#!/usr/bin/python
#
# Dynamically update DNS records when the EC2 instance boots.
# Requires boto3 installed on the instance.
# Requries an IAM policy attached to an instance role.
#

import urllib2
from boto3.session import Session
import boto3
import datetime

def getRegion():
    url = "http://169.254.169.254/2009-04-04/meta-data/placement/availability-zone"
    try:
        resp = urllib2.urlopen(urllib2.Request(url))
        # Region will be a string like eu-central-1a. This chops the last character off.
        region = resp.read()[:-1]
    except urllib2.HTTPError as e:
        if e.code == 404:
            return None
        if e.code == 500:
            return None
        raise

    return region

def getInstanceId():
    url = "http://169.254.169.254/2009-04-04/meta-data/instance-id"
    try:
        resp = urllib2.urlopen(urllib2.Request(url))
        instanceid = resp.read()
    except urllib2.HTTPError as e:
        if e.code == 404:
            return None
        if e.code == 500:
            return None
        raise

    return instanceid

def getPublicIP():
    url = "http://169.254.169.254/2009-04-04/meta-data/public-ipv4"
    try:
        resp = urllib2.urlopen(urllib2.Request(url))
        value = resp.read()
    except urllib2.HTTPError as e:
        if e.code == 404:
            return None
        if e.code == 500:
            return None
        raise

    return value

# Get region and instance ID
myregion = getRegion()
myinstance = getInstanceId()
myipv4 = getPublicIP()

if( myregion == None or myinstance == None or myipv4 == None):
    print "ERROR: Could not get instance metadata. Abort."
    exit( -1 )

# Connect to EC2 and get the instance handle for this instance
ec2 = boto3.resource('ec2', region_name=myregion)
instance = ec2.Instance(myinstance)

# Grab the tags associated with this instance
mytags = instance.tags
dnsname = None

# Get host tag or bail
for tag in instance.tags:
    if( tag['Key'] == 'dnsname'):
        dnsname = tag['Value']

if (dnsname == None):
    print "Did not find dnsname tag. "
    exit( -1 )

# Get just the domain part of dnsname
myzone = dnsname.partition(".")[2]

# Get zoneid for zone or bail
session = Session(region_name=myregion)
client = session.client('route53')
# Note: if you have more than 100 hosted zones, there's a chance that
# the zone you need won't be returned in response to the first call
# to list_hosted_zones(). The next loop will fail to find your zone.
#
# If this applies to you, you have to rewrite this section to use
# pagination to iterate over the collection of returned domains,
# looking for the one you want. That extension is left as an
# exercise for the reader.
myzoneid = None
zones = client.list_hosted_zones()

# Route53 will return zones in canonical form with the trailing dot.
# e.g.: "ec2.example.com."
# It's likely that people will enter dnsnames in the tags without the
# trailing dot, so this helps make the match either way.
for zone in zones['HostedZones']:
    if( (zone['Name'] == myzone) or (zone['Name'] == myzone + ".")):
        myzoneid = zone['Id']

if( myzoneid == '' ):
    # didn't find the zone
    print "ERROR: Did not find " + myzone + " among the hosted zones."
    exit( -1 )

# Create record set update
print "setting " + dnsname + " to " + myipv4
now = datetime.datetime.now()

response = client.change_resource_record_sets(
    HostedZoneId=myzoneid,
    ChangeBatch={
        'Comment': 'Instance ' + myinstance + ' boot at ' + now.strftime("%Y-%m-%d %H:%M"),
        'Changes': [
            {
                'Action': 'UPSERT',
                'ResourceRecordSet': {
                    'TTL': 300,
                    'Name': dnsname,
                    'Type': 'A',
                    'ResourceRecords': [
                        {
                            'Value': myipv4
                        },
                    ]
                }
            },
        ]
    }
)

# Check for error.
if( response['ResponseMetadata']['HTTPStatusCode'] != 200 ):
    print "ERROR updating Route53"
    print response
    exit( -1 )