[maltalex]

I don't think there's a reason to use TCP for this unless you want to check for TCP connectivity specifically.

Just do a DNS lookup against a list of well-known public DNS servers.

[oconnor663]

One reason not to do DNS exclusively is that many networks allow DNS requests (and intercept them) but block HTTP requests.

[jesusprubio]

BTW, there're parts of DNS over TCP.