Friday, December 31, 2004

How to use nslookup to troubleshoot DNS

DNS (Domain Name Service or System) is the name resolution technology used on the global Internet and in most local and wide area networks. It resolves hostnames and fully qualified domain names (such as to an IP address. DNS uses various types of Resource Records to associate hosts and services with IP addresses. Some of the most common are A records which map a host name to an IP address, NS records which identify name servers, SOA records which identify the name server that is “start of authority” for a domain (the primary name server), MX records which are mail exchanger records, CNAME records which are canonical name or alias records, and PTR records which are pointer records that map a known IP address to a name. There are others, but that’s beyond the scope of this article.

A widely-used tool for querying DNS servers is nslookup. nslookup is supported in Windows and Linux and offers a variety of options related to the type of record to be retrieved.

To use nslookup, at a command prompt, type “nslookup”. The prompt will change to a greater-than symbol (>). Try these commands:

  • >set type=soa
  • Next, enter your domain name.>
  • nslookup will return information about the server that is SOA for the domain in question including timeout values, IP addresses, host names, etc.
  • Try the same thing substituting mx, ns, and other Resource Record types for soa.
  • Help is available by typing help at an nslookup prompt.
  • Exit nslookup by typing exit.

nslookup has fallen out of favor in the Linux/Unix world. Many Linux/Unix admins prefer to use dig or host. We cover the use of dig and host in our 2-day Linux training seminar and our 1-day BIND/DNS seminar.

Thursday, December 30, 2004

How to use the Windows Recovery Console

Beginning with Windows 2000, Microsoft included the Recovery Console as one of the troubleshooting and system restore tools. The Recovery Console is a “last-resort” tool you can use when safe mode and other startup measure don’t work. Using the Recovery Console, you can copy files from other media (such as floppy disks or CD-ROMs), enable and disable services, perform drive formatting, read and write a local drive (including NTFS-formatted drives), and perform numerous other administrative tasks.

Two examples of where we’ve used the Recovery Console are when required boot files become corrupted and need to be replaced in order to boot the system and when automatically started services become corrupted and hog system resources to the point where the system becomes unresponsive. The Recovery Console is supported in Windows 2000, Windows XP, and Windows Server 2003.

Here’s how to use the Recovery Console:

  1. Boot the computer using the Windows installation CD

  2. When you see the Welcome to Setup screen, press the R key

  3. Choose the Windows installation you wish to access from the Recovery Console (this step is required even if you have only a single Windows installation)

  4. Enter the Administrator password, and then press the ENTER key

  5. You will then presented with a command prompt where you can enter the various commands you wish to use to repair the Windows installation (You can see a list of available commands by typing help, then pressing the ENTER key)

  6. When you’re finished, type exit at the command prompt, and press the ENTER key

The Recovery Console can also be installed as a startup option to simplify system recovery. For more information about using the Recovery Console, including installation options, see Microsoft Technet article number 326215.