On the Internet, all computers have a unique IP address which allows packets of data to be correctly routed to the correct host. IP addresses are a 32-bit number, often represented in dotted decimal notation (134.126.10.30). To make things easier on the user, DNS allows a name such as "falcon.jmu.edu" to be associated with an IP address.
Using DNS is a fairly simple procedure, and totally transparent to the user. Every domain has a primary DNS server and a secondary DNS server. The primary DNS server contains a table of all the hosts in the domain, and the corresponding DNS name. It also contains a list of root servers, which are servers at the top of the DNS hierarchy server information about which servers are primary for which domains. The secondary DNS server contains this same information, which it gets from the primary server. When a host makes a request for a DNS lookup, it sends its query to the primary DNS server for its domain. If the request is in the table on the primary server, meaning that the host is in the same domain, then it returns the IP address. If the query is not in the table on the primary server, then the server must goto a root server, and get the address of the primary server for that domain, and then query that server, and then return the IP address
DNS servers can do much more than just translate hostnames to IP addresses, though. They can also translate IP addresses to names, define mail servers, and provide aliases for hosts. This is done by using different record types in the DNS table.
Most applications have built in functions to query DNS servers for information. There is also a command-line utility called nslookup which you can use to query a DNS server and obtain information about a host.
The DNS servers for JMU are:
| IP Address | Hostname | Server Type |
|---|---|---|
| 134.126.10.11 | doc.jmu.edu | primary |
| 134.126.10.30 | falcon.jmu.edu | secondary |
| 134.126.10.40 | raven.jmu.edu | secondary |
The DNS table is nothing more than a series of records which define a particular domain. Here are some common record types:
| Record Name | Record Type | Description |
|---|---|---|
| Start of Authority | SOA | Marks the begining of a zone, defines parameters for the entire zone |
| Nameserver | NS | Identifies a domain's nameserver |
| Address | A | Converts a hostname to an IP address |
| Pointer | PTR | Converts an IP address to a hostname |
| Mail Exchange | MX | Defines where mail is to be delievered for a domain |
| Canonical Name | CNAME | Defines aliases |
| Host Information | HINFO | Describes information about a host |
| Well-Known Service | WKS | Advertises network services |
| Text | TXT | Any text string |
NSLOOKUP is a utility which can be used to query nameservers for information. It is packaged standard with most network operating systems (Windows NT, UNIX) but is avaliable for almost any platform. NSLOOKUP can be used directly from the command prompt by specifying the hostname or IP address to be queried.
$ nslookup falcon.jmu.edu
Server: doc.jmu.edu
Address: 134.126.10.11
Name: falcon.jmu.edu
Address: 134.126.10.30
$ nslookup 134.126.10.30
Server: doc.jmu.edu
Address: 134.126.10.11
Name: falcon.jmu.edu
Address: 134.126.10.30
NSLOOKUP can also be started in comand mode by simply typing 'nslookup' at a command prompt. In this mode, different name servers may be set, and different types of records may be specified.
$ nslookup
Default Server: doc.jmu.edu
Address: 134.126.10.11
> set type=MX
> falcon.jmu.edu
Server: doc.jmu.edu
Address: 134.126.10.11
falcon.jmu.edu preference = 0, mail exchanger = mail.jmu.edu
jmu.edu nameserver = doc.jmu.edu
jmu.edu nameserver = falcon.jmu.edu
mail.jmu.edu internet address = 134.126.10.50
doc.jmu.edu internet address = 134.126.10.11
falcon.jmu.edu internet address = 134.126.10.30
> set type=SOA
> falcon.jmu.edu
Server: doc.jmu.edu
Address: 134.126.10.11
jmu.edu
origin = doc.jmu.edu
mail addr = minniccr.jmu.edu
serial = 2858
refresh = 1800 (30 mins)
retry = 900 (15 mins)
expire = 12096000 (140 days)
minimum ttl = 86400 (1 day)
> exit
E-mail comments to Lon Jarvis jarvislb@jmu.edu
last updated 3/17/00