DNS

dig

dig txt example.com @8.8.8.8
dig example.com | grep -v ";" | grep A
dig -x 8.9.10.11 | grep IN

Flush DNS cache

sudo systemd-resolve --flush-caches

SystemV

# flush DNS
systemctl restart nscd
 
# resolvconf
sudo systemctl enable resolvconf

SPF
http://wiki.hetzner.de/index.php/DNS_SPF

DIG
https://www.cyberciti.biz/faq/linux-unix-dig-command-examples-usage-syntax/
https://intodns.com/

dig @10.0.0.22 -p 8500 +short A www1.example.com

DNSSEC
https://www.scip.ch/?labs.20140911
# validator
https://dnsviz.net/
https://dnssec-analyzer.verisignlabs.com/

Public recursive name server
https://en.wikipedia.org/wiki/Public_recursive_name_server