Retrieving public dns of EC2 instance with BOTO3

William Rudisill picture William Rudisill · Jan 11, 2016 · Viewed 8.4k times · Source

I'm using ipython to get an understanding of Boto3 and interacting with EC2 instances. Here is the code I'm using to create an instance:

import boto3

ec2 = boto3.resource('ec2')
client = boto3.client('ec2')


new_instance = ec2.create_instances(
    ImageId='ami-d05e75b8',
    MinCount=1,
    MaxCount=1,
    InstanceType='t2.micro',
    KeyName=<name_of_my_key>,
    SecurityGroups=['<security_group_name>'],
    DryRun = False
    )

This starts an EC2 instance fine, and I can get the public DNS name, ip and other info from the AWS console. But, when I try to get the public DNS using Boto, by doing this:

new_instance[0].public_dns_name

Returns blank quotes. Yet, other instance details, such as:

new_instance[0].instance_type

Returns the correct information.

Any ideas? Thanks.

EDIT:

So if I do:

def get_name(inst):
    client = boto3.client('ec2')
    response = client.describe_instances(InstanceIds = [inst[0].instance_id])
    foo = response['Reservations'][0]['Instances'][0]['NetworkInterfaces'][0]['Association']['PublicDnsName']
    return foo


foo = get_name(new_instance)
print foo

Then it will return the public DNS. But it doesn't make sense to me why I need to do all of this.

Answer

Jordon Phillips picture Jordon Phillips · Jan 12, 2016

The Instance object you get back is only hydrated with the response attributes from the create_instances call. Since the DNS name is not available until the instance has reached the running state [1], it will not be immediately present. I imagine the time between you creating the instance and calling describe instances is long enough for the micro instance to start.

import boto3

ec2 = boto3.resource('ec2')
instances = ec2.create_instances(
    ImageId='ami-f0091d91',
    MinCount=1,
    MaxCount=1,
    InstanceType='t2.micro',
    KeyName='<KEY-NAME>',
    SecurityGroups=['<GROUP-NAME>'])
instance = instances[0]

# Wait for the instance to enter the running state
instance.wait_until_running()

# Reload the instance attributes
instance.load()
print(instance.public_dns_name)