How to use FQDN in firewall rules for GNU/Linux?

"As per Darren's suggestion, I wrote up the a shell script that looks up the IP, then adjusts firewall rules as necessary (and, by necessary, I meant delete everything from earlier and replace with the right IP). Here's the script:

#!/bin/bash

target_hosts=""dynhost.does-not-exist.com another-host.does-not-exist.com""

if [ -f ""/root/dynblock-curr"" ]; then
mv /root/dynblock-curr /root/dynblock-prev
fi

touch /root/dynblock-curr

if [ -f ""/root/dynblock-prev"" ]; then
# Remove previously set firewall allows
for prev_ip in `cat /root/dynblock-prev`; do
ufw delete allow from $prev_ip to any app OpenSSH > /dev/null
done
fi

for target_host in $target_hosts; do
# Look up IP per host
# echo ""Looking up IP for host:"" $target_host
target_ip=`host $target_host | cut -d ' ' -f 4`
if [ $? -eq 0 ]; then
echo $target_ip >> /root/dynblock-curr
ufw allow from $target_ip to any app OpenSSH > /dev/null
fi
done

Obviously, I didn't intend to spend more than the required number of brain cells on this. This has been tested and guaranteed to Work For Me™. This is executed through cron every 15 minutes.

Another (just as obvious) note: I ended up using ufw to manage iptables rules for me (as I said, minimum brain cell count)."