Researching and Bypassing DNS Issues

So we have almost finished transition three out of four. Move to a domain - check. Move to a new file/print server - check. Number three was the biggie.

Move the servers to an off-site data center - almost check.

We moved thirty seven servers to a data center last weekend, changed all of the external and internal IP addresses, and planned to be up and running on last Monday morning. I can hear you chuckling as you read, and yes it was probably too ambitious, but that was the directive so we did it.

DNS issues, both internal and external, are one of the big problems in a transition like this. The DNS server must be properly updated with all IP changes, the local office DNS server must be updated as well, and the external registrar must be updated with new external IP addresses, which can take a few days to propagate.

So if you are like me, a non-networking guy supporting the transition, what are some of the best ways to work on connectivity issues for users? Start with DNS. Here are some of the main testing steps I took to evaluate problems.

1) Ping the name you are trying to reach. If it resolves to the correct IP address but times out, there may be a real connection issue. If it resolves to the old IP address or something else unexpected...

2) Flush the DNS. From the command prompt, just run "ipconfig /flushdns" and it will remove any cache of DNS resolution so you can verify the IP is coming from the latest changes on the DNS server. Also this is a good time to check the DNS1 and DNS2 IP addresses in the NIC properties to verify the PC/Server is pointing at the correct DNS server. Then Ping again. If you get the same problem, move on to the next step.
3) Ping the IP address. If you can't Ping it, it doesn't always mean anything since some servers are configured to prevent pinging them. But if you know you could ping it from another machine, you should be able to ping it from the current machine.

4) Check DNS Suffixes. If you have multiple domains like we do, during this type transition you could have some resources showing up under the wrong zone due to config errors. So perhaps SERVER1.domain1.com now is setup under SERVER1.domain2.com. If you add DNS suffixes to the suffix list in the NIC properties, you can allow more opportunity for short names to resolve. By that I mean when you commonly go to SERVER1 rather than typing SERVER1.domain1.com, the suffix list will kick in to help.

5) For file shares, use the IP. If you can't map a drive or reach a resource through \\SERVER1 then just type \\IPADDRESS and see if you can reach the file share. When DNS issues are in play, often it is best to just work around it until the dust settles. Make sure you document the places where you do this type of thing so you can go back and change them to names when the issues are resolved.

6) If you must, change the hosts file. I save this for a last resort, but especially on servers, it is sometimes necessary in order to get processes working quickly. The hosts file (on Windows XP/2003, find it at Windows\system32\drivers\etc\hosts) allows you to bypass the DNS server. You type in an IP address followed by the name you want to use to resolve to that IP address. For example, adding "1.1.1.1 MyPC" to the hosts file means that if you ping or reference "MyPC" it will resolve to IP 1.1.1.1. It is like a personal DNS server for your PC/Server managed only by you.

WARNING: If you make changes to this file, you are overriding the real DNS server so do it with caution, and preferably only do it on a temporary basis. If you leave in your changes, you risk having the IP changing again in the future, the DNS server updating, and you never getting the changes.

Those are some of the highlights of repeating issues and resolutions I used this last week. Hope it helps someone else in their time of transition.