DynDNS with Curl and dig.

curldyndns.sh

#!/bin/bash

DNS="
  firstdns.com
  seconddns.com
"
SERVER=dyndns.xxx/nic/update
USERNAME=xxx
PASSWORD=xxx

# arguments:
# update_dyndns.sh version=4 or 6
if [[ $# -eq 0 ]] ; then
	echo "No version specified"
	exit
elif [[ $1 -ne 4 && $1 -ne 6 ]] ; then
	echo "IP-Version can be only 4 or 6"
	exit
fi

# init
UPDATE=false
VER=$1
IP=$(dig -$VER TXT +short o-o.myaddr.l.google.com @ns1.google.com)
IPFILE="/tmp/current_ip"$VER

echo IPv$VER: $IP

# check for update
if [ ! -f $IPFILE ] ; then
    UPDATE=true
else
    IPOLD=$(cat $IPFILE)
    if [ "$IPOLD" != "$IP" ] ; then
        UPDATE=true;
    fi
fi

# no need for update
if [ $UPDATE == "false" ] ; then
	echo "No UPDATE needed"
	exit
fi

# do update
echo $IP > $IPFILE
for dns in $DNS; do
	echo UPDATING IPv$VER: $dns;
	curl -D - --user $USERNAME:$PASSWORD https://$SERVER?hostname=$dns
done