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I currently work for an medium-sized institution whose DNS management is questionable at best. Often, there are DNS-related failures causing downtime for the entire building and our public-facing website.

Though I am not directly in charge of the zone file or the DNS servers, I'm interested in tracking down any possible excessive/unnecessary work these servers are doing that contribute to downtime.

Our servers during peak/business hours get between 80-160 active users at a time (according to realtime GA). With TTL set at 5 minutes, I'm wondering about the following:

  1. For all users remaining on the website > 5 minutes, are they requiring additional refreshes to dns? (thus increased website responsiveness)
  2. considering two dedicated DNS machines (no insight on specs), is this negligible activity?
  3. is it likely that the 5 minute TTL is being ignored outright, because of fast flux countermeasures?

We are not currently in any migration processes that warrant a low TTL--this has been the case for years now.

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TTL isn't a burden per se, but it can cause a serious increase in replies that it that it has to process, as the caching that the user's recursive DNS servers does is rendered useless. –  NickW Mar 25 at 16:45

2 Answers 2

up vote 5 down vote accepted

FWIW, many browsers cache the DNS info for a site and don't honor TTL limits. But some do so

  1. Yes, a compliant browser/OS should refresh the info after 5 minutes. However, it's unlikely they would contact your servers directly. The server they use would contact your server.

  2. Yes, that's a trivial load for 2 servers even if all the browsers are contacting your site directly.

  3. DNS records get cached in all different kinds of software, some of which honor TTL's and some don't, setting TTL's only works reliably for other DNS servers.

IMHO, a 5 min TTL is well within standard practice these days and shouldn't cause problems. Things that hammer DNS servers enough to cause them problems are likely broken in ways that setting a longer TTL would not fix.

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Would be nice if it would be correct. Many companies with a lot lower TTL manage that fine (Cloudflare) and without problems, indicating that the answer is wrong in all 3 points. –  TomTom Mar 25 at 19:14
    
I've been running a sub-domain with a 10 sec TTL for over 10 years. Mostly it just works, but I've seen all the ways clients can go wrong. –  Fred the Magic Wonder Dog Mar 25 at 22:51

Well, let me formulate it like that.

Ever heard of cloudflare?

30 second TTL and they live with it.

So, the answer is: Yes.

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for cloudflare that is a feature almost -- they want to be able to flip you over to different hosts very quickly to mitigate DDOS attacks and that pretty much requires very low TTL. –  Wyatt Barnett Mar 25 at 19:12
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I'd wager they also have a much larger DNS infrastructure than 2 servers. –  fukawi2 Mar 25 at 21:57
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Explain your answer? "Is low TTL an appreciable burden on DNS?".... "So, the answer is: Yes". Seems incongruent with "30 second TTL and they live with it." –  hexparrot Mar 25 at 23:29

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