[dnsdist] Add EDNS Client Subnet

[Daniel Stirnimann]

Dear all,

Is it possible to add/overwrite an EDNS option for EDNS Client Subnet
(ECS) [1] with dnsdist?

I see that dnsdist supports something similar using MAC addresses:
https://blog.powerdns.com/2016/01/27/per-device-dns-settings-selective-parental-control/

Our resolver is currently running BIND and we would like to implement
view statements and provide different responses based on the source IP
address of the client.

BIND 9.11 supports ECS so this seems like a clever way to do it. Of
course, that means that the ECS netmask should be /32 not /24 for IPv4.

[1] https://tools.ietf.org/html/draft-ietf-dnsop-edns-client-subnet-07

Daniel