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Can I configure my domain, example.com's name servers as:

ns1.dyndns.com ns2.dyndns.com ns1.opendns.com ns2.opendns.com

That is, combining free dns services to create a redundant name server array? Note these name servers from different companies are not aware other companies' name servers also serve our domain.

In case one company, say, ns1(2).dyndns.com is down, will people experience interruption when visiting my example.com? If one name server is unreachable, the next name server will be tried, or?

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Correct, if a nameserver failes to respond another one will be tried. It'll be double work keeping both the free services up to date. That's no problem if you dont do alot of changes. –  Mark Mar 13 '11 at 8:14
    
I'm sorry I don't quite understand what Jonnyuser said above and then voretaq7 agreed. I do created free accounts on every name server dyndns.com, dyndns.com, etc and configured ips for something.mydomain.com there. Therefore I believe they are all authoritative name servers for something.mydomain.com, not cache servers. So something.mydomain.com won't vanish as long as one name server still works, right? Slashdot just ran a story about Emergency Internet Bunkers: it.slashdot.org/story/11/03/12/1521258/… . I was just thinking anyone can at least creat –  John Mar 13 '11 at 18:14
    
The problem is, since I entered all those dyndns and opendns etc name servers in the registrar (networksolutions), the .com DNS servers know all these name servers: "dig ns @a.gtld-servers.net example.com" produced all these dyndns and opendns name servers in the "authority section". So that means, if any of them is down, then the .com DNS servers will go to the remaining dyndns and opendns etc name servers for the records, right? –  John Mar 14 '11 at 1:35
    
It depends on, where you register your domain. If you register your domain at no-ip.com for example, your domain's SOA is the no-ip.com of course. But if I understand the situation well, you have a DNS server, and you want to create backup for it at other DNS providers. It's possible, you need a trusted connection with an another DNS provider, who accept you as the owner of your DNS record, and accept zone updates from you. With a little search I found many of them. http://www.twisted4life.com/ for example. –  Jonnyuser Mar 14 '11 at 12:46
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3 Answers

You can configure in that way, but it's not really effective. Your subdomains something.yourdomain.com only cached for short time on public domain servers. After it, only the yourdomain.com will be resolved, because it's stored on .com domain. You can really count on your ns2 if your ns1 is down to solve your internal domain.

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Specifying multiple authoritative DNS servers IS redundancy. Most public providers of authoritative primary DNS have good networks, often geographically and topologically distributed, to prevent outages. They're very good at what they do.

If you're paranoid you can add a secondary from another company, possibly on another continent (Google "Free Secondary DNS" - there's a ton of them).

Simply pointing your authoritative nameservers at public CACHING nameservers is a Bad Idea. They won't return authoritative results (some resolver libraries may not like the authoritative NS returning cached results) and as Jonnyuser pointed out you've got no control on how long the domain data will be cached for (besides hoping that TTL is respected), after which your domain will vanish.

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Sorry, I didn't want to be too technical. You should consider how DNS works. If you create a zone (yourdomain.com), this zone will be yours on your DNS server. The NS2 and other backup servers will be get it with zone transfer and they think them as own zone, like yours. This servers are the authoritative servers. If they need this zone, they know this zone is ours, and look them locally. But there is a parent DNS server for the zone. In this example the .com. The .com DNS servers know you are the authoritative DNS owner of the yourdomain.com, and if the yourdomain.com serial number is grown, they get the zone with zone transfer, to have the new version. They cache the zone, for other servers, but with a TTL. After some time, the TTL expires, and they get the zone again from the authoritative DNS server, in this example, yours. If they can't, they think this zone is no longer available, and delete it from their zones. That's the reason, you can't use this pubic DNS servers as backups. Only authoritative DNS servers can work as a backup. Every DNS record knows who are their authoritative servers, it's in the zone. (The SOA record). This explanation could be lead far :)

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