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From Marcus Bointon <mar...@synchromedia.co.uk>
Subject Re: EC2 node adding trouble
Date Thu, 26 May 2011 07:36:32 GMT
On 26 May 2011, at 00:17, aaron morton wrote:

> I've seen discussion of using the EIP but I do not have direct experience. 

The idea is not to use the external IP, but the external DNS name because of this very useful
trick (please excuse me if you already know this!):

Say the DNS name of an elastic IP assigned to an instance is ec2-50-18-223-109.compute-1.amazonaws.com,
then from outside EC2:

#host ec2-50-18-223-109.compute-1.amazonaws.com
ec2-50-18-223-109.compute-1.amazonaws.com has address 50.18.223.109

But from inside EC2:

#host ec2-50-18-223-109.compute-1.amazonaws.com
ec2-50-18-223-109.compute-1.amazonaws.com has address 10.126.13.22

If you suspend and resume an instance, its internal IP will change, but the external will
not (if it's assigned the same elastic IP), but if you use the external name, you'll get consistent
behaviour whatever happens. This is extremely useful!

Of course it would be extremely useful if we could get behaviour like this without having
to assign an elastic IP, as this is a waste of IPs otherwise.

Marcus
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