tumblr_n04rer7wl91trr08so1_500Internet users around the world, but mostly in the United States, reported that top websites are not loading on Friday morning.

The affected sites include Amazon, Twitter, Etsy, Github, Spotify, and others.

The situation is developing, but it appears to have something to do with DNS hosts.

Domain Name Servers (DNS) are a core part of the internet’s backbone. They translate what you type into your browser–www.businessinsider.com, for example -into IP addresses that computers can understand.

And as it happens, DYN, a major DNS host, is suffering a DDoS attack. DDoS attacks basically mean that hackers are overwhelming their servers with useless data and repeated load requests, which means that useful data–the Twitter IP address–can’t get through.

GitHub also says that it seems to be having problems with its DNS host.

DYN posted on Friday morning:

Starting at 11:10 UTC on October 21th-Friday 2016 we began monitoring and mitigating a DDoS attack against our Dyn Managed DNS infrastructure. Some customers may experience increased DNS query latency and delayed zone propagation during this time. Updates will be posted as information becomes available.

It later updated its status with:

This attack is mainly impacting US East and is impacting Managed DNS customer in this region. Our Engineers are continuing to work on mitigating this issue.

Dyn updated to say services are back to normal as of 9:37 a.m. ET:

Services have been restored to normal as of 13:20 UTC.

CNBC reports that Amazon is investigating the internet outage too. “Amazon & DynDNS investigating internet outage reports on east coast of U.S. amid reports of major websites not working properly,” it somehow tweeted.

Earlier this month, the United States transferred its oversight of DNS to an international non-profit group, a move that had been more than 20 years in the making.

Here’s a map of outages as of 9:20 a.m. ET, via Level3:

CREDIT: Business Insider

According to Hacker News and reports, these sites have been affected:

  • - DYN
  • - Twitter
  • - Etsy
  • - Github
  • - Soundcloud
  • - Spotify
  • - Heroku
  • - Pagerduty
  • - Shopify
  • - Okta
  • - Zendesk
  • - Business Insider

This post originally appeared on Business Insider.