Some people new to the internet have a hard time understanding why an off-site backup DNS server is necessary. After all, if you have two machines each running DNS you’re set, right?
Actually, there are plenty of reasons why you need to have an off-site DNS server. These include:
- Power Outages: If you have an extended backup, it can affect all servers at a specific site.
- Circuit Outages: If your circuit to your ISP goes down, your servers are no longer reachable from the internet, thus as far as the internet is concerned they are not there.
- ISP/Internet Routing Problems: If your ISP or major points on the internet have problems connecting to each other, chances are that a portion of the internet cannot see your DNS servers. Again, that means that those servers which can’t see you think you don’t exist.
The related question goes something like: “Well, so what if our name servers our down for a while? Everything will just work when they come up, right?” Well, yes and no. When your name servers aren’t available, two main problems occur:
- Mail Bounces. Most mail servers will actually return mail if they can’t resolve the DNS name of the system they are going to. If they can resolve the DNS name, then they will usually hold onto the mail until you come back up.
- Web users get messages saying you don’t exist, instead of getting messages indicating your server is “down or not responding”.
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