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How Stack Overflow plans to survive the next DNS attack

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[系统(windows) 所属分类 系统(windows) | 发布者 店小二04 | 时间 2017 | 作者 红领巾 ] 0人收藏点击收藏

Let’s talk about DNS. After all, what could go wrong? It’s just cache invalidation and naming things .

tl;dr

This blog post is about how Stack Overflow and the rest of the Stack Exchange network approaches DNS:

By bench-marking different DNS providers and how we chose between them By implementing multiple DNS providers By deliberately breaking DNS to measure its impact By validating our assumptions and testing implementations of the DNS standard

Thegood stuff in this post is in the middle, so feel free to scroll down to “The Dyn Attack” if you want to get straight into the meat and potatoes of this blog post.

The Domain Name System

DNS had its moment in the spotlight in October 2016, with a major Distributed Denial of Service (DDos) attack launched against Dyn , which affected the ability for Internet users to connect to some of their favourite websites, such as Twitter, CNN, imgur, Spotify, and literally thousands of other sites.

But for most systems administrators or website operators, DNS is mostly kept in a little black box, outsourced to a 3rd party, and mostly forgotten about. And, for the most part, this is the way it should be. But as you start to grow to 1.3+ billion pageviews a month with a website where performance is a feature , every little bit matters.

In this post, I’m going to explain some of the decisions we’ve made around DNS in the past, and where we’re going with it in the future. I will eschew deep technical details and gloss over low-level DNS implementation in favour of the broad strokes.

In the beginning

So first, a bit of history: In the beginning, we ran our own DNS on-premises using artisanally crafted zone files with BIND. It was fast enough when we were doing only a few hundred million hits a month, but eventually hand-crafted zonefiles were too much hassle to maintain reliably. When we moved to Cloudflare as our CDN, their service is intimately coupled with DNS, so we demoted our BIND boxes out of production and handed off DNS to Cloudflare.

The search for a new provider

Fast forward to early 2016 and we moved our CDN to Fastly . Fastly doesn’t provide DNS service, so we were back on our own in that regards and our search for a new DNS provider began. We made a list of every DNS provider we could think of, and ended up with a shortlist of 10:

Dyn NS1 Amazon Route 53 Google Cloud DNS Azure DNS (beta) DNSimple Godaddy EdgeCast (Verizon) Hurricane Electric DNS Made Easy

From this list of 10 providers, we did our initial investigations into their service offerings, and started eliminating services that were either not suited to our needs, outrageously expensive, had insufficient SLAs, or didn’t offer services that we required (such as a fully featured API). Then we started performance testing. We did this by embedding a hidden iFrame on 5% of the visitors to stackoverflow.com, which forced a request to a different DNS provider. We did this for each provider until we had some pretty solid performance numbers.

Using some basic analytics, we were able to measure the real-world performance, as seen by our real-world users, broken down into geographical area. We built some box plots based on these tests which allowed us to visualise the different impact each provider had.

If you don’t know how to interpret a boxplot, here’s a brief primer for you . For the data nerds, these were generated with R’s standard boxplot functions, which means the upper and lower whiskers are min(max(x), Q_3 + 1.5 * IQR) and max(min(x), Q_1 1.5 * IQR), where IQR = Q_3 Q_1

This is the results of our tests as seen by our users in the United States:


How Stack Overflow plans to survive the next DNS attack

You can see that Hurricane Electric had a quarter of requests return in < 16ms and a median of 32ms, with the three “cloud” providers (Azure, Google Cloud DNS and Route 53) being slightly slower (being around 24ms first quarter and 45ms median), and DNS Made Easy coming in 2nd place (20ms first quarter, 39ms median).

You might wonder why the scale on that chart goes all the way to 700ms when the whiskers go nowhere near that high. This is because we have a worldwide audience, so just looking at data from the United States is not sufficient. If we look at data from New Zealand, we see a very different story:


How Stack Overflow plans to survive the next DNS attack

Here you can see that Route 53, DNS Made Easy and Azure all have healthy 1st quarters, but Hurricane Electric and Google have very poor 1st quarters. Try to remember this, as this becomes important later on.

We also have Stack Overflow in Portuguese , so let’s check the performance from Brazil:


How Stack Overflow plans to survive the next DNS attack

Here we can see Hurricane Electric, Route 53 and Azure being favoured, with Google and DNS Made Easy being slower.

So how do you reach a decision about which DNS provider to choose, when your main goal is performance? It’s difficult, because regardless of which provider you end up with, you are going to be choosing a provider that is sub-optimal for part of your audience.


How Stack Overflow plans to survive the next DNS attack
You know what would be awesome? If we could have two DNS providers, each one servicing the areas that they do best! Thankfully this is something that is possible to implement with DNS. However, time was short, so we had to put our dual-provider design on the back-burner and just go with a single provider for the time being.

Our initial rollout of DNS was using Amazon Route 53 as our provider: they had acceptable performance figures over a large number of regions and had very effective pricing (on that note Route 53, Azure DNS, and Google Cloud DNS are all priced identically for basic DNS services).

The DYN attack Roll forwards to October 2016. Route 53 had proven to be a stable, fast, and cost-effective DNS provider. We still had dual DNS providers on our backlog of projects, but like a lot of good ideas it got put on the back-burner until we had more time. Then the Internet ground to a halt. The DNS provider Dyn had come under attack, knocking a large number of authoritative DNS servers off the Internet, and causing widespread issues with connecting to major websites. All of a sudden DNS had our attention again. Stack Overflow and Stack Exchange were not affected by the Dyn outage, but this was pur

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