In October 2021, a misconfiguration at a DNS provider called Fastly took down a significant chunk of the internet — including Reddit, Twitch, the UK government's website, and dozens of major news outlets — for about an hour. The root cause was a single customer triggering a bug in Fastly's DNS software.
DNS failures are uniquely disruptive because they are upstream of everything. If your domain cannot resolve, it does not matter how reliable your servers are.
What DNS does
DNS (Domain Name System) translates human-readable domain names into IP addresses. When you type websitedown.com into your browser, your computer asks a DNS resolver: "What IP address does websitedown.com point to?" The resolver returns an IP address like 104.21.45.67, and your browser connects to that server.
This lookup happens every time you visit a new website (cached lookups are faster), and the entire chain — from your device to your ISP's resolver to authoritative nameservers — has to work correctly for the connection to succeed.
How DNS failures cause outages
There are several ways DNS causes downtime.
Expired or deleted records: If a domain's DNS records are not renewed or are accidentally deleted, the domain stops resolving. Visitors get "server not found" errors even though the server is perfectly healthy.
Nameserver outages: If the company hosting a domain's DNS records (the authoritative nameserver) goes down, DNS lookups for all their customers fail simultaneously. This is how the Fastly incident took down so many unrelated sites at once.
DNS propagation delays: When records are changed, the update spreads gradually across the internet's DNS infrastructure. During this propagation window (which can last up to 48 hours for high-TTL records), some users resolve to the old destination while others get the new one.
Why some users are affected and others are not
DNS lookups are cached at multiple layers — your device, your browser, your router, and your ISP's resolver all cache recent lookups. The cache duration is set by the TTL (Time to Live) value in the DNS record.
During a DNS outage, users who have a recent cached copy of the DNS record can still reach the site. Users whose cache has expired, or who have never visited the site, cannot. This creates the puzzling situation where a site is "down for some but not others" for the duration of the TTL.
How to check if a site is down due to DNS
If your browser says "server not found" or "DNS_PROBE_FINISHED_NXDOMAIN," DNS is likely the issue. Try running a direct DNS lookup with a tool like nslookup (built into Windows and Mac) to see if the domain resolves.
WebsiteDown's checks include DNS resolution as part of the probe. If our check shows a site as unreachable with a DNS failure signature, you will see that in the error details.