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The DNS request process is somewhat complicated due to the hierarchical nature of the DNS name space. This layout has performance advantages though, as every request on the Internet goes through a fairly small group of root name servers. Caching also alleviates some of this load. The first step in the process is when the client sends a request for a DNS name resolution to the local DNS server. The local DNS server first checks to see if it is authoritative for the domain, and checks for a cached copy of the requested information. In either case it will return the response to the client and the process ends. If the request cannot be fulfilled locally the DNS server must retrieve the information for the client from other DNS servers. We use the DNS name www.vtc.com for example. This process is called recursion; recursion is the process of looking up each section of a name, starting form the top of the hierarchy; the first step in recursion is to contact one of the Internets root name servers. Every DNS server that is capable of recursion maintains a list of these servers. It sends a request to the root server to retrieve the authoritative name server for the next part of the name, in this case .com. The root server returns a list of authoritative DNS servers for .com to the server performing the recursion. The DNS server now contacts one of the servers in the list returned by the root server. It will request the next portion of the DNS name being queried, in this case vtc.com. The queried server replies with the authoritative DNS server for vtc.com. The DNS server doing the recursion finally has the authoritative DNS server for the domain being requested. It can now query that authoritative server for the record - www.vtc.com. The hierarchical lay out is important in DNS; this layout ensures that no DNS server needs to hold the entire DNS database. Instead the root DNS servers hold a list of servers authoritative for each TLD. The servers authoritative for the TLDs, hold lists of servers authoritative for the domains under the TLDs; no servers hold the actual records. It is also possible that the servers under the TLDs can point to other lower level servers; in fact this hierarchy can go as many as hundred and twenty seven levels below the root. This hierarchy ensures that the load on the servers is not enormous, but also ensures that the DNS name space is maintainable across the root servers and TDL servers. Caching is another feature of DNS that helps to alleviate the load on the servers near the top of the DNS hierarchy. Cashing can happen at any point; for example, the local DNS server can cache the addresses for the TLD authoritative servers as well as the servers lower in the hierarchy. Once the server caches this information, it does not need to perform the entire recursion process again. We can look at an example of how the process works using NS look up. The first step is to query a root name server. NS lookup is covered in detail later. Start NS lookup from a command line window, type server, space, a.root-servers.net. All of the root servers are named with a single letter then .root-server.net; type the name to query in this case www.vtc.com. The response states served by, followed by a list of servers. These are a list of the authoritative servers for the .com TLD. Now change the server to one in the list by typing server space a.gtld-servers.net. The root server and TLD server names are similar because they are all maintained by ICAN; type the name to query again. Again the response is served by, followed by a list of servers; these are the authoritative servers for the VTC.com domain. Choose one and use the server command to select it. Type the name again and finally you should see the record being queried as you've requested it from the authoritative server. Of course, this whole procedure is purely academic, because querying your DNS server would result in this series of requests happening automatically and the result would be the same, but this is a good exercise to better understand recursion.