Fedora Linux

Once the nameserver is working, you can configure your local clients to use it:

? If the computers on your local network have been manually configured with their IP information, change the nameserver IP address to that of the machine running named . Edit the file /etc/resolv.conf , or for Fedora Linux systems, use the menu option System>Administration>Network(the system-config-network tool, also known as neat) and adjust the entries in the DNS tab. For other operating systems, use the appropriate network configuration tool (such as the Microsoft Windows Control Panel).

? If the computers on your local network are configured to get their IP information through the DHCP protocol, and the DHCP server is on a gateway or router device, program the DHCP settings on that gateway or router so that the IP address of your named server is used as the domain name server. Consult the device documentation for configuration information.

? If the computers on your local network are configured to get their IP information through the DHCP protocol, and you're using your Fedora system as the DHCP server, add the nameserver to your DHCPD configuration file.

If you configure only your local clients to use your nameserver, any zones that you have configured are accessible only to those clients.

7.3.1.6. Serving domain information to the Internet

In order to make your domain information accessible to other systems on the Internet, it is necessary to register your domain and give the IP address of your nameserver(s) to your domain registrar.

There are many registrars available; to find one, simply search for 'domain registration' on any search engine. Be sure to read the fine print of the registrar's contract because some registrars will try to lock you into their service by charging you exorbitant transfer fees if you try to switch to another registrar at a later date.

Most registrars now offer a myriad of different packages with domain forwarding, web hosting, or email management features. If you are planning to do your own web serving and email hosting, you can forgo those features and sign up for the most basic registration service. Give the IP address of all of your named servers to your registrar as the nameservers for your domain.

Once your domain registration is complete, the nameservers for your top-level domain (TLD)such as .com or .org will start forwarding queries about your domain to your nameservers. It takes a short while for your domain information to circulate to all of the nameservers for your TLD, so be patient!

To test whether your domain name service is accessible to the Internet, use the dig command with your ISP's nameserver:

$ dig somehost.yourdomain.com @nameserver.yourisp.com

7.3.1.7. Reverse mapping

DNS is also capable of performing reverse mapping , which translates an IP address into a domain name. However, unless your ISP has provided you with a block of IP addresses that is a power of 256that is, either 256, 65,536, or 16,777,216 addressesreverse mapping is particularly difficult to set up. If you really need reverse mapping controlled by your nameserver, you'll need to find out how your ISP has configured this and whether they are willing to delegate the reverse mapping to you.

If you have a small number of Internet-accessible hosts, most ISPs prefer to enter your hostnames and IP addresses into their reverse-mapping tables rather than go through the arduous task of connecting a portion of their reverse map to your nameserver.

7.3.2. How Does It Work?

Domain name service is based on the concept of referrals . When a client program (such as a web browser) needs to convert a hostname into an IP address, it uses query functions in a resolver library . The resolver looks in the local hosts file ( /etc/hosts on Fedora, other Linux, Unix, and Mac OS X systems; c: windowssystem32driversetchosts on Windows 2000, Windows Server 2003, and Windows XP), and if the hostname is not found in that file, it queries one of the caching nameservers specified in the operating system's network configuration. On a Fedora system (like most other Linux and Unix systems) the nameservers to be used are listed in /etc/resolv.conf.  

The Linux resolver uses the file /etc/nsswitch to determine possible ways of resolving a hostname to an IP address. The default configuration is to check /etc/hosts first, and then try DNS.

The caching nameserver first checks its cache to see if it already has the answer to the query, and if it does, it returns that answer to the client. Otherwise, it contacts one of the root nameservers (listed in /var/named/named.ca ) by sending a UDP packet to port 53. The root nameserver sends back a reply referring the caching nameserver to the authoritative nameserver for the appropriate top-level domain (TLD). The caching nameserver then sends another query, this time to the TLD nameserver, which replies with a referral to the next nameserver down the chain. This happens recursively until a nameserver that knows the answer is foundor until the possibilities are exhausted and a nameserver finally returns an NXDOMAIN (nonexistent domain) response.

Slave zones are transferred from master zones when the master zone notifies the slave of the need for an update, or when an update is mandated by the refresh time value in the zone's SOA record. The transfer is always initiated by the slave side.

In addition to the resource records discussed in this lab, DNS supports a number of other record types that can be used to serve information, such as host hardware and OS configuration, geographical locations, email server authorization (Sender Policy Framework), and more, but these records are much less commonly used.

Reverse address resolution is performed by reversing the bytes of the dotted-quad IP address and using that as a domain name within the in-addr.arpa TLD. The resource record returned is a pointer (PTR) record.

For example, to discover the hostname of 216.183.93.224 , a query is made for 224.93.183.216.in-addr.arpa :

$ dig 224.93.183.216.in-addr.arpa ptr
; <<>> DiG 9.3.1 <<>> 224.93.183.216.in-addr.arpa ptr
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 10860
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 1, ADDITIONAL: 1