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Using Elastic IP to Identify Internal Instances on Amazon EC2

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Using Elastic IP to Identify Internal Instances on Amazon EC2

Amazon EC2 supports Elastic IP Addresses to implement the effect of having a static IP address for public servers running on EC2. You can point the Elastic IP at any of your EC2 instances, changing the active instance at any time, without changing the IP address seen by the public outside of EC2.
This is a valuable feature for things like web and email servers, especially if you need to replace a failing server or upgrade or downgrade the hardware capabilities of the server, but read on for an insiders’ secret way to use Elastic IP addresses for non-public servers.

Internal Servers

Not all servers should be publicly accessible. For example, you may have an internal EC2 instance which hosts your database server accessed by other application instances inside EC2. You want to architect your installation so that you can replace the database server (instance failure, resizing, etc) but you want to make it easy to get all your application servers to start using the new instance.
There are a number of design approaches which people have used to accomplish this, including:

  1. Hard code the internal IP address into the applications and modify it whenever the internal server changes to a new instance (ugh and ouch).
  2. Run your own DNS server (or use an external DNS service) and change the IP address of the internal hostname to the new internal IP address (extra work and potentially extra failover time waiting for DNS propagation).
  3. Store the internal IP address in something like SimpleDB and change it when you want to point to a new EC2 instance (extra work and requires extra coding for clients to keep checking the SimpleDB mapping)

The following approach is the one I use and is the topic of the rest of this article:

  1. Assign an Elastic IP to the internal instance and use the external Elastic IP DNS name. To switch servers, simply re-assign the Elastic IP to a new EC2 instance

This last option uses a little-known feature of the Elastic IP Address system as implemented by Amazon EC2:

When an EC2 instance queries the external DNS name of an Elastic IP, the EC2 DNS server returns the internal IP address of the instance to which the Elastic IP address is currently assigned.

You may need to read that a couple times to grasp the implications as it is non-obvious that an “external” name will return an “internal” address.

Setting Up

You can create an Elastic IP address in an number of ways including the EC2 Console or the EC2 API command line tools. For example:

$ ec2-allocate-address 
ADDRESS 75.101.137.243

The address returned at this point is the external Elastic IP address. You don’t want to use this external IP address directly for internal server access since you would be charged for network traffic.
The next step is to assign the Elastic IP address to an EC2 instance (which is going to be your internal server):

$ ec2-associate-address -i i-07612d6e 75.101.137.243
ADDRESS 75.101.137.243  i-07612d6e

Once the Elastic IP has been assigned to an instance, you can describe that instance to find the external DNS name (which will include the external Elastic IP address in it):

$ ec2-describe-instances i-07612d6e | egrep ^INSTANCE | cut -f4
ec2-75-101-137-243.compute-1.amazonaws.com

This is the permanent external DNS name for that Elastic IP address no matter how many times you change the instance to which it is assigned. If you query this DNS name from outside of EC2, it will resolve to the external IP address as shown above:

$ dig +short ec2-75-101-137-243.compute-1.amazonaws.com
75.101.137.243

However, if you query this DNS name from inside an EC2 instance, it will resolve to the internal IP address for the instance to which it is currently assigned:

$ dig +short ec2-75-101-137-243.compute-1.amazonaws.com
10.254.171.132

You can now use this external DNS name in your applications on EC2 instances to communicate with the server over the internal EC2 network and you won’t be charged for the network traffic as long as you’re in the same EC2 availability zone.

Changing Servers

If you ever need to move the service to a new EC2 instance, simply reassign the Elastic IP address to the new EC2 instance:

$ ec2-associate-address -i i-3b783452 75.101.137.243
ADDRESS 75.101.137.243  i-3b783452

and the original external DNS name will immediately resolve to the internal IP address of the new instance:

$ dig +short ec2-75-101-137-243.compute-1.amazonaws.com
10.254.171.132

Existing connections will fail and new connections to the external DNS name will automatically be opened on the new instance and

Using CNAME

It is not entirely intuitive to have your application use names like ec2-75-101-137-243.compute-1.amazonaws.com but you can make it clearer by creating a permanent entry in your DNS which points to that name with a CNAME alias. For example, using bind:

db.example.com.    CNAME    ec2-75-101-137-243.compute-1.amazonaws.com.

You can then use db.example.com to refer to the server internally and still not have to update your DNS when you change instances.

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