Basics first: If I'm trying to access www.superuser.com my Computer first tries to resolve the FQDN to an IP address. If that address isn't in its host file or cache it asks the configured DNS-Server for the IP. In most cases that would be the preconfigured DNS supplied by your ISP.

The ISP then looks up its own cache and let's just say that domain does not exist in the cache either. Now the ISP DNS-Server searches on one of the 13 global Root-DNS-Servers for the DNS-Server for all .com-Domains. The Root-Server replies with an IP and the ISP starts asking this server and so on.. but why is the first part needed?

The Root-DNS-Server only contains IP addresses to all the TLDs. But those aren't that much. There might be 300 to 400 TLDs out there. Why can't the ISP DNS-Server handle those addresses by himself.

So if the ISP DNS syncs its IP addresses to all TLDs every hour or so with the Root-Server there would be a huge performance improvement for each request because the ISP has to send one request less.

Does that even make sense or am I just wrong? Is there a reason for that?


The main reason for the existing architecture is that it follows the DNS hierarchy, which makes it scalable and somewhat secure. The 12 root servers that are owned RIPE, ICANN, NASA, US DOD etc but they cannot edit the content of the root zone. They all get ti from http://www.iana.org/domains/root

This means that when you register new .com domain no changes are needed to the root files. And most correctly configured servers connect to the root every 48 hours or so.

More details and FAQ: http://www.isoc.org/briefings/020/

  • Hi Tom, thanks for your answer but that still does not solve my question. I'm aware of the benefits this system brings and I understand why they designed it like that but I'm wondering why the first step is necessary. Of course the ISP needs the DNS-Server-IP for the next level but why aren't these IPs synced from the Root-DNS-Server to the ISP-DNS-Server? There is no good reason why they have to fetch the IP every time. – Karim Geiger Jan 20 '14 at 18:12
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    Sorry - they don't fetch the IP every time - due to the cache mechanism it only happens 1x every 48 hours or close to that. – TomEus Jan 20 '14 at 19:54
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    Also keep in mind that the DNS design is from time where transfer speeds were nowhere near to what we have today. The system is designed in as DISTRIBUTED - if you would follow your suggestion there would be 12 servers that provide info about all DNS servers. While doable not as distributed as passing it down 1 level int eh hierarchy - makes sense? – TomEus Jan 20 '14 at 20:00
  • ah all right sorry I missed that. Thanks for your help! – Karim Geiger Jan 20 '14 at 20:16
  • The right answer is that the resolving proxy DNS server can indeed handle this without contacting the ICANN root content DNS servers. That is what private root content DNS servers are all about. See superuser.com/q/390390/38062 for example. – JdeBP Dec 17 '19 at 6:15

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