A utility to find subdomains for a domain.

Also see https://crt.sh.

Covers a lot of content about DNS lookup, including these:

nsswitch
/etc/hosts
/etc/resolv.conf
ping vs host style lookups
systemd and its networking service
ifup and ifdown
dhclient
resolvconf
NetworkManager
dnsmasq

<blockquote>ARIN is a nonprofit, member-based organization that administers IP addresses & ASNs in support of the operation and growth of the Internet.</blockquote>

A free DDNS provider supporting custom domain name or a secondary level domain name.