How to find the IP address of an amazon EC2 instance on reboot

Question

On reboot, the IP address of an amazon instance changes. How to find the new IP address using java API?

Pattison

Why don't you assign a fixed elastic IP? – Satterfield

Answer 1

Assuming you don't want to assign an Elastic IP address (and there are reasons why this is not always a solution) then simply call DescribeInstances on the rebooted instance, which returns a bunch of information including Public IP Address.

Here's the AWS EC2 Java API Documentation on the topic.

Hypnotherapy

I upvoted you Eight-Bit-Guru. Do you know of any easy way to get the InstanceID of the last created instance for immediately associating an ElasticIP? To my knowledge it requires parsing the returned info from ec2-run-instances or parsing JSON using bash with the return JSON from aws ec2 run-instances. Trying to avoid additional dependencies. – Conciliar

To clarify the DescribeInstances command above, I like to use this: aws ec2 describe-instances | grep "Value\|PublicIpAddress" (of course you'll need the aws cli installed) – Squint

Why thank you, kind anonymous downvoter. Your reason for chipping my rep down slightly by chastising my six-year-old accepted answer is so blatantly obvious that no comment is required! – Hypnotherapy

Answer 2

On reboot, the IP addresses of an EC2 instance do not change. They do generally change on stop/start of a non-VPC EBS boot instance.

See my answer to your related question here:

https://stackoverflow.com/questions/7533871/difference-between-rebooting-and-stop-starting-an-amazon-ec2-instance

That said, you can find the private and public IP addresses through the API call for DescribeInstances in your particular language.

If you are on the instance itself, you can also find the IP addresses through the user-data API using simple HTTP:

http://instance-data/latest/meta-data/local-ipv4
http://instance-data/latest/meta-data/public-ipv4

For example,

wget -qO- http://instance-data/latest/meta-data/public-ipv4

Elastic IP addresses are recommended for keeping a consistent (static) externally facing IP address for a particular service or server. These need to be re-assigned to an instance after a stop/start (but not after a reboot).

Brey

This is a solid complete answer. The "wget" provides the specific correct answer and the surrounding prose is also helpful/useful. – Antigen

Answer 3

curl http://169.254.169.254/latest/meta-data/public-ipv4

Spectrometer

This should be the accepted answer, and is also what they say in the docs. docs.aws.amazon.com/AWSEC2/latest/UserGuide/… – Periodontal

What is that weird IP? Is it a security threat to call this unknown IP from one's server? :D I suggested an edit explaining that IP and looks like it wasn't accepted – Aphesis

@RavindranathAkila no, you can use this command from inside your EC2 instance – Whiteness

This no longer works, they've updated the metadata API apparently. – Amphibole

Answer 4

The public IPv4 address is also available from the EC2 instance like so:

curl checkip.amazonaws.com

And the public hostname is:

dig -x $(curl -s checkip.amazonaws.com) +short

Demure

this actually worked better. The above answer /public-ipv4 was working until recently, unsure if AWS changed something or restricting traffic through the ELB made a difference, but I was getting my elastic IP reported back on both instances. The curl checkip however did it correctly, my elastic ip on one and a random assigned on the other. – Indra

Answer 5

this is how I did it on an Ubuntu 12.04 EC2 instance within a VPC:

curl ifconfig.me

not sure why but public-ipv4 was not found under the meta-data url

Bea

Works on CentOS 7 – Vindication

Answer 6

In order to fetch the public IP of the instance you first need to get the instance id of that instance. You can get the instance id of the instance by using the following java code.

 List<Instance> instances = runInstancesResult.getReservation().getInstances();

 String instanceId = instances.get(0).toString().substring(13, 23);

And now in order to get the public IP you can use the following java code.

public void fetchInstancePublicIP() {
    DescribeInstancesRequest request = new   DescribeInstancesRequest().withInstanceIds("i-d99ae7d2");
    DescribeInstancesResult result= ec2.describeInstances(request);
    List <Reservation> list  = result.getReservations();

    for (Reservation res:list) {
         List <Instance> instanceList= res.getInstances();

         for (Instance instance:instanceList){

                 System.out.println("Public IP :" + instance.getPublicIpAddress());     
                 System.out.println("Public DNS :" + instance.getPublicDnsName());
                 System.out.println("Instance State :" + instance.getState());
                 System.out.println("Instance TAGS :" + instance.getTags());
         }     
    }
}

Sirkin

Answer 7

They have util class which facilitates access to EC2 metadata. For instance

EC2MetadataUtils.getNetworkInterfaces().get(0).getPublicHostname();
EC2MetadataUtils.getNetworkInterfaces().get(0).getPublicIPv4s();

Nidus

Answer 8

Assuming your rebooting the instance and not launching from scratch than you can assign an elastic IP which always remains with the ec2 instance(unless you move the IP to another server). This allows you to point all your DNS to that one IP and not worry that a reboot will cause you issues.

I think thats what your asking but there are other things you could be asking. The internal IP of the server changes(if you relaunch the instance not reboot) and you can't 'reserve' it so you may need to create a script to keep you the new IP(if your pointing internal services to it).

hope that helps

Harlie

Answer 9

you Can Use This.

                var ipofnewServer = string.Empty;    
                while (string.IsNullOrEmpty(ipofnewServer))
                {
                    var statusRequest1 = new DescribeInstancesRequest
                    {
                        InstanceIds = new List<string>() { instanceId }
                    };

                    var result = amazonEc2client.DescribeInstances(statusRequest1);
                    var status = result.Reservations[0].Instances.FirstOrDefault();
                    if (status != null)
                    {
                        ipofnewServer = status.PublicIpAddress;
                    }
                }

Mechanician

Answer 10

You can use CLI CRUD aplication to AWS resources. AWS-CRUD-Manager

To find all ec2 instances

./awsconsole.py ec2 all

To find all of data for one ec2 instances (including IP priv and public)

./awsconsole.py ec2 find -i <id-insta`ce>

Unfriended

Answer 11

I was using domain=$(hostname | cut -d "." -f1) however that was getting the ip as ip-1-2-3-4. Are you not able to use hostname -i

Luteous