Colons are not allowed in hostnames, but they are allowed in domain
names. Try "dig FFFF::1.midwestcs.com txt +short".
Yes, it works. But still inet_ntop() or similar should not be used for
creating a domain name because its result can have several different forms
(e.g. "0:0:0:0:0:0:FFFF:102:304", "0:0:0:0:0:0:FFFF:1.2.3.4",
"::FFFF:102:304", or "::FFFF:1.2.3.4").
Also, is foo=%{i} a domain name, or a string?
If "foo=%{i}" is the whole directive, then it is an unknown modifier.
But if you meant for e.g. "exists:foo=%{i}", then it is a domain name.
What would "exists:%{c}.foo.example.com" do when you are using IPv4?
The same as "exists:%{i}.foo.example.com".
Roger