Please Whitelist This Site? I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :) If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads. If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$document". Then just click OK. Thanks for your understanding! Sincerely, Charles Kozierok Author and Publisher, The TCP/IP Guide

NOTE: Using software to mass-download the site degrades the server and is prohibited. If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  Name Systems and TCP/IP Name Registration and Name Resolution            9  TCP/IP Name Systems: Host Tables and Domain Name System (DNS)                 9  TCP/IP Domain Name System (DNS)                      9  DNS Name Servers and Name Resolution                           9  DNS Resolution Concepts and Resolver Operations

DNS Resolver Functions and General Operation 1 2 3 4 DNS Name Resolution Efficiency Improvements: Caching and Local Resolution

Contrasting Iterative and Recursive Resolution

To help explain the difference between these methods, let's take a side-trip to a real-world case. Suppose you are trying to find the phone number of your old friend Carol, with whom you haven't spoken in years. You call your friend Joe; he doesn't have Carol's number, but he gives you John's number, suggesting you call him. So you dial up John; he doesn't have the information but he knows the number of Carol's best friend, Debbie, and gives that to you. You call Debbie and she gives you Carol's information. This is an example of an iterative process. In contrast, suppose you called Joe and Joe said “I don't know, but I think I know how to find out “. He called John and then Debbie and called you back with the phone number. That would be like recursive resolution.

So in essence, iteration is doing the job yourself, while recursion is “passing the buck”. You might think that everyone would always want to use recursion since it makes “the other guy” do the work. This is true, but “passing the buck” is not considered good form if it is not done with permission. Not all name servers support recursion, especially servers near the top of the hierarchy. Obviously, we don't want to bog down the root name servers and the ones that handle “.COM” and other critical TLDs with doing recursion. It is for this reason that clients must request that name servers perform recursion for them.

One place where recursion is often used is with the local name server on a network. Rather than making client machine resolvers perform iterative resolution, it is common for the resolver to generate a recursive request to the local DNS server, which then generates iterative requests to other servers as needed. As you can see, recursive and iterative requests can be combined in a single resolution, providing significant flexibility to the process as a whole. This is demonstrated in a more realistic example in the topic detailing the DNS name resolution process.

Again, remember that for the purpose of understanding resolution, a DNS server can in fact act as a client. As soon as a DNS server accepts a recursive request for resolution on a name it cannot resolve itself, it becomes a client in the process. I should also point out that it is common for resolvers to know the names of not one but two local DNS servers, so if a problem occurs reaching the first they can try the second.

DNS Resolver Functions and General Operation 1 2 3 4 DNS Name Resolution Efficiency Improvements: Caching and Local Resolution