DNS Prefetching

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Despite anchor tags having HREF attributes which lead to other host names, browsers do not execute DNS lookups on those domains. Content prefetching can be invaluable in speeding up your websites, but did you know that you can also implement DNS prefetching? It's as easy as simple LINK element:

<link rel="dns-prefetch" href="//somehost.tld" />

This technique can be very useful when your website links to related host names. Take Twitter for example; they implement two DNS prefetches:

<link rel="dns-prefetch" href="https://si0.twimg.com" />
<link rel="dns-prefetch" href="https://api.twitter.com" />

I'd be willing to bet that most of you didn't know this tag existed. It's an interesting idea with a very simple execution. What do you think? Do you manage websites that prefetching could be helpful for?

Cloudinary

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Discussion

  1. Peter Dubrovski

    Hi David,

    it seems that this example also works with html4. isn’t it so? do you know wich browser can work with this tag?

    greetings
    peter

  2. That’s a pretty cool idea.

    I think you should make clear that this code needs to go at the bottom of your HTML (rather than in the HEAD) otherwise it seems like it will actually slow down your page load, rather than speed it up.

    • update: oh, in retrospect not so sure that’s an issue. I assumed if it’s in the HEAD then the browser would do the pre-fetch before continuing with the rest of the page load. Now, I’m not so sure :/ ?

    • According to the HTML5 Boiler plate you should attempt to initiate this as early as possible.

      http://html5boilerplate.com/docs/DNS-Prefetching/

      I guess the act of doing this doesn’t block.

  3. Edible Droppings

    How many outstanding DNS prefetch requests can be extant at any one time? You could hang up your browsers UI thread if you spin up too many of these requests. It is also another attack vector into your web application.

  4. I use DNS prefetches on many sites (examples being http://invx.com or http://currencyfeed.com).

    In my experience, with hosts that have a good DNS setup, DNS resolution frequently completes before the remote resource is loaded, shaving 50-150 ms of its loading time.

    I don’t do this for all resources though, http://www.google-analytics.com is one that I don’t preload because of its ubiquity.

    Also, if you load resources from a different host in a blocking fashion from the <head> (which you probably shouldn’t), DNS prefetches are useless. There needs to be a little bit of time (loading, parsing, rendering…) between the prefetch and the loading of the resource.

  5. You are right david, I don’t know about this tag. But I am still not clear about the use of this.

  6. "dns-prefetch" is basically a suggestion to the browser that it should consider prefetching that DNS lookup as soon as it’s practical to do so. Therefore, it won’t be a blocking action, and in general the browser (if doing things sanely) would not tie up the valuable bandwidth/connections pool during critical page load path with those “extra” DNS lookups, but would rather defer that work until the main resource loading is complete.

    On the other hand, if your site has lots of different DNS items you are prefetching, I think you are definitely creating wasted bandwidth for the user (lots of mobile users are still on metered/limited bandwidth), as they are not likely to visit all those extra domains (maybe only 1 or 2 max). So I’d call it a “smell” if your markup had more than one or two at most of those prefetches listed explicitly.

    Also, AFAIK, Chrome automatically prefetches even stuff it finds in the markup (and I imagine this is actually the future direction of most browsers, with implicit lookups rather than having to explicitly indicate). You can actually disable the DNS prefetching in this scenario, by sending an HTTP response header to tell Chrome not to do that. I have several sites I disable it on, because I know there may be many different hosts (like on a big blog post, for instance), and I don’t want to generate a lot of DNS noise unnecessarily.

    One example where DNS prefetching can actually create lots of unnecessary DNS traffic to a server is if that server has wildcard (*.) DNS records, because they are not cacheable (or at least, they aren’t shared cacheable), so if you employ wildcard DNS records and you have many of them listed in a page, you may very well want to disable Chrome from automatically fetching all those records separately and hammering your DNS server.

  7. Bit late to the conversation, but:
    I’ve a few websites with Google Analytics and Adwords installed (javascript) and I’m trying to shave a second or so off the page load time.

    I have messed around with hosting the urchin code within my blogs and linking internally but it still doesn’t overcome the inevitable goodle adwords dns lookup. Now, I know googles servers and dns’s are fast but, I like to speed things up a little more.

    Will the link rel="dns-prefetch" code added in front of their js help things along? Or am I making a mountain out of a mole hill on this one? Even half a second improvement on page load time is a worthwhile exercise to undertake across all of my websites.

    Any ideas? Comments? Steer me in the right direction?
    Thanks

  8. I have seen a couple of references that indicate that assets on https are not prefetched. Seems it would make many of the benefits of dns-preftch unrealized. Is this the case? Is there a browser breakdown of default dns-prefetch support of HTTPS?

  9. amoc

    What the point of this tag, and how it actually works? I don’t really get it.

  10. DNS resolution time can lead to a significant amount of user perceived latency. The time that DNS resolution takes is highly variable. Latency delays range from around 1ms (locally cached results) to commonly reported times of several seconds.

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