From a cloudflare blog post on this, their public DNS resolvers were getting 30x their usual traffic. Presumably that was the case for DNS resolvers everywhere, so that would explain a slowdown:

> But that's not all. Now human behavior and application logic kicks in and causes another exponential effect. A tsunami of additional DNS traffic follows.

> This happened in part because apps won't accept an error for an answer and start retrying, sometimes aggressively, and in part because end-users also won't take an error for an answer and start reloading the pages, or killing and relaunching their apps, sometimes also aggressively.

... > So now, because Facebook and their sites are so big, we have DNS resolvers worldwide handling 30x more queries than usual and potentially causing latency and timeout issues to other platforms.

https://blog.cloudflare.com/october-2021-facebook-outage/

Edit: Although looking at the graph on that page, it appears that only DNS requests for FB properties increased 30x, so that last sentence appears to be misleading. Regardless, a significant overall increase.

> their public DNS resolvers were getting 30x their usual traffic

But why does my browser need to do a DNS lookup every time I request a page from HN? It's not like the IP address changes every 5 minutes.

Short answer is because they've set their DNS TTL to 5 minutes: https://mxtoolbox.com/SuperTool.aspx?action=a%3anews.ycombin...

As for why, you'd have to ask them. It could be to allow for relatively quick DNS failover in the case of an outage. Or if using a proxy like Cloudflare, that provider could prefer short DNS TTLs to have flexibility in their routing.

Because hackernews sets their TTL to 5 minutes:

dig +noauthority +noquestion +nostats news.ycombinator.com @ns-225.awsdns-28.com.

news.ycombinator.com. 300 IN A 209.216.230.240

Cloudflare and similar companies only work if the TTL is very short. If they set it to some infinite number, they wouldn't be do even the most basic load balancing at the global scale in which they operate.

Surely the moment facebook comes back up would be higher traffic as people rush to comment on it (see this thread's rapid rise).

Best guess: there's a DNS dependency in HN's critical path somewhere and the DNS server they used was being hammered with the facebook dns lookup retries.

Nah, unauthenticated HN was working very fast. It was simply the amount of traffic HN got while FB was down.

Here's a crazy idea: maybe a lot of HN people also spend time on FB during the day. When FB was down, they had more time to spend on HN!

Lots of facebook employees with nothing to do. We have this impression of HN being a huge internet property because of the importance of the news here in our lives, but in the great scheme of things, probably it has pretty modest traffic. 60.000 more people hammering it probably is a good load.