Looks like Heroku uses NS1 as the upstream DNS provider. you can find the information like this dig NS @1.1.1.1 test.herokuapp.com -> fail dig @1.1.1.1 te

Looks like Heroku uses NS1 as the upstream DNS provider. you can find the information like this dig NS @1.1.1.1 test.herokuapp.com -> fail dig @1.1.1.1 test.herokuapp.com -> fail dig NS @dns1.p03.nsone.net test.herokuapp.com -> works dig @dns1.p03.nsone.net test.herokuapp.com -> works So my conclusion is that NS1 is having issue responding DNS queries from other DNS servers. Interestingly, there is no public information on heroku being dependent on NS1 or any current outages from NS1 status page.