This is a Canonical Question about DNS geo-redundancy.
It's extremely common knowledge that geo-redundant DNS servers located at separate physical locations are highly desirable when providing resilient web services. This is covered in-depth by document BCP 16, but some of the most frequently mentioned reasons include:
Protection against datacenter disasters. Earthquakes happen. Fires happen in racks and take out nearby servers and network equipment. Multiple DNS servers won't do you much good if physical problems at the datacenter knock out both DNS servers at once, even if they're not in the same row.
Protection against upstream peer problems. Multiple DNS servers won't prevent problems if a shared upstream network peer takes a dirt nap. Whether the upstream problem completely takes you offline, or simply isolates all of your DNS servers from a fraction of your userbase, the end result is that people can't access your domain even if the services themselves are located in a completely different datacenter.
That's all well and good, but are redundant DNS servers really necessary if I'm running all of my services off of the same IP address? I can't see how having a second DNS server would provide me any benefit if no one can get to anything provided by my domain anyway.
I understand that this is considered a best practice, but this really seems pointless!
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