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I have a project wherein I need to update configuration files each time an EC2 instance is booted with the Public DNS address of the current instance. I'll be using Perl or Sed for this, so that's not really the question, but the real question is: is there a way that I can determine the instance's public DNS address? Is there an EC2 api that I can access from the instance to determine it?

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up vote 22 down vote accepted

There is. From inside the instance, you can run:


To get the public DNS hostname, you can change that to:


You can get the private IP for the instance, too:


As a side note, you can double-check it against a non-AWS site on the internet, like http://ip4.me


pubip=$( curl http://ip4.me 2>/dev/null | sed -e 's#<[^>]*>##g' | grep '^[0-9]' )

echo $pubip

That will work, generally, to check the "public IP" of any NATed system, or to find your public proxy IP, etc.

And here's a good link to read up on the types of information you can get from Amazon's API: http://www.ducea.com/2009/06/01/howto-update-dns-hostnames-automatically-for-your-amazon-ec2-instances/

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PUBLIC_HOSTNAME="$(curl 2>/dev/null)" :) You, sir, are metal. – Naftuli Tzvi Kay Nov 11 '11 at 3:26
glad to help :) – Tim Kennedy Nov 11 '11 at 3:28

I define this function inside my .bashrc to retrieve the public ip and dns:

export PUBLIC_DNS=`curl 2>/dev/null`
export PUBLIC_IP=`curl 2>/dev/null`

function get-pub() {
  if [ $# -ne 1 ]; then
    echo "Invalid number of arguments"
    return 1
    case $1 in
        echo $PUBLIC_DNS
        echo $PUBLIC_IP
        echo $"Usage: get-pub {dns|ip}"
        return 2
  return 0
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