Shell script to fetch public ip address and update Amazon Rout53 DNS. Requires cli53 (https://github.com/barnybug/cli53). If run from crontab you might need to include the full path to cli53 in line 31. Type `which cli53` to get full path.

ddns.sh

#!/bin/sh

TTL="300"
ZONE="example.com"
HOSTS="@ subdomain0 subdomain1" #subdomains seperated by spaces
LASTIP_CHECK="example.com" #domain to check to se if current IP matches public DNS records

# Use -f to force ip change
if echo "$1" | grep -q "\-f" ; then
    FORCE=true
else
    FORCE=false
fi

 # Get Public DNS A record IP 
LASTIP=`dig +short $LASTIP_CHECK`
# Get WAN IP address from OpenDNS
WAN=`dig +short myip.opendns.com @resolver1.opendns.com`

# Check if we need to update the DNS records
echo "The current IP is: $WAN"
if [ "$LASTIP" != "$WAN" ] || [ $FORCE = true ]; then
    echo "IP changed from $LASTIP to $WAN"
else
    echo "WAN IP has not changed"
    echo "Cowardly refusing to proceed!"
    exit
fi

for h in $HOSTS
do
    echo $h
    cli53 rrcreate --replace $ZONE "$h $TTL A $WAN"
done

ddnscrontab

SHELL=/usr/local/bin/bash
PATH=/etc:/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin
# Order of crontab fields
# minute        hour    mday    month   wday    command
*/5             *       *       *       *       /home/ddns/bin/ddns.sh

iam policy

{
    "Statement": [
        {
            "Effect": "Allow",
            "Action": [
                "route53:GetHostedZone",
                "route53:ListResourceRecordSets",
                "route53:ChangeResourceRecordSets"
            ],
            "Resource": "arn:aws:route53:::hostedzone/<zone id>"
        },
        {
            "Effect": "Allow",
            "Action": [
                "route53:ListHostedZones",
                "route53:ListHostedZonesByName"
            ],
            "Resource": "*"
        }
    ]
}

Nice mate!