Domain Name Server (DNS)

The Internet DNS service is responsible for converting Internet domain names (such as www.rebel.com) to IP addresses (such as 192.168.42.13). Less often, the DNS is also asked to convert in the other direction - IP addresses to domain names.

Your NetWinder provides a DNS so you can publish your own name-to-address mappings. If you already have a DNS server on your network, the NetWinder can be configured to use that instead -- see the Ethernet Settings section.

You should only use the NetWinder's built-in DNS if you don't already have a DNS on your network. If you will not be using your NetWinder's DNS, you can safely skip the rest of this section.

If you click DNS manager from the Network settings screen, you will see a page that looks like this:

This is the list of domain names that your NetWinder's DNS server is currently publishing. The exact process of choosing, registering and publishing domain names is quite complicated, and is beyond the scope of this manual; in fact, entire books have been written on the subject. If you need to publish your own domain names, you should either obtain one of those books, or ask your internet service provider (ISP) for help.

To publish a new domain on your NetWinder's DNS server, click on Add a new domain. You will see the following page:

There are two kinds of domains that a DNS server can handle: forward domains and reverse domains. Forward domains have regular names, such as rebel.com; their job is to convert Internet domain names, such as www.rebel.com, into IP addresses.

Reverse domains do the opposite – they convert IP addresses into domain names. They have unusual names, which are a series of numbers followed by the letters .in-addr.arpa.

NOTE: Simply adding a domain on your NetWinder will not make it available to the world. You need to register your domain with the appropriate authorities. If you don't know how to do this, you should ask your internet service provider for help.

Forward DNS Domains

If you create or edit a forward DNS domain, you will see a screen that looks like this:

A table of values appears. Each line in the table has three parts:

A forward DNS map will normally contain several A records, one or two NS records, and one or two MX records. However, you can have any number of any of these records. A forward map will never contain a PTR record.

You can add entries to your DNS map by clicking Add a new record, or edit existing entries by clicking the word Edit next to the appropriate entry. A page like this will appear:

A records provide the main purpose of DNS: to convert domain names to IP addresses. They work as you might expect; in the DNS map for rebel.com,

server1  A  192.168.42.13

means that server1.rebel.com has the address 192.168.42.13.

On the other hand, the NS records provide additional information that help the DNS system run smoothly. Normally, the Name field of an NS line is blank; since it has the domain name (e.g. rebel.com) appended automatically, it matches the whole domain, not any particular host. The Value field specifies the domain name of a master name server for this domain. By checking the NS entries in a domain, a program querying the DNS map can determine if a particular domain name server is authoritative -- that is, whether it is the master for the domain.

Finally, MX entries specify mail exchangers for the domain. This is how mail programs know, for example, that mail for ffiddle@rebel.com should be handled by a computer called server1.rebel.com. As with NS entries, the Name field is blank, and the Value field is a domain name. So,

MX  server1.rebel.com.

means that server1.rebel.com is a mail exchanger for rebel.com. Notice the trailing dot; this is to prevent the name server from appending rebel.com to the name a second time.

Reverse DNS Domains

Reverse DNS domains work in much the same way as forward DNS domains, except that their purpose is to convert IP addresses to domain names, instead of the other way around.

The table for reverse DNS maps follows the same format as for forward maps, except this time only two record types should be used: Nameserver (NS), and Pointer (PTR).

As with forward maps, the NS entry specifies the name of a domain name server that is authoritative for the domain. Usually, your own DNS will be at least one of these.

A PTR record actually does the work of mapping an IP address to a name. The Name field consists of the last part of the IP address, while the Value field is the hostname itself. Since the domain we are defining is an .in-addr.arpa domain, we certainly don't want the target names to have the current domain appended; thus, we always provide the full domain name, and follow it with a dot. For example, in the 42.168.192.in-addr.arpa domain, the entry

12  PTR  server1.rebel.com.

means that 192.168.42.12 has the name server1.rebel.com.

You can add entries to your DNS map by clicking Add a new record, or edit existing entries by clicking the word Edit next to the appropriate entry.