Ask HN: Best way to monitor external sites uptime?

I am interested in verifying uptime for various cors services like google, icloud, microsoft, bank of America,DNS root servers, whitehouse.gov, ect. Both due to the uptick in ddos attacks and general reluability.

what design considerations should I be cognizant of?

- is a simple ping enough?

- should I implement a crawler and test http status codes?

- is it necessary to go further and use something like phantomjs?

Also, some sites and services do this and I would love to lean on external (free) sources and parse rss/atom feeds but this will be very granular

Any data sources or design tips would be helpful