefetching for unencrypted IRC connection is already open for mass surveillance and there is no telling who triggered a DNS query there anyway.
- Business/association/enterprise/whatever device: there may be free political/regulational/bureaucratic/whatever brownie points for using DNS0.eu with the queries not being transmitted outside of the EU.
_Private DNS_ server as [they have special handling](https://cs.android.com/android/platform/superproject/main/+/main:packages/modules/DnsResolver/PrivateDnsConfiguration.h) and are thus DNS over
