Saturday, June 26, 2010
Domain Name System (DNS) is an Internet Engineering Task Force (IETF) standard name service that allows your computer to register and resolve domain names. The DNS makes it possible to assign domain names to organizations independent of the routing of the numerical IP address. In other words, DNS is a system that translates domain names into IP addresses. This is necessary because computers only make use of IP addresses yet we use only human readable names since the names are easier to remember than IP addresses. Without this DNS resolution, the internet would be a very inconvenient place. DNS resolution is therefore a very important task. However, you may sometimes try to connect to a system by name and get a DNS error because the name did not resolve to the proper IP address. There can be several causes for this, such as:
* The DNS server is down.
* IP connectivity gets lost and thus the DNS cannot resolve
* DNS cache poisoning.
* Update and zone issues.
* The DNS server does not have network connectivity to the root servers.
There are a number of ways to find out whether your system is resolving properly. You can use nslookup to verify name resolution. The nslookup command can be used to find various details relating to a particular DNS (Domain Name System) such as IP address, MX records etc.
Go to the command prompt and type in nslookup host_name server_IP_address. Replace the actual host name that you are trying to resolve for host_name and the IP address of the DNS server for server_IP_address then press enter.
This will allow you to verify if an error is on the server, if there is a widespread resolution error, or if the server is possibly down. Nslookup will also display the various types of DNS records, not just primary (A) records, or all records for a domain. You can then ping with the -a switch to also verify if DNS resolution is working fine.
Troubleshoot the dns client since most problems start with failed queries at the client. If a dns server provides incorrect data to queries that it successfully answers, then the most likely causes would be:
* Resource records (RRs) were not dynamically updated in a zone.
* An error was made when manually adding or modifying static resource records in the zone.
* Stale resource records in the DNS server database, left from cached lookups or zone records not updated with current information or removed when they are no longer needed.
If the DNS server does not resolve names for external networks, then the possible causes could be:
* The recursive query times out before it can be completed.
* A remote DNS server fails to respond.
* A remote DNS server provides incorrect data.
* DNS server recursion has been disabled.
Troubleshoot also the connectivity to the root servers. Verify that the DNS server used in a failed query can ping its root servers by IP address. If a ping attempt to one root server fails, it might indicate that an IP address for that root server has changed.
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