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Those short TTLs that FB likely has on its DNS records probably bit it in the ass today. If it had longer TTLs, caching would have helped it. Curious though that recursive resolvers won't serve an expired cached record when they can't reach the authoritative server. (I know that unbound can be configured to do so, but not sure about others.)

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I don't think so. The root cause (as I understand) was that FB stopped advertising it's IP address space to the world. Even if you had the IP addresses of the FB servers, you would not find any route to access them.

The fact that DNS was also not resolving was a symptom of the DNS servers also being unavailable since they were part of the same IP address space that was unadvertised.

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