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My problem was low download speed from Mac App Store. I changed my DNS configuration to Google DNS 8.8.8.8 and it started to download over 10x faster with the same WiFi. How is this actually possible?

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It can indirectly.

Example, downloading a file from Akamai. You're in Germany say. DNS A resolves you to a French node, connection is good, download is fast. DNS B resolves you to a US node, connection is poor, download is significantly slower.

Slow DNS queries themselves wont slow your download speed, but will cause pages to wait longer before starting to download.

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    Note that it is more likely that your local ISP's DNS will resolve Akamai to the address in France, and Google's DNS resolves it to the US address, because for DNS-based anycast the location of the DNS server that performs the recursive resolution is what counts. – Simon Richter Dec 16 '15 at 9:12
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    @SimonRichter actually the google DNS service goes to great lengths to resolve local IPs. They even send your country in the query to the upstream servers. – Josef Dec 16 '15 at 9:56
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It has some effect, but not that much.
A DNS does nothing more (in this case) than translating an domainname to an IP. They do this really really fast, often under 5ms. Changing to another DNS could be benchmarked to notice a change in timing, but we're talking milliseconds here. The changes for that part will be minimal.

The difference is in where the DNS points to. If one DNS points you to a server on the other side of the planet for a resource, that'll result in a slow download. If the other DNS points you to a server nearby, that'll download a lot faster.

  • Using AppStore or iTunes on OS X the DNS settings do have a major effect on my download speeds, i.e. 500kbps vs. 50Mbps; this is roughly a factor of 100. Does Apple utilize that many requests during a single download? I am getting the same results on WiFi and cable. To bring it back to the original question; how is this possible? – ben Apr 16 '16 at 19:58
  • Keeping in mind DNS is cached on the client, how could it be possible? Perhaps the DNS is putting you to the wrong (read: far away) resource – Jonathan Jul 11 '17 at 1:00

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