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Old 08-27-2015, 09:23 AM   #1
biosboy4
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Round Robin DNS


Hello,

Does anyone here have experience in using RR DNS for High Availability?

I'm not talking geography here, just a simple server failover using dns.
I'm setting up an email server for educational purposes and I want to see if I can get a failover without going with some expensive solution like Doubletake, or VMwareFT.

Also, if there are other free solutions besides dns, I'm all ears for that too.

Thanks,

PS: the servers will most likely be backing up consistently using a plain ole xover cable. (Haven't figured out how to accomplish that for free yet but that's the plan.)
 
Old 08-27-2015, 11:31 AM   #2
lazydog
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Using RR DNS for HA is a bad idea. Here is my reasoning why:

1. DNS has no idea of when a server goes down so it is going to continue to server the bad ip address.

2. You have to manually change DNS to get it to stop issuing the bad ip address.

3. DNS records have a TTL on them. Depending on the TTL other clients/server will keep the bad record until the TTL expires.

4. Setting a short TTL on DNS record causes unnecessary traffic on the internet and your DNS server to get hammered.

5. Round Robin != Fail over.

What you could do is add a third box. This box would run iptables and forward your traffic on to the main server and once that server is off line forward the traffic onto the other box. This would be done with a monitoring script that would watch the main server and when it goes down re-write your rule so that traffic is router the failover server.
 
Old 08-27-2015, 01:54 PM   #3
brebs
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Quote:
Originally Posted by lazydog View Post
Round Robin != Fail over.
Yes, but it *is* a simple, easy, reasonable approximation, when it is assumed that the client will *retry*, and that the retry will not stubbornly always stick to the same IP address as previously.

Therefore your arguments are invalid
 
Old 08-28-2015, 08:36 AM   #4
lazydog
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Quote:
Originally Posted by brebs View Post
Yes, but it *is* a simple, easy, reasonable approximation, when it is assumed that the client will *retry*, and that the retry will not stubbornly always stick to the same IP address as previously.

Therefore your arguments are invalid
There is where you are mistaking. It will stick with the same address until the TTL has expired or cached information has been manually cleared. Only then will the client request the ip address again from DNS and then you cannot be sure they will get the working one unless you have changed your DNS setting before they made the new request.
 
Old 08-28-2015, 08:56 AM   #5
brebs
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Quote:
Originally Posted by lazydog View Post
Only then will the client request the ip address again
So? The 2 IP addresses should *already* be in the DNS - it's not a case of the DNS *changing*.

It is up to the *client* to perform the "round-robin" switcharound.
 
Old 08-28-2015, 09:57 AM   #6
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Round Robin for Mail?
http://classicyuppie.com/idea-dns-cr...-2-mx-records/
 
  


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