DNS Resolution, Step by Step

when you type a web address into a web browser like WWE yahoo comm your computer needs to turn that into an IP address so that it can contact that web server and deliver the webpage to you this is called a forward lookup because we're turning a hostname into an IP address there's also a reverse lookup that does the opposite it takes an IP address and turns it back into a hostname we'll cover that in a different video I'm going to show you in this video the steps that the domain name system goes through in order to do a forward lookup for you it all starts with your computer your computer is a DNS client a DNS client is one that issues a request to do a forward lookup and a DNS server is one that answers that request in the form of a response in order to do any kind of DNS lookup your client computer needs to contact a DNS server and usually the one it contacts is one that's fairly nearby like when it's supplied by your internet service provider or maybe it's on your local or your network in your router or in the IT department at your business the client computer that's issuing a DNS request needs to know what DNS server to contact act on its behalf in order to fulfill that request so somewhere on that computer is a configuration that just consists of an IP address that tells it what DNS server to contact and it might have obtained this IP address from the router when it booted up or someone might have manually entered it into the DNS configuration control panel let's say that the IP address in this configuration is 10.5 one eight which is the IP address of a DNS server for the internet service provider so your ISP has a DNS server which is going to act on your behalf to do DNS lookups for you this is called a recursive query you're going to issue a request to the ISP the ISP is going to in turn reissue that request to other DNS servers in order to get the answer for you so your client computer starts out by viewing a request to the ISP and it asks a simple question what is the address or WWI yahoo.com if this is the first time this DNS server has seen this request it's not going to know the answer so it needs to ask someone else and that someone else is going to be a DNS server for the top-level root zone let's say its IP address is 198 about 41 at 0.4 which is an actual IP address of one of the root DNS servers your ISP is now going to forward your request to this DNS server and it's going to ask the same question what is the address for wwu comm and most likely this name servers not going to know the answer it's not responsible for knowing the answer to any query that's outside of its zone it's going to come back with a response that says I don't know the answer why don't you go ask the name server for the com domain it's going to start from the right hand side and since this is the root name server it's going to add on one little piece and that's going to be the next DNS server to ask what comes back and it says go ask the comm domain name server it is cg TLD servers net whose address is 1 9 to 26 90 to 30 the ISPs name server is going to take that response and reissue it to the comm DNS server so it asks the same question again what is the address for WWI yahoo.com and once you know it this DNS server is going to come back and say I don't know the answer either why don't you go ask the yahoo.com name server the yahoo.com name server is ns-one yahoo.com and it's address is 68 180 dot 1 31 16 so once again your ISP reissues the request again this time to the yahoo.com name sir so it asks the same question again what is the address for WW yahoo.com and this time the DNS server has an answer it says the address is 70 2.30 243 now your ISP has the answer and it can return that response back to you and your computer is now able to contact the ww-who comm web server in order to deliver a webpage to you now your ISP is DNS server having seen the answer to the response is going to do one more thing for you it's going to keep track of that answer so it's going to put down in a cache the answer to the question what is the address for you WW yahoo.com now the next time a client asks that same question your ISPs DNS server just has to look in its cache find the answer and we turn it straight back to the client this will save some steps in the future because it doesn't have to go back and ask over and over again all these other DNS servers and it takes some of the load off of them so the caching is a really important part of DNS it both solves a problem in that it reduces the traffic on the net but it can also cause some problems because if this address ever changes your ISPs DNS server is going to need to know to to flush that answer out and we're going to see in a future video exactly how that happens so as you can see the steps need to do a forward lookup of a hostname into an IP address is rather complicated your client asks the question your ISP in the form of a recursive query goes then it asks that question again to a whole bunch of DNS servers until it finally gets the answer and then it gives that answer back to you this all happens in a fraction of a second it's really quite amazing that it all works so well and really this is why I want to put this series of videos together because I want you to understand how this works so that you are in a position to be able to not only configure your own DNS server but to debug problems when they happen you

Channel: Barry Brown

Views: 155,199

Rating: 4.9582248 out of 5

Keywords: dns, computer science, networking, network

Id: 3EvjwlQ43_4

Length: 6min 42sec (402 seconds)

Published: Sun Jun 24 2012