Question : DNS Server on SBS2k3 "Clear DNS Cache"

Hi,

Recently staff complained that they cannot browse on the internet from their windows PC. I checked the the SBS Server;DNS Management/DHCP and everything was running as per normal. I contacted our ISP tech and he said everything is good on his end.

I tried to ping www.google.com and received an error: on name resolution.  I rebooted the Server thinking it might get rid of some niggling issues and we still could not access the internet.

So I went into DNS Management again and right clicked on our server and selected clear dns cache.......by fluke....we could then browse on the internet again.

Can someone tell me how this fixed the problem?? And also how I can learn more about dns/dhcp troubleshooting or to get general knowledge(websites).

Many Thanks

Jedi

Answer : DNS Server on SBS2k3 "Clear DNS Cache"

Your problem was due to caching more then likely due to a misconfig which corrected but dns still cached:

All Internet hosts, including your computer when it is connected to the Internet, use a DNS server. Every time you go to a website, you need to look up the site's IP address using the domain name of the website. Your request for this lookup is eventually passed to a DNS server somewhere.

But your request is one of thousands, even millions of requests being made at any one time across the Internet. The DNS lookup process requires that if your local DNS server is not Authoritative for the domain that contains the domain name you are trying to reach, it should ask other servers to get an answer. Your local server could get quite busy performing these lookup requests, and this could slow down its performance if it is Authoritative for a domain name.

To combat this the answers that a DNS server gets from another DNS server can be added to their own internal database and retained for a period of time equal to the time to live (ttl) value set on the record stored on the Authoritative DNS server.

Storing these responses is called caching, and allows a DNS server to respond more quickly to multiple queries for the same domain or host. If you are on a website, and want to retrieve the next page on the site, the local DNS server does not have to look up the host again, provided the time to live (ttl) value has not expired and caused the local DNS server to delete the information. This is why it takes so long to contact a website at first, but subsequent requests for pages on the same site are somewhat faster.

Caching DNS servers are configured for recursive lookup as well. This creates a server that will respond to lookup requests by delivering answers from its cache, or looking them up on other servers. It is the job of a caching DNS server to handle general lookups of Internet domains. A caching DNS server reduces the load placed on an Authoritative DNS server by handling the requests that don not pertain to the local domain.

Almost all Internet Service Providers (ISPs) operate some kind of caching DNS server.

Unfortunately DNS caching is a double-edged sword. It speeds up resolution by storing recent answers, and short-circuiting the normal resolution process. However there is a down side. Because DNS servers cache answers, and don't delete these answers until the time to live (ttl) expires, it can take hours or days for the entire Internet to recognize changes to DNS information for your domain name.

Understanding DNS:

http://www.windowsnetworking.com/articles_tutorials/Understanding-DNS-Protocol-Part1.html

Understanding DHCP:

http://www.windowsnetworking.com/articles_tutorials/Understanding-DHCP-Protocol-Part1.html