In-addr.arpa There’s one major difficulty with PTRrecords: Often,
In-addr.arpa There’s one major difficulty with PTRrecords: Often, when they appear, they’re listed backwards. You see, DNS checks hosts from left to right. When you check for the host http://www.absolutebsd.com/, the nameserver first looks for a nameserver for .com. It then checks under .com for http://absolutebsd.com/, then under http://absolutebsd.com/ for http://www.absolutebsd.com/. The biggest units are on the left, but in an IP address, the biggest unit is on the right. To check the IP address, we have to reverse it. For example, we turn 66.80.60.21 into 21.60.80.66. It’s very easy to confuse a forward IP address with a reversed IP address, so DNS uses a special marker to indicate that an IP address is reversed. Reversed IP addresses have the string in-addr.arpa on the end of them. (The reasons for this date back several years and are quite boring, so we won’t go into them.) The bottom line is that our 66.80.60.21 becomes 21.60.80.66.in-addr.arpa. So why not just leave the IP address forward, and use the in-addr.arpato indicate it’s a reverse DNS check? Glad you asked. The preceding address is a simple one, and if you ran dig, it would check a very limited space. If you’re running a large network, you might need to run a DNS query of a much larger range of IP addresses, like 118.168.192.in-addr.arpa, which would translate to everything under 192.168.118. You might even need to run 168.192.in-addr.arpa, or even 192.in-addr.arpa. Each is a check of an increasingly large space much like doing dig .com. (You’ll probably never need to run dig .com, but Internet backbone engineers do, and backbone engineers are the ones who write this sort of program. One of the problems with using professional-strength tools is that they’re geared toward, well, professionals.) Note If you’re looking for quick-and-dirty answers, host(1) does this reversal for you. Dig also does this for you, if you use the -x option. Don’t be confused when you see in-addr.arpa, however. Configuring Named Before you can start named, you need to set it up. The directory /etc/namedb contains the basic named configuration files. named.root One file that must be present, but that doesn’t need editing, is named.rooti, which lists the root nameservers. If a nameserver receives a query for a site it doesn’t have in its cache, it asks these nameservers. (This file changes rarely the last update was in August 1997.) You may need to edit this file if your system is not on the Internet and if you have a private root server. named.conf The other important file is named.conf, named’s central file. If your named.conf file is broken, your nameserver is hosed. The syntax of named.conf resembles C code. If you don’t know C, though, don’t worry, because the rules are very simple, and the examples demonstrate everything you need to know. Any line beginning with two slashes (//) is a comment. Similarly, any text contained within old-fashioned C comment marks (/* and */) is a multi-line comment. 278
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