Allow host with dynamic ip through iptables firewall

I have a telnet enabled service, which doesn't have any form of authentication. Naturally I don't want to expose this to the public internet - so it's firewall. Now I want to allow my host to access it, but my host have a dynamic ip address. Iptables only support static ips and ip ranges - but for my use case I only wanted a single ip.

My solution is to update rules in iptables using cron, such that only the ip found at my dynamic dns is allowed through the firewall.

HOSTNAME=$1
IP=$(host $HOSTNAME | grep -iE "[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+" |cut -f4 -d' '|head -n 1)

# If chain for remote doesn't exist, create it
if ! /sbin/iptables -L $HOSTNAME -n >/dev/null 2>&1 ; then 
  /sbin/iptables -N $HOSTNAME >/dev/null 2>&1
fi

# Flush old rules, and add new
/sbin/iptables -F $HOSTNAME
/sbin/iptables -I $HOSTNAME -s $IP -j ACCEPT

# Add chain to INPUT filter if it doesn't exist
if ! /sbin/iptables -C INPUT -t filter -j $HOSTNAME >/dev/null 2>&1 ; then
  /sbin/iptables -t filter -I INPUT -j $HOSTNAME
fi

Example usage: 

./dnsallow.sh my.dynamic.dns.com