[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 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).
_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
