How Stack Overflow plans to survive the next DNS attack

Re:How Stack Overflow plans to survive the next DNS attack by matt4077 on Tuesday January 10, 2017 @02:35AM (#13363323)

The calculation regarding the ideal number of name servers to list needs some empirical data regarding the likelihood of provider and server outages and the client reactions to it, right? Because otherwise 2 would must be the best number, if I'm not mistaken (Chance of hitting the provider that's offline is always 0.5 on the first try, but the second try would be guaranteed to hit the other).

Here's the math for expected number of tries if half of the servers are offline. (It's a hypergeometric distribution but I couldn't find a closed formula)

E(2 server) = 1 * 1/2 + 2 * 1/2 = 1.5

E(4 server) = 1 * 2/4 + 2 * 2/4 * 2/3 + 3 * 2/4 * 1/3 = 1.67

E(8 server) = 1 * 4/8 + 2 * 4/8 * 4/7 + 3 * 4/8 * 3/7 * 4/6 + 4 * 4/8 * 3/7 * 2/6 *4/5 = 1.73

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Re:How Stack Overflow plans to survive the next DNS attack by thefarseeker on Tuesday January 10, 2017 @09:56AM (#13365199)

Author here. One thing I didn't cover in the post was about how to have Google and AWS honour their SLA's is that you have to use _all 4_ of the nameservers they provide. Because we're not doing that (only using half), we have to balance the chance of an outage versus the impact of an outage.

You are correct in saying that more empirical data could be used here. We might even end up changing our minds. I'm not much of a numbers person but I might pass this onto some of the people in our company who love solving problems like this.

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