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DNS Prefetching
07-01-2010, 07:37 AM
Post: #3
RE: DNS Prefetching
(07-01-2010 06:26 AM)green-watch.org Wrote:  #1 - Google Chrome looks like they look up domain names, translate them into IP addresses, and then save the results somewhere. This would mean that it would only have to be resolved once correct? So once you navigate to other webpages, the DNS resolution time would be just the time it takes to read the file and retrieving the saved entry.

All browsers cache DNS results in RAM for a period of time and there are caches at both the OS level and upstream as well. For the in-RAM cache you won't even see a DNS lookup time.

(07-01-2010 06:26 AM)green-watch.org Wrote:  #2 - Do other browsers do any sort of prefetching like this? Looking at webpagetest.org test results on repeat view, it looks like there can be significant time for DNS resolution even though the DNS was resolved on the first view. Any comments on this?

Yes, the browsers all pick up techniques from each other and keep implementing (and improving on) the features from the others though it's really only the really recent browsers (IE8-9, Firefox 3.6, Chrome 5) where they started getting really fancy and pre-fetching all sorts of things from DNS to even javascript code.

WebPagetest does a few things to make the "repeat view" more like a user coming back later in a new browser session rather than immediately:

1 - WebPagetest closes the browser between the first view and repeat view so any in-memory caches are lost (browsers cache requests for a session as well so you'd see very different behavior regardless of expires headers if the browser isn't closed).

2 - We disable the OS DNS cache so that lookups go to the first level cache (ISP) and are more representative of someone coming back later after the TTL has expired.

(07-01-2010 06:26 AM)green-watch.org Wrote:  #3 - If IE does not save DNS lookups, is there anyway you can tell the browser to start resolving lookups for a particular page without downloading a resource? For instance, let's say a user comes to my webpage. When they first get there, it would be nice to let the browser know what they need to resolve for the base document to work.

There was a thread on it a little while back - http://www.webpagetest.org/forums/showth...hp?tid=169

Does look like (at least at the time) that support for explicit prefetching was pretty much limited to Chrome.

(07-01-2010 06:26 AM)green-watch.org Wrote:  #4 - Do you think other browsers will catch on to this concept if they do not already?

Absolutely. They are all taking innovations from each other pretty aggressively at this point so it's just a matter of what release will pick up the improvements (particularly the implicit/automatic ones) and how long it takes for that browser to get decent market share (that last bit is always a killer).
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Messages In This Thread
DNS Prefetching - green-watch.org - 07-01-2010, 06:26 AM
RE: DNS Prefetching - sajal - 07-01-2010, 06:44 AM
RE: DNS Prefetching - pmeenan - 07-01-2010 07:37 AM
RE: DNS Prefetching - pmeenan - 07-14-2010, 10:28 PM

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