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In Mac OS X Server 10.6 Snow Leopard: DNS and Network Services, instructor Sean Colins introduces the networking services available in Snow Leopard Server. This course covers setting up a DNS server to provide network resources, using firewalls to protect systems against intrusion and to route traffic, using DHCP to automatically configure network settings for computers when they join a network, and accessing a network securely via a remote VPN (virtual private network) connection. Exercise files accompany the course.
There are many misconceptions about DNS. Do you need or want DNS? Funny thing about the word need. You don't really need DNS unless you're running mail or directory services. But when you start talking about want, now there is an interesting word. You might want DNS for several reasons. But I bet the best reason is to make something easier. That's what DNS does; it makes much of what you do with a computer easier.
Have you noticed how much your iPad wants to be connected to a network? How about web browsing or getting onto a social service like Facebook so you can easily interact with people? None of that would be easy without DNS. So what is DNS? Really, it's just a system to match numbers to names and names to numbers on a computer network. It's that simple. Computers find their way around using numbers. People find it a lot easier to work with names.
So, DNS was invented to make life easier for people. Now, when I say numbers, I mean addresses. At the moment, we all use IPv4 addresses on our computers, our routers, our printers, etcetera. So, the number part of the equation is going to be something that looks like this, where we replace the pound symbols with actual numbers. Each of those four segments is called an octet, and each octet can only be the number 0-255. There is a lot more to it than that, but this gives you the basic idea.
When I say names in this context, I mean fully qualified domain names. Fully qualified is a very specific way to refer to a name that means full name. It's kind of like saying my name is Sean. That's my host name. Or saying my name is Sean Matthew Colins. That's my fully qualified domain name. Well, not really, but you get the idea. The fully qualified domain name is the complete name with nothing more to add. Now, if I had a dog named Scruffy Colins, Scruffy would be the dog's host name, Scruffy Colins would be my dog's fully qualified domain name, and Colins would be the domain or the zone name.
So, within the domain Colins, I have hosts named Sean, Matthew, and Scruffy, and I could have more if I wanted to add my wife and kids to the zone file. If I changed that analogy to a more literal representation, I could actually name computers after everyone in my family. So, mine could be Sean.Matthew.Colins, and my dog's could be Scruffy.Colins. So, how does your request for Scruffy Colins get to where it's intended? That has more to do with the DNS system than the names and the numbers.
You see, to make all of those matches, something needs to have the matching names and numbers in a system that can answer your questions. That system is the DNS system and it's huge. The DNS or Domain Name System starts at 13 root servers, which are actually clusters of servers. Those 13 root servers are managed by the organization responsible for DNS on the Internet.
Everything in the DNS system is listed at various levels of a chain of interconnected DNS servers, each DNS server talking to servers above and below in the hierarchy. It's very complicated, but actually, kind of cool when you think about it. The DNS system is a fantastic example of international cooperation and adherence to rules that make a very complicated system very stable. I don't know about you, but I don't know of too many complicated things that are also very stable.
So, a zone is just your little corner of the DNS universe. It's going to be a name that is either private to only your network, or it's something that is public that you purchased from a domain name registrar. Sometimes, you can have two zones with the same name but different information. That's called Split DNS. And we'll talk about that later on. A DNS zone is essentially a file on a computer that contains records. Your DNS zone is a namespace for which you own authority.
So you can do whatever you want with it. An A record is one entry, or item in that zone. An A record just maps a name to a number. A PTR record does the opposite, mapping a number to a name. An MX record is responsible for sending mail to a specific machine for delivery. A CNAME is kind of like an alias. It lets the machine it references go by another name, but that machine keeps its original identity too. Now that you know what DNS is, how it fits into your world, and what the major pieces and bits look like, let's dig in and see how to make it work.
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