BROCADE IP PRIMER

Chapter 18: Global Server Load Balancing (GSLB)• The local DNS server, once again, sends a query for “www.brocade.com.”It sends the query to one of the IP addresses of the servers that manageDNS for “brocade.com” (the addresses it got from its last reply).• The servers that manage DNS for “brocade.com” do know how to resolve“www.brocade.com,” and they reply with “63.236.63.244” (the requestedIP resolution).Now, that seems like a lot of work (and it is), but it also decentralizes the data.The only servers that know how to resolve “www.brocade.com” are the DNSservers that manage that domain. Now wouldn't it be more efficient for thelocal DNS server to just start by querying the “brocade.com” DNS servers?Sure, but how would it know to start there? Every DNS server on the Internetwould have to have the IP address for every domain name (and sub-domains)out there. Again, this would be a monstrous file and an administrative nightmareto make sure that it was up to date on all DNS servers. The Root serversdon't change that often. They stay pretty constant. TLD servers change morefrequently, but still, not that often. Parent domain and sub-domain DNS serverschange all the time. This is fine, because they only have to update theirinformation in one place: the TLD servers. This is usually done by the registrar(the company that registered the domain name).When the local DNS server receives this resolution, it will keep it in its localcache. That way, if someone else makes a query asking for “www.brocade.com,”it will simply hand the client the answer, rather than initiateanother recursive query. But how long will the local DNS server keep thatname resolution? What if it changes? That's up to the administrator of DNS.When the DNS record is defined, there is a Time-To-Live (TTL) value assignedto it. This tells any other DNS server how long this resolution is good for. If theTTL expires, the “other” DNS servers will know that they have to perform therecursive query again, should anyone request the name.DNS Domains/DNS ZonesDNS Domains and DNS Zones are often used interchangeably. It's hard toblame people who do this. It's often difficult to define the difference. We'vestarted the DNS hierarchy with the “.” (root). The next level (to the left) is theTLD (“com,” “net,” “org,” etc.). The next division in a name (to the left) is usuallywhat people refer to as the “domain.” In the example “www.brocade.com,”the right-hand “.” is the root, “com” is the TLD, and “brocade” is the domain(often, this would be read as “brocade.com”).Now, there are those that consider a “zone” to be wherever administrationchanges. This would make the Root domain a zone. The TLD domain wouldalso be a zone. In our example, “brocade.com” could also be considered azone. How is it different from a domain? What if you saw this name:“www.internal.research.brocade.com.” Wait a minute. So, “brocade.com” isthe domain, but what's “research,” “internal,” and “www?” They are zones.They still belong to brocade.com, but they have their own tables of names andresolution. Some even have their own DNS servers that host the “subdomain.”390 Brocade IP Primer