Scan the DNS top-level domains and see which support IPv6.
In order for an IPv6-only resolver to work, the authority servers that it queries must also support IPv6. Since DNS is hierarchical, all of the zones above a given domain must also support IPv6. The root DNS servers have had IPv6 support for many years now, but there are still some TLD domains which do not have any IPv6 servers. This program finds them.
Most importantly you need a Unix machine with working IPv6.
You also need Python 3 and dnspython. You can install dnspython via:
$ pip install -r requirements.txt
If you do not want to install dnspython system-wide you can also do it only for your current user:
$ pip install --user -r requirements.txt
You can also use a virtual environment (my preferred method):
$ python3 -m venv venv
$ . venv/bin/activate
(venv) $ pip install -r requirements.txt
It takes quite a while to run, since every lookup is done
sequentially. The program outputs progress to the console via
stderr, so you can know that it is not just stuck.
Typical execution looks like this:
$ python3 tldIPv6.py > results-2017-11-27
No answer or RRset not for qname; Unable to XFR from 2001:500:2d::d
No answer or RRset not for qname; Unable to XFR from 198.41.0.4
No answer or RRset not for qname; Unable to XFR from 199.7.83.42
No answer or RRset not for qname; Unable to XFR from 198.97.190.53
[ 48/1541 3%] alsace
As you see here, there are a few errors as it tries to AXFR the root zone from a root server or two that does not support AXFR, but eventually it will work.
You will get a result file that looks something like this:
ai no IPv6 nameservers
bb no IPv6 nameservers
bf no IPv6 nameservers
bh no IPv6 nameservers
ck no IPv6 nameservers
dj no IPv6 nameservers
fk no IPv6 nameservers
.
.
.
The program downloads the current root zone via an AXFR from one of the root servers that allows zone transfers. It does not keep a list of which servers support this, but just tries each root server IP address until the transfer works. The order attempted is whatever the local resolver returns for the NS for root.
Next it goes through each TLD and does an NS lookup to get the authoritative list of name servers. It then tries each name server returned until it finds one with an AAAA record. If none of the name servers for a TLD have an AAAA record, then it is reported as not supporting IPv6.