def is_ec2_instance():
    """Try fetching instance metadata at 'curl http://169.254.169.254/latest/meta-data/'
    to see if host is on an ec2 instance"""

    # Note: this code assumes that docker containers running on ec2 instances
    # inherit instances metadata, which they do as of 2016-08-25

    global IS_EC2_INSTANCE

    if IS_EC2_INSTANCE != -1:
        # Returned the cached value
        return IS_EC2_INSTANCE

    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    s.settimeout(0.2)
    try:
        s.connect(("169.254.169.254", 80))
        IS_EC2_INSTANCE = 1
        return True
    except socket.timeout:
        IS_EC2_INSTANCE = 0
        return False
    except socket.error:
        IS_EC2_INSTANCE = 0
        return False