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Lesson 7

DNS nslookup Program Conclusion

Resource Records

This module explored DNS and used the nslookup program to explore the DNS database. Now that you have completed this module, you should be able to:
  1. Explain the basic use of the nslookup program
  2. Use nslookup to read other types of Resource Records
  3. Use nslookup to use a different name server and to list an entire zone
  4. Describe the components of the BIND package
  5. Define the elements of the name server configuration file

Address Entries and Resource Records

The entries in a DNS server's cache are indeed known as Resource Records (RRs).
Resource Records are data elements that define DNS server responses, including name resolution information. Each RR identifies a specific resource, such as a website or a mail server, associated with a domain. Every Resource Record contains specific fields: a domain name, a type identifier, a class, a time-to-live (TTL) value, and the resource data itself.
There are several types of Resource Records, including, but not limited to:
  1. A (Address) Record: The most common type, it maps a hostname to an IPv4 address.
  2. AAAA (Quad A) Record: Similar to the A record, but it maps a hostname to an IPv6 address.
  3. CNAME (Canonical Name) Record: This record is used to alias one name to another.
  4. MX (Mail Exchange) Record: Used in routing requests to email servers.
  5. NS (Name Server) Record: Points to a DNS server for the domain.
  6. PTR (Pointer) Record: Provides the inverse function of an A or AAAA record, mapping an IP address back to a hostname for reverse DNS lookup.
  7. SOA (Start of Authority) Record: Contains administrative information about the DNS zone, including the primary name server, the email of the domain administrator, the domain serial number, and several timers relating to refreshing the zone.

When a DNS query is made, the DNS server checks its cache for any matching Resource Records. If it finds a match, the server responds to the query with the data from the RR. This information is cached and comes with a TTL value, which determines how long the RR can be kept before it must be discarded or refreshed.

Key terms

In this module, we used the following terms:
  1. BIND (Berkeley Internet Name Daemon) is the software that sets up a name server on a UNIX system.
  2. name server configuration file: The name server configuration file (/etc/named.boot) is a simple text file in which comment lines are indicated with a semicolon (;).
    Each line of the file addresses some aspect of name server configuration.
  3. root cache file: A root cache file is one of the name server data files and is used to prime the name server with the addresses of the root servers.

Commands and files

In this module, we discussed the following commands and files:

Command Purpose
ls Used by the nslookup program to list a domain
nslookup Debugging tool for generating DNS queries
server Used by the nslookup program to allow a different name server from the one listed in the /etc/resolv.conf file
set type Used by the nslookup program to retrieve other kinds of Resource Records from the DNS database besides A and PTR records

Files Purpose
/etc/resolv.conf Used by the nslookup program to locate the name server
/etc/named.boot Configuration file for the name server daemon

Examining DNS Quiz

Click the Quiz link below to take a multiple-choice quiz covering what you learned in this module.
Examining DNS Quiz