Simple script to check specified domains dynamic IP & update iptable ssh rule.

dock_myip.sh

#!/bin/bash
# file: dock_myip.sh

#----
# Simple script to check specified domains dynamic IP & update iptable ssh rule
#----

#---- 
# To Use:
# Make this file executable:    chmod +x dock_myip.sh
# Copy to scripts folder:       cp dock_myip.sh /usr/local/bin
# Update the "Editable Vars".
# Run crontab:                  sudo crontab -e
# Set crontab for every hour:   0 * * * *  /usr/local/bin/dock_myip.sh
#----

#----
# Helpers For Debugging:
# Check iptables rules:             sudo iptables -L --line-numbers -n
# Delete iptables rule by number:   sudo iptables -D INPUT 1
#----

#----BEGIN EDITABLE VARS----

# Domain prefix to get IP from (without the .com)
domain_prefix=digitalsailboat

# Output file to write the IP to (example.txt)
output_txt_file=$domain_prefix.txt

# Output path for the IP text file (/var/example.txt)
output_txt_path='/tmp'

# SSH Port being used
ssh_port=22

# Used for logging
is_debug=true

#----END EDITABLE VARS-------

# Looking up IP & writing the ip to txt file
nslookup $domain_prefix.com resolver1.opendns.com | awk -F': ' 'NR==6 { print $2 } ' > $output_txt_path/$output_txt_file

# Abandon everything if any of the above didn't work for some reason
if [ $? -eq 1 ] ; then
    exit 1
fi

# Creating an ipset group if it doesn't already exist (-!)
sudo ipset create -! $domain_prefix hash:ip
# Adding IP from txt file to ipset group if it doesn't exist (-!)
while read ip; do
    sudo ipset add -! $domain_prefix $ip
    if [ "$is_debug" = true ] ; then
        logger -i "dock_myip.sh: Added ${ip} to ipset if it doesn't already exist."
    fi
done < $output_txt_path/$output_txt_file

# Using the -C flag to check the existence of iptable rule via exit-code.
sudo iptables -C INPUT -m set --match-set $domain_prefix src -p tcp --dport $ssh_port -j ACCEPT
RESULT=$?
if [ $RESULT -eq 0 ]; then
    if [ "$is_debug" = true ] ; then
        logger -i "dock_myip.sh: Skipped adding iptable...iptable rule already exists."
    fi
else
    if [ "$is_debug" = true ] ; then
        logger -i "dock_myip.sh: Adding iptable rule."
    fi
    # Rule doesn't already exist, so setting a new rule to allow all IP's from ipset group to ssh port
    sudo iptables -A INPUT -m set --match-set $domain_prefix src -p tcp --dport $ssh_port -j ACCEPT
fi

if [ "$is_debug" = true ] ; then
    logger -i "dock_myip.sh: Finished. Check ipset group: ${domain_prefix} & iptable rule."
fi

exit 0