Listing 1, page 115, shows an example zone file mapping host names to IP addresses in the fictitious example.com domain. In this example, the domain hosts Web, FTP, email, and name servers, and the ISP (ispexample.net) hosts the secondary DNS and backup mail server. The IP address in this example is actually a private IP address. So what do all these records mean? The SOA record includes the name of the primary DNS server for this domain and the email address associated with this domain's naming administrator; note that the record lists the DNS administrator's email with a period (.) instead of the at sign (@) because @ has special meaning in DNS files.
This resource record also contains five other parameters. First, a serial number identifies the version of this information and tells a secondary server that new information exists to download. Second, a refresh value tells the secondary name servers how often to check for updated information. Third, a retry value tells the secondary servers how often to reattempt connections to the primary server. Fourth, an expire value tells the secondary servers when the information in databases is old and unreliable. And fifth, a Time to Live (TTL) value tells a requester how long you can safely cache the information.
The NS records contain the names of the name servers for this domain. The first server listed is the primary name server because it's the server that the SOA record names. The MX records contain the names of the email servers for this domain. The number in front of the address is the preference value and is most useful when the domain has two or more email servers.
When a remote user sends mail to user@example.com, the remote mail system looks up the MX record for the example.com domain. The remote mailer then attempts to establish an SMTP connection with the mail server that has the lowest preference value. Thus, an organization can specify multiple mail servers with the same preference level for load balancing, or specify servers with different preference levels to provide a backup. The A records contain the IP addresses to associate with each of the listed host names in the example.com domain. The CNAME record contains alias host names. In this example, the FTP and Web services are on the same server but have two different names that map to the same IP address. Using two separate names ensures that if the FTP service moves to another system, external users never need to know about the move.
Each domain contains another important zone file: the reverse lookup file. This file maps an IP address to a host name. The zone file for the fictitious 192.168.210.0 address space (assuming that this entire Class C address has one owner) might look like the file in Listing 2, page 116. The only new RRs in this listing are the PTR records, which associate an IP address with a host name.
You'll find DNS server software under a variety of names, depending on the OS you use. Windows NT simply refers to DNS, but UNIX calls the software named (i.e., name daemon) or Berkeley Internet Name Domain; BIND is the most common name. Although each DNS software package is slightly different from others, it's useful to understand RRs and file formats so that you know how the packages lay out domain name information. For information about products that let you provide a DNS service on an NT server, see the sidebar "DNS Resources," page 116.
All DNS software uses the same terminology to refer to DNS information, whether the software uses regular DNS text-file format or a proprietary-file format. Furthermore, although individual sites might not use UNIX-based DNS software, the major ISPs do, so you need to use the correct terminology when discussing DNS with your ISP.
Last but not least, UDP datagrams on port 53 carry DNS queries. DNS zone transfers between primary and secondary name servers use TCP on port 53. If you run the primary DNS server on your network and connect to an outside secondary DNS server, configure your firewall so that zone transfers can occur only between the designated name servers.
Tools to Examine DNS and Domain Information
The most basic tool you can use to search a domain name or IP number database is NSI's Whois. Although you'll find Whois on all UNIX systems and some Windows systems, accessing the database is easiest on the Internet. You can find NSI's Web interface to Whois at http://www.networksolutions.com/cgi-bin/whois/whois. Screen 1, page 116, shows a Whois query for the win2000mag.com domain. The response shows contact information for the domain, when the record was last updated, and the associated name servers (in preference order). You can use Whois to look up information based on domain names and contact names. Whois can also tell you whether a particular domain name is available.
Nslookup is a handy TCP/IP utility for examining the DNS database. This utility is a standard part of NT (and UNIX) systems, and versions of Nslookup for Windows 9x also exist. You can use the Nslookup utility as a test aid to examine DNS.
Figure 1 shows an example Nslookup session. As callout A in Figure 1 shows, the user invokes the program by typing the command nslookup. The program responds by listing the name and address of the user's default name server. The first command, Help, which callout B in Figure 1 shows, lists all Nslookup commands and functions. Callout C in Figure 1 shows that the user next enters host name www.win2000mag.com. The program responds by listing the host's IP address (204.56.55.202). The set type=MX command, which callout D in Figure 1 shows, tells the program to display MX information. The following command, Win2000mag.com, asks for information about the win2000mag.com domain. The program responds with the names and addresses of the domain's three mail servers (and two name servers). Callout E in Figure 1 shows the Set Type=SOA and Win2000mag.com commands, which tell the program to display SOA information about the win2000mag.com domain. Finally, the program responds with the SOA parameter information, as well as the names and addresses of the domain's name servers.
The NS host information that Nslookup shows (ns1.duke.com and ns2.duke.com) doesn't match the host information that the Whois lookup shows (ns1.rockymtn.net and ns2.rockymtn.net). This result is unusual but merely signals that the Whois database isn't synchronized with the name server information advertised on the Internet. Given this discrepancy, the display from nslookup is more definitive than the display from Whois.
In the past, Internet connections, IP addresses, and domain names were the responsibility of UNIX systems administrators. However, because NT represents a growing percentage of servers on the Internet, these details are important to systems administrators who have traditionally concentrated on the LAN. DNS is one of the most important aspects of your site's Internet connection. If you don't set it up correctly, your public hosts might be unreachable and your users might not be able to reach hosts on the Internet. If you want to know the nitty-gritty behind DNS, the industry-standard text is Paul Albitz and Cricket Liu's DNS and BIND, 3rd edition (O'Reilly & Associates, 1998). For NT-specific information, see Paul Albitz, Matt Larson, and Cricket Liu, DNS on Windows NT (O'Reilly & Associates, 1998).
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