Welcome Overview Up and Running Technical Assistance Support Links Site Map     updated: 4/10/98

DNS Technical Sidebar What is DNS? Purpose Sample Lookup DNS Hierarchical Structure Sample Recursive Lookup Authoritative vs. Non-Authoritative Responses Setting up DNS services: Overview Secondary Servers and Zone Transfers Mail Exchangers Glossary What is DNS? Purpose Domain Name Service is a distributed database system used to map hard-to-remember IP addresses to easy-to-remember names. In many ways, it's similar to a phone directory: when you want to call someone, you know their name, and look up their number. When you want to connect to a certain type of IP service, whether it be a Web Server or a mail server or some other kind of service, you usually know its name, but need to look up its IP address (because computers need the IP address in order to communicate with other computers over the network) . That's where DNS comes in. The domain name server keeps a table of hosts (i.e., computers) and their IP addresses; when someone needs to resolve a name to an IP address, they do what's called a "lookup", which is basically sending the name to the name server (NS); and the NS responds with the IP address. Sample Lookup The lookup process is typically handled behind the scenes, using information entered in your TCP/IP control panel (Name Servers field). This field contains the IP addresses of name servers. Here's an example, in simplistic terms, of how such a transaction might go: You are using a Web browser and trying to connect to www.apple.com. Your machine asks your DNS server (the one entered in your TCP/IP control panel), what's the IP address of www.apple.com? If your DNS server doesn't have that information in its own tables or cached away from a previous lookup, it knows how to find out by communicating with other DNS servers; in this case, it would want to talk to the DNS server that has authority over the domain "apple.com". It learns the IP address of www.apple.com, and returns that information to your machine. Once your machine has the IP address it needs, it can now send a request to that server to download a page. Of course, it can get more complicated, but this is basically how it works. DNS Hierarchical Structure In order to keep domain names unique across the Internet (which encompasses the world), a central organization called "InterNIC" controls domain names (they are also the organization that assigns IP addresses to organizations). To use our telephone analogy again, think of the how irritating it is when you need to phone someone and find multiple listings in the phone book for the same name. You might have to call each one of them before you locate the correct person. Because DNS lookup should be transparent to the end user, this type of trial-and-error method of locating the right number would not be acceptable. So the InterNIC makes sure that all registered domain names are unique. The domains are also organized into a hierarchy, like an upside-down tree. At the root of the hierarchy is the "." (dot) domain, also just called "root". It is the parent of all other domains. The top level domains are organized either by function, or by geographical area. For example, the following top-level domains are organized by function. The table below describes the other functional top-level domains. (NOTE: A second way domains are organized at the top level is geographically, for example, sites in Canada would be in domain .ca, Japan in .jp, Kentucky in .ky.) Function Domain Government .gov Military .mil Not-for-Profit .org Educational .edu Commercial .com Network Services .net A commercial institution, applying for a domain name, would be given something within the .com domain. It could then create subdomains within its domain. For example, Apple is in the .com domain, and has subdomains such as "corp" (corp.apple.com), and "austin" (austin.apple.com). The name server for "austin.apple.com" keeps a table for the computers within that domain name space, or "zone." This table, which includes the host names, host static IP addresses, aliases, mail exchangers, time-to-live values, and other information, is also called a "zone file" and the information it contains is sometimes referred to as "zone information."   Sample Recursive Lookup To continue with our earlier example of trying to resolve www.apple.com, we said that your acme.com name server didn't have the IP address for www.apple.com in its own tables or in its cache, but it was able to find out the address from other name servers. It does this by asking the appropriate name server at the top level where it can find this information. That name server will most likely give it a pointer to another name server to try. And the name server will keep asking other name servers till it finds an answer to return to you, the client. This is called recursive lookups. The interaction might go something like this: Your domain name server (acme.com) only has entries in its zone file for hosts within acme.com, and finds it doesn't have "www.apple.com" cached from an earlier inquiry. The acme.com NS asks the .com NS, what's the IP address for www.apple.com? The .com NS replies, "I don't know...ask the apple.com NS, its IP address is 17.104.26.2." The acme.com NS asks the apple.com NS, what's the IP address for www.apple.com? The apple.com NS has this address in its tables, and responds with the correct IP address. The acme.com NS returns the IP address to your machine, and caches it for a period of time, specified by the Time-to-Live (TTL) value in the www.apple.com host's record, so that next time it's asked, it can answer more quickly. Authoritative vs. Non-authoritative responses When the apple.com name server gives out an IP address for a host within the apple.com domain, this is an "authoritative" response, because the information is coming straight from the database where it is defined. If a response comes from a cached entry in another name server, such as our acme.com NS, its called a non-authoritative response, because there's a possibility, albeit slight, that the IP address was changed at apple.com since the address was cached. That's why TTL values are kept short. When the TTL expires, the entry is flushed from the NS' cache. Setting Up DNS The first step in setting up your own DNS services is to contact the InterNIC to register a domain name, if you don't already have one. See www.internic.net for the details in doing so. If you also need a range of IP addresses for your organization, you could obtain those at the same time. Registering your domain name is critical; failure to do so will result in the top-level domain name servers not knowing about your domain, and thus being unable to point other name servers to your DNS to get names resolved. Your DNS server would then be the "authority" over names in your domain. It would contain a table, mapping machines in your organization to their domain names, and it would give this information to other DNS servers on the Internet, trying to connect with someone in your domain. When setting up DNS entries for your hosts (i.e., computers at your site), each host entered must have a static IP address, one that will not change from day to day, such as might happen if you were using IP server addressing to assign an IP address somewhat randomly from a group of addresses. So be sure that your computers offering IP services (web server, mail server, ftp server, etc.) have static IP addresses. You can still use server addressing, such as BootP or DHCP addressing, as long as the server gives these machines the same address each time. Not every machine in your organization would need to have an entry in DNS, but you would certainly want to enter every machine that was providing an IP service, such as your ftp server, your web server, your mail server, your AppleShare IP server. You would also include information on aliases that could be used for each machine, and information on what machine provides mail service. Note that DNS services is not mandatory for communicating with hosts on the Internet. The IP address could be used instead of a domain name to communicate with a web server, for example. However, using domain names makes it much easier for users to find your services; many people actually guess at an organization's web server name by simply trying "www.companyname.com". Think of how many fewer visitors you would have if each had to successfully guess the machine's IP address! Another advantage is that you can move services from place to place, without having to inform all your users of the changed IP addresses; just make the change at your DNS server. The sections on setting up MacDNS will bring you through these steps, and explain more about the information that is entered for each host. Secondary Servers and Zone transfers MacDNS acts as a primary name server. That means that it is the “true” authority for a particular zone. For robustness, the Domain Name System allows secondary servers to keep a copy of the zone’s information. The secondary server obtains its information from the primary server for a zone. Rather than attempting to query the primary server for each of its hosts, one at a time, there is a zone transfer mechanism that moves all the information in a single operation. (You cannot configure MacDNS to be a secondary server, though you can control how often MacDNS distributes information to secondary servers.) The zone transfers happen on a periodic basis. The secondary server uses three parameters to control the timing: The refresh timer tells the secondary how often to query the primary server to see if new information is available. If something has changed, the secondary then does a zone transfer. The retry timer tells how long the secondary should wait before reattempting to contact the primary if the first attempt failed. The expire timer tells how long to cache the zone information in the event that the primary cannot be reached. If this timer lapses, then the secondary server removes all information about the primary from its database. Mail Exchangers DNS provides a facility for designating that E-mail for a site should be sent to one of several hosts. When E-mail is to be delivered to a site, the sender attempts to deliver it to the ultimate destination. If that computer is down, or if it is not available at the moment (possibly because it establishes a dial-up connection to receive mail), the sender can deliver messages to an alternate host that will, in turn, attempt to deliver the message at a later time. The site’s administrator could define a series of these hosts (known as “mail exchangers” or “MX hosts”) with different priorities. In MacDNS, mail exchangers are entered this way: 10 mail.acme.com, 20 mail.myISP.com The numbers 10 and 20 are preference value, indicating the priority. The lower number means higher priority. So in this case, the mail would be delivered to mail.myISP.com only if mail.acme.com was not available for whatever reason. (Arrangements should be made with the ISP for mail forwarding service.) Here is an example of how mail is sent across the Internet: You compose a letter in your mail software, address it to "john@acme.com" and click "send" The mail is sent to the SMTP (Simple Mail Transfer Protocol) server you have configured in your mail program. Let's say it's called "smtp.mycompany.com" smtp.mycompany.com now has the mail and needs to deliver it to the mail server at acme.com. It contacts the acme.com DNS server and says, basically, who does mail for acme.com ? acme.com NS returns the Mail Exchangers for acme.com: 10 mail.acme.com, 20 mail.myISP.com smtp.mycompany.com tries mail.acme.com first, so now it asked the acme.com NS, who does mail for mail.acme.com ? acme.com NS returns the Mail Exchangers for mail.acme.com: 10 mail.acme.com, 20 mail.myISP.com Note that mail.acme.com has an entry that simply points back to itself; this says in effect, "I am the mail server". smtp.mycompany.com then sends the mail to mail.acme.com, where the mail server software checks to make sure he has a user called "john", and puts the mail in the equivalent of john's PO box, waiting for user John to connect and pick it up. If no response came from mail.acme.com, smtp.mycompany.com would attempt to get the mail to mail.myISP.com instead, and that mail server would then continue making attempts to deliver the mail to mail.acme.com (note that you must make arrangements with your ISP for this type of mail forwarding). The set up instructions within this training guide walk you through setting up mail exchangers for your permanent hosts, and setting up an MX-only host record, which allows you to specify mail exchangers for a domain (such as acme.com). This is used when there is no real permanent host named acme.com. Glossary (courtesy of Network Solutions, Inc.) com The top level domain originally intended for "commercial" entities. One of the worldwide top level domains, in which any person or organization may register a domain name. [see also: Top Level Domain] Domain A subset of the total domain name space. A domain represents a level of the hierarchy in the Domain Name Space, and is represented by a domain name. For example, the domain name netsol.com represents the second level domain netsol which is a subset, or sub-domain, of the top level domain com, which is in turn a larger subset of the total Domain Name Space. If you think of the Domain Name Space as a tree, a domain would be analogous to a limb and would encompass all of the branches off of that limb. [see also: Domain Name Space, Second Level Domain, Top Level Domain, Third Level Domain, Domain Name] Domain Name An addressing construct used for identifying and locating computers on the Internet. Domain names provide a system of easy-to-remember Internet addresses, which can be translated by the Domain Name System (DNS) into the numeric addresses (Internet Protocol (IP) numbers) used by the network. A domain name is hierarchical and often conveys information about the type of entity using the domain name. A domain name is simply a label that represents a domain, which is a subset of the total domain name space. Domain names at the same level of the hierarchy must be unique, for example there can be only one com at the top level of the hierarchy, and only one netsol.com at the next level of the hierarchy. [see also: Domain Name System (DNS)] Domain Name Space An Internet addressing scheme that is hierarchical in nature and uses a "tree" structure to organize information that describes networks and computers. The domain name space is the sum total of domain names that currently represent networks and computers, as well as all of the possible domain names - not yet in use - which may potentially represent networks and computers. Also, the realm of networks, computers, and other network devices that can be described and represented by the Domain Name System (DNS). [see also: Domain Name, Domain Name System (DNS)] Domain Name System (DNS) A distributed database of information that is used to translate domain names, which are easy for humans to remember and use, into Internet Protocol (IP) numbers, which are what computers need to find each other on the Internet. People working on computers around the globe maintain their specific portion of this database, and the data held in each portion of the database is made available to all computers and users on the Internet. The DNS comprises computers, data files, software, and people working together. dot or "." The top of the hierarchy in the Domain Name System (DNS). [See Root] edu The top level domain designated for four-year, degree-granting colleges and universities. [see also: Top Level Domain] gov The top level domain designated for agencies and branches of the United States Federal Government. State and local governments in the United States should use the us top level country code domain. Prior to October 1, 1997, registration in gov was handled by Network Solutions; the General Services Administration (GSA) began handling registration in the top level domain gov starting October 1, 1997. [see also: Top Level Domain] hosts Used here to refer to computers on a network. Inverse Address Resolution (IN-ADDR) The process used to resolve Internet Protocol (IP) number(s) to their corresponding domain name(s), as opposed to the more familiar type of resolution that starts with a domain name and translates it into the corresponding IP numbers. The Domain Name System (DNS) was designed to translate domain names into IP numbers; to enable an IP number to be translated to a domain name, both the domain name and its corresponding IP number(s) must be registered in a special domain - in-addr.arpa - using the IN-ADDR registration form. This means that the IP address 198.41.0.52 would be listed as 52.0.41.198 in the in-addr.arpa domain, so that when read from left to right it moves from specific to general in the hierarchy, the same way that domain names do. Inverse address resolution can be helpful when using tools such as log files, because it enables the Internet Protocol (IP) number(s) logged by the network and computers to be related to the corresponding domain names, which are more easily recognized by humans. [see also: Domain Name System (DNS)] Internet Protocol (IP) Numbers (IP addresses) A unique, numeric identifier used to specify hosts and networks. Internet Protocol (IP) numbers are part of a global, standardized scheme for identifying machines that are connected to the Internet. Technically speaking, IP numbers are 32 bit addresses that consist of four octets, and they are expressed as four numbers between 0 and 255, separated by periods, for example: 198.41.0.52. Internet Service Provider (ISP) A person, organization, or company that provides access to the Internet. In addition to Internet access, many ISPs provide other services such as Web hosting, Domain Name Service, and other proprietary services. [see also: Name Service] InterNIC The name given to a project that originated and operates under acooperative agreement with the National Science Foundation (NSF). Under this agreement, Network Solutions, Inc. provides domain name registration services in com, net, org, and edu; Internet Protocol (IP) network number allocation; and information and education services. AT&T provides directory and database services InterNIC Directory and Database Services A National Science Foundation (NSF) cooperative agreement awarded to and operated by AT&T. Database and Directory services provides: Directory of Directories, containing lists of FTP sites, servers, white and yellow page directories, library catalogs and data archives. White and yellow pages-type Directory Services. InterNIC Registration Services A National Science Foundation (NSF) cooperative agreement awarded to and operated by Network Solutions, Inc.. Registration Services administers the registration of second level domain names under the following top level domains: com, org, net, and edu. Network Solutions also has been responsible for Internet Protocol (IP) number allocation for the Americas, the Caribbean, and sub-Saharan Africa throughout the duration of the cooperative agreement, but will be transitioning this responsibility to the American Registry for Internet Numbers (ARIN). mil The top level domain designated for United States military entities. Name Server Also called a host or a name server. A computer that has both the software and the data (zone files) needed to resolve domain names to Internet Protocol (IP) numbers. [see also: Host (Name Server), Zone File, Resolve] Name Service Providing individuals or organizations with domain name-to-Internet Protocol (IP) number resolution by maintaining and making available the hardware, software, and data needed to perform this function. Many Internet Service Providers (ISPs) operate name servers and provide their customers with name service when they register a domain name. Most individuals are not in a position to operate a name server on their own and will need to make arrangements for name service with an ISP or some other person or organization. [see also: Resolve, Internet Service Provider (ISP), Name Server] net The top level domain designated for entities and computers that represent part of the Internet’s infrastructure. Originally intended for use by Network Information Centers (NICs), Network Operations Centers (NOCs), administrative computers (such as a name server) and network node computers. One of the worldwide top level domains. [see also: Top Level Domain] org The top level domain designated for miscellaneous entities that do not fit under any of the other top level domains. Typically used for non-profit organizations. One of the worldwide top level domains. [see also: Top Level Domain] Primary Server The host name and Internet Protocol (IP) number of the name server that will contain authoritative information for the domain name and will be used to resolve that domain name to its corresponding IP number(s). The designation of "primary" means that this name server will be used first and will be relied upon before any of the other name servers that may be listed on the Domain Name Registration Agreement. Request For Comments (RFCs) The official document series of the Internet Engineering Task Force (IETF) that discusses many aspects of computing and computer communication focusing on networking and Internet protocols, procedures, programs, and concepts. All Internet protocols are written up as RFCs, but not all RFCs describe Internet standards. Resolve The term used to describe the process by which domain names are matched with corresponding Internet Protocol (IP) numbers. "Resolution" is accomplished by a combination of computers and software, referred to as name servers, which use the data in the Domain Name System to determine which IP numbers correspond to a particular domain name. [see also: Domain Name System (DNS), Name Server] Root The top of the Domain Name System (DNS) hierarchy. Often referred to as the "dot." [see dot or ".", Domain Name System (DNS)] Root server A machine that has the software and data needed to locate name servers that contain authoritative data for the top level domains (e.g., root servers know which name servers contain authoritative data for com, net, fr, uk. etc.). The root servers are, in fact, name servers and contain authoritative data for the very top of the Domain Name System (DNS) hierarchy. Currently, technical specifications limit the number of root servers to 13. These machines are located around the globe, in the U.S., the U.K., Sweden, and Japan. [see also: Root, Top Level Domain, Name Server, Domain Name System (DNS)] RWhois A protocol under development at Network Solutions that helps find domain names and other network data on a large number of servers, rather than on just one server. RWhois is a directory services protocol which extends and enhances the Whois protocol in an effort to provide a scaleable, decentralized, and efficient means of storing and retrieving information related to hosts, network information systems, and the individuals associated with those systems. Borrowing from earlier established Internet protocols, other directory service efforts, and the architecture of the Domain Name System (DNS), RWhois uses the hierarchical nature of the information related to network "objects" (domains, Internet Protocol (IP) numbers, hosts, people) to provide the shortest and most efficient path between network data and the person who needs it. [see also: Whois, Domain Name System (DNS)] Second Level Domain In the Domain Name System (DNS), the next highest level of the hierarchy underneath the top level domains. In a domain name, that portion of the domain name that appears immediately to the left of the top level domain. For example, the netsol in netsol.com. Second level domain names are often descriptive and have come to be used increasingly to represent businesses and other commercial concerns on the Internet. [see also: Domain Name System (DNS), Top Level Domain] Secondary Server The hostname and Internet Protocol (IP) number of a name server that will contain authoritative data for a domain name and will resolve that domain name to its corresponding IP number(s). The designation of "secondary" indicates that the name server will be used in addition to and as a backup for the primary name server. Start of Authority (SOA) Resource Record A type of record used in the distributed database that is the Domain Name System (DNS) to indicate that a particular name server contains authoritative data for a particular domain. [see also: Domain Name System (DNS), Name Server] Third Level Domain In the Domain Name System (DNS), the next highest level of the hierarchy underneath the second level domains. In a domain name, that portion of the domain name that appears two segments to the left of the top level domain. For example, the reston in reston.va.us. Third level domains are not the portion of an email address that appears in front of the @ symbol - for example, the webmaster in webmaster@netsol.com is not a third level domain. [see also: Second Level Domain, Top Level Domain, Domain Name System (DNS)] Top Level Domain (TLD) In the Domain Name System (DNS), the highest level of the hierarchy after the root. In a domain name, that portion of the domain name that appears furthest to the right. For example, the com in netsol.com. [see also: Domain Name System (DNS), Root, Domain Name] Whois A searchable database maintained by Network Solutions, which contains information about networks, networking organizations, domain names, and the contacts associated with them for the com, org, net, edu, and ISO 3166 country code top level domains. Also, the protocol, or set of rules, that describes the application used to access the database. Other organizations have implemented the Whois protocol and maintain separate and distinct Whois databases for their respective domains. Zone A portion of the total domain name space that is represented by the data stored on a particular name server. The name server has authority over the zone - or the particular portion of the domain name space - described by that data. [see also: Zone File, Name Server, Domain Name Space] Zone File A file that contains data describing a portion of the domain name space. Zone files contain the information needed to resolve domain names to Internet Protocol (IP) numbers. [see also: Zone, Domain Name Space, Resolve] For glossary: Permission is granted to quote, copy, or otherwise reproduce this material, provided that the following copyright notice is retained on each and every copy: © Copyright 1997 Network Solutions, Inc. All rights reserved.   [ Welcome | Overview | Up&Running | Tech Assist | Support |Links | | SiteMap |